printk.c 74.6 KB
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/*
 *  linux/kernel/printk.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 * Modified to make sys_syslog() more flexible: added commands to
 * return the last 4k of kernel messages, regardless of whether
 * they've been read or not.  Added option to suppress kernel printk's
 * to the console.  Added hook for sending the console messages
 * elsewhere, in preparation for a serial line console (someday).
 * Ted Ts'o, 2/11/93.
 * Modified for sysctl support, 1/8/97, Chris Horn.
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 * Fixed SMP synchronization, 08/08/99, Manfred Spraul
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 *     manfred@colorfullife.com
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 * Rewrote bits to get rid of console_lock
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 *	01Mar01 Andrew Morton
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 */

#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/console.h>
#include <linux/init.h>
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#include <linux/jiffies.h>
#include <linux/nmi.h>
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#include <linux/module.h>
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#include <linux/moduleparam.h>
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#include <linux/interrupt.h>			/* For in_interrupt() */
#include <linux/delay.h>
#include <linux/smp.h>
#include <linux/security.h>
#include <linux/bootmem.h>
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#include <linux/memblock.h>
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#include <linux/aio.h>
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#include <linux/syscalls.h>
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#include <linux/kexec.h>
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#include <linux/kdb.h>
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#include <linux/ratelimit.h>
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#include <linux/kmsg_dump.h>
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#include <linux/syslog.h>
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#include <linux/cpu.h>
#include <linux/notifier.h>
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#include <linux/rculist.h>
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#include <linux/poll.h>
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#include <linux/irq_work.h>
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#include <linux/utsname.h>
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#include <asm/uaccess.h>

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#define CREATE_TRACE_POINTS
#include <trace/events/printk.h>

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#include "console_cmdline.h"
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#include "braille.h"
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/* printk's without a loglevel use this.. */
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#define DEFAULT_MESSAGE_LOGLEVEL CONFIG_DEFAULT_MESSAGE_LOGLEVEL
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/* We show everything that is MORE important than this.. */
#define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */
#define DEFAULT_CONSOLE_LOGLEVEL 7 /* anything MORE serious than KERN_DEBUG */

int console_printk[4] = {
	DEFAULT_CONSOLE_LOGLEVEL,	/* console_loglevel */
	DEFAULT_MESSAGE_LOGLEVEL,	/* default_message_loglevel */
	MINIMUM_CONSOLE_LOGLEVEL,	/* minimum_console_loglevel */
	DEFAULT_CONSOLE_LOGLEVEL,	/* default_console_loglevel */
};

/*
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 * Low level drivers may need that to know if they can schedule in
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 * their unblank() callback or not. So let's export it.
 */
int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);

/*
 * console_sem protects the console_drivers list, and also
 * provides serialisation for access to the entire console
 * driver system.
 */
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static DEFINE_SEMAPHORE(console_sem);
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struct console *console_drivers;
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EXPORT_SYMBOL_GPL(console_drivers);

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#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_lock_dep_map = {
	.name = "console_lock"
};
#endif

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/*
 * Helper macros to handle lockdep when locking/unlocking console_sem. We use
 * macros instead of functions so that _RET_IP_ contains useful information.
 */
#define down_console_sem() do { \
	down(&console_sem);\
	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
} while (0)

static int __down_trylock_console_sem(unsigned long ip)
{
	if (down_trylock(&console_sem))
		return 1;
	mutex_acquire(&console_lock_dep_map, 0, 1, ip);
	return 0;
}
#define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_)

#define up_console_sem() do { \
	mutex_release(&console_lock_dep_map, 1, _RET_IP_);\
	up(&console_sem);\
} while (0)

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/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it are racing, but it helps tracking those weird code
 * path in the console code where we end up in places I want
 * locked without the console sempahore held
 */
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static int console_locked, console_suspended;
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/*
 * If exclusive_console is non-NULL then only this console is to be printed to.
 */
static struct console *exclusive_console;

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/*
 *	Array of consoles built from command line options (console=)
 */

#define MAX_CMDLINECONSOLES 8

static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
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static int selected_console = -1;
static int preferred_console = -1;
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int console_set_on_cmdline;
EXPORT_SYMBOL(console_set_on_cmdline);
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/* Flag: console code may call schedule() */
static int console_may_schedule;

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/*
 * The printk log buffer consists of a chain of concatenated variable
 * length records. Every record starts with a record header, containing
 * the overall length of the record.
 *
 * The heads to the first and last entry in the buffer, as well as the
 * sequence numbers of these both entries are maintained when messages
 * are stored..
 *
 * If the heads indicate available messages, the length in the header
 * tells the start next message. A length == 0 for the next message
 * indicates a wrap-around to the beginning of the buffer.
 *
 * Every record carries the monotonic timestamp in microseconds, as well as
 * the standard userspace syslog level and syslog facility. The usual
 * kernel messages use LOG_KERN; userspace-injected messages always carry
 * a matching syslog facility, by default LOG_USER. The origin of every
 * message can be reliably determined that way.
 *
 * The human readable log message directly follows the message header. The
 * length of the message text is stored in the header, the stored message
 * is not terminated.
 *
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 * Optionally, a message can carry a dictionary of properties (key/value pairs),
 * to provide userspace with a machine-readable message context.
 *
 * Examples for well-defined, commonly used property names are:
 *   DEVICE=b12:8               device identifier
 *                                b12:8         block dev_t
 *                                c127:3        char dev_t
 *                                n8            netdev ifindex
 *                                +sound:card0  subsystem:devname
 *   SUBSYSTEM=pci              driver-core subsystem name
 *
 * Valid characters in property names are [a-zA-Z0-9.-_]. The plain text value
 * follows directly after a '=' character. Every property is terminated by
 * a '\0' character. The last property is not terminated.
 *
 * Example of a message structure:
 *   0000  ff 8f 00 00 00 00 00 00      monotonic time in nsec
 *   0008  34 00                        record is 52 bytes long
 *   000a        0b 00                  text is 11 bytes long
 *   000c              1f 00            dictionary is 23 bytes long
 *   000e                    03 00      LOG_KERN (facility) LOG_ERR (level)
 *   0010  69 74 27 73 20 61 20 6c      "it's a l"
 *         69 6e 65                     "ine"
 *   001b           44 45 56 49 43      "DEVIC"
 *         45 3d 62 38 3a 32 00 44      "E=b8:2\0D"
 *         52 49 56 45 52 3d 62 75      "RIVER=bu"
 *         67                           "g"
 *   0032     00 00 00                  padding to next message header
 *
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 * The 'struct printk_log' buffer header must never be directly exported to
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 * userspace, it is a kernel-private implementation detail that might
 * need to be changed in the future, when the requirements change.
 *
 * /dev/kmsg exports the structured data in the following line format:
 *   "level,sequnum,timestamp;<message text>\n"
 *
 * The optional key/value pairs are attached as continuation lines starting
 * with a space character and terminated by a newline. All possible
 * non-prinatable characters are escaped in the "\xff" notation.
 *
 * Users of the export format should ignore possible additional values
 * separated by ',', and find the message after the ';' character.
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 */

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enum log_flags {
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	LOG_NOCONS	= 1,	/* already flushed, do not print to console */
	LOG_NEWLINE	= 2,	/* text ended with a newline */
	LOG_PREFIX	= 4,	/* text started with a prefix */
	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
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};

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struct printk_log {
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	u64 ts_nsec;		/* timestamp in nanoseconds */
	u16 len;		/* length of entire record */
	u16 text_len;		/* length of text buffer */
	u16 dict_len;		/* length of dictionary buffer */
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	u8 facility;		/* syslog facility */
	u8 flags:5;		/* internal record flags */
	u8 level:3;		/* syslog level */
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};

/*
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 * The logbuf_lock protects kmsg buffer, indices, counters.
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 */
static DEFINE_RAW_SPINLOCK(logbuf_lock);
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#ifdef CONFIG_PRINTK
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DECLARE_WAIT_QUEUE_HEAD(log_wait);
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/* the next printk record to read by syslog(READ) or /proc/kmsg */
static u64 syslog_seq;
static u32 syslog_idx;
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static enum log_flags syslog_prev;
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static size_t syslog_partial;
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/* index and sequence number of the first record stored in the buffer */
static u64 log_first_seq;
static u32 log_first_idx;

/* index and sequence number of the next record to store in the buffer */
static u64 log_next_seq;
static u32 log_next_idx;

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/* the next printk record to write to the console */
static u64 console_seq;
static u32 console_idx;
static enum log_flags console_prev;

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/* the next printk record to read after the last 'clear' command */
static u64 clear_seq;
static u32 clear_idx;

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#define PREFIX_MAX		32
#define LOG_LINE_MAX		1024 - PREFIX_MAX
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/* record buffer */
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#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
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#define LOG_ALIGN 4
#else
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#define LOG_ALIGN __alignof__(struct printk_log)
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#endif
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#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
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static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
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static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;

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/* human readable text of the record */
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static char *log_text(const struct printk_log *msg)
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{
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	return (char *)msg + sizeof(struct printk_log);
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}

/* optional key/value pair dictionary attached to the record */
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static char *log_dict(const struct printk_log *msg)
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{
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	return (char *)msg + sizeof(struct printk_log) + msg->text_len;
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}

/* get record by index; idx must point to valid msg */
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static struct printk_log *log_from_idx(u32 idx)
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{
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	struct printk_log *msg = (struct printk_log *)(log_buf + idx);
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	/*
	 * A length == 0 record is the end of buffer marker. Wrap around and
	 * read the message at the start of the buffer.
	 */
	if (!msg->len)
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		return (struct printk_log *)log_buf;
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	return msg;
}

/* get next record; idx must point to valid msg */
static u32 log_next(u32 idx)
{
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	struct printk_log *msg = (struct printk_log *)(log_buf + idx);
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	/* length == 0 indicates the end of the buffer; wrap */
	/*
	 * A length == 0 record is the end of buffer marker. Wrap around and
	 * read the message at the start of the buffer as *this* one, and
	 * return the one after that.
	 */
	if (!msg->len) {
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		msg = (struct printk_log *)log_buf;
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		return msg->len;
	}
	return idx + msg->len;
}

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/*
 * Check whether there is enough free space for the given message.
 *
 * The same values of first_idx and next_idx mean that the buffer
 * is either empty or full.
 *
 * If the buffer is empty, we must respect the position of the indexes.
 * They cannot be reset to the beginning of the buffer.
 */
static int logbuf_has_space(u32 msg_size, bool empty)
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{
	u32 free;

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	if (log_next_idx > log_first_idx || empty)
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		free = max(log_buf_len - log_next_idx, log_first_idx);
	else
		free = log_first_idx - log_next_idx;

	/*
	 * We need space also for an empty header that signalizes wrapping
	 * of the buffer.
	 */
	return free >= msg_size + sizeof(struct printk_log);
}

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static int log_make_free_space(u32 msg_size)
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{
	while (log_first_seq < log_next_seq) {
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		if (logbuf_has_space(msg_size, false))
			return 0;
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		/* drop old messages until we have enough continuous space */
		log_first_idx = log_next(log_first_idx);
		log_first_seq++;
	}
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	/* sequence numbers are equal, so the log buffer is empty */
	if (logbuf_has_space(msg_size, true))
		return 0;

	return -ENOMEM;
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}

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/* compute the message size including the padding bytes */
static u32 msg_used_size(u16 text_len, u16 dict_len, u32 *pad_len)
{
	u32 size;

	size = sizeof(struct printk_log) + text_len + dict_len;
	*pad_len = (-size) & (LOG_ALIGN - 1);
	size += *pad_len;

	return size;
}

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/*
 * Define how much of the log buffer we could take at maximum. The value
 * must be greater than two. Note that only half of the buffer is available
 * when the index points to the middle.
 */
#define MAX_LOG_TAKE_PART 4
static const char trunc_msg[] = "<truncated>";

static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
			u16 *dict_len, u32 *pad_len)
{
	/*
	 * The message should not take the whole buffer. Otherwise, it might
	 * get removed too soon.
	 */
	u32 max_text_len = log_buf_len / MAX_LOG_TAKE_PART;
	if (*text_len > max_text_len)
		*text_len = max_text_len;
	/* enable the warning message */
	*trunc_msg_len = strlen(trunc_msg);
	/* disable the "dict" completely */
	*dict_len = 0;
	/* compute the size again, count also the warning message */
	return msg_used_size(*text_len + *trunc_msg_len, 0, pad_len);
}

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/* insert record into the buffer, discard old ones, update heads */
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static int log_store(int facility, int level,
		     enum log_flags flags, u64 ts_nsec,
		     const char *dict, u16 dict_len,
		     const char *text, u16 text_len)
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{
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	struct printk_log *msg;
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	u32 size, pad_len;
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	u16 trunc_msg_len = 0;
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	/* number of '\0' padding bytes to next message */
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	size = msg_used_size(text_len, dict_len, &pad_len);
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	if (log_make_free_space(size)) {
		/* truncate the message if it is too long for empty buffer */
		size = truncate_msg(&text_len, &trunc_msg_len,
				    &dict_len, &pad_len);
		/* survive when the log buffer is too small for trunc_msg */
		if (log_make_free_space(size))
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			return 0;
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	}
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	if (log_next_idx + size + sizeof(struct printk_log) > log_buf_len) {
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		/*
		 * This message + an additional empty header does not fit
		 * at the end of the buffer. Add an empty header with len == 0
		 * to signify a wrap around.
		 */
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		memset(log_buf + log_next_idx, 0, sizeof(struct printk_log));
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		log_next_idx = 0;
	}

	/* fill message */
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	msg = (struct printk_log *)(log_buf + log_next_idx);
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	memcpy(log_text(msg), text, text_len);
	msg->text_len = text_len;
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	if (trunc_msg_len) {
		memcpy(log_text(msg) + text_len, trunc_msg, trunc_msg_len);
		msg->text_len += trunc_msg_len;
	}
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	memcpy(log_dict(msg), dict, dict_len);
	msg->dict_len = dict_len;
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	msg->facility = facility;
	msg->level = level & 7;
	msg->flags = flags & 0x1f;
	if (ts_nsec > 0)
		msg->ts_nsec = ts_nsec;
	else
		msg->ts_nsec = local_clock();
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	memset(log_dict(msg) + dict_len, 0, pad_len);
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	msg->len = size;
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	/* insert message */
	log_next_idx += msg->len;
	log_next_seq++;
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	return msg->text_len;
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}
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#ifdef CONFIG_SECURITY_DMESG_RESTRICT
int dmesg_restrict = 1;
#else
int dmesg_restrict;
#endif

static int syslog_action_restricted(int type)
{
	if (dmesg_restrict)
		return 1;
	/*
	 * Unless restricted, we allow "read all" and "get buffer size"
	 * for everybody.
	 */
	return type != SYSLOG_ACTION_READ_ALL &&
	       type != SYSLOG_ACTION_SIZE_BUFFER;
}

static int check_syslog_permissions(int type, bool from_file)
{
	/*
	 * If this is from /proc/kmsg and we've already opened it, then we've
	 * already done the capabilities checks at open time.
	 */
	if (from_file && type != SYSLOG_ACTION_OPEN)
		return 0;

	if (syslog_action_restricted(type)) {
		if (capable(CAP_SYSLOG))
			return 0;
		/*
		 * For historical reasons, accept CAP_SYS_ADMIN too, with
		 * a warning.
		 */
		if (capable(CAP_SYS_ADMIN)) {
			pr_warn_once("%s (%d): Attempt to access syslog with "
				     "CAP_SYS_ADMIN but no CAP_SYSLOG "
				     "(deprecated).\n",
				 current->comm, task_pid_nr(current));
			return 0;
		}
		return -EPERM;
	}
	return security_syslog(type);
}


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/* /dev/kmsg - userspace message inject/listen interface */
struct devkmsg_user {
	u64 seq;
	u32 idx;
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	enum log_flags prev;
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	struct mutex lock;
	char buf[8192];
};

static ssize_t devkmsg_writev(struct kiocb *iocb, const struct iovec *iv,
			      unsigned long count, loff_t pos)
{
	char *buf, *line;
	int i;
	int level = default_message_loglevel;
	int facility = 1;	/* LOG_USER */
	size_t len = iov_length(iv, count);
	ssize_t ret = len;

	if (len > LOG_LINE_MAX)
		return -EINVAL;
	buf = kmalloc(len+1, GFP_KERNEL);
	if (buf == NULL)
		return -ENOMEM;

	line = buf;
	for (i = 0; i < count; i++) {
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		if (copy_from_user(line, iv[i].iov_base, iv[i].iov_len)) {
			ret = -EFAULT;
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			goto out;
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		}
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		line += iv[i].iov_len;
	}

	/*
	 * Extract and skip the syslog prefix <[0-9]*>. Coming from userspace
	 * the decimal value represents 32bit, the lower 3 bit are the log
	 * level, the rest are the log facility.
	 *
	 * If no prefix or no userspace facility is specified, we
	 * enforce LOG_USER, to be able to reliably distinguish
	 * kernel-generated messages from userspace-injected ones.
	 */
	line = buf;
	if (line[0] == '<') {
		char *endp = NULL;

		i = simple_strtoul(line+1, &endp, 10);
		if (endp && endp[0] == '>') {
			level = i & 7;
			if (i >> 3)
				facility = i >> 3;
			endp++;
			len -= endp - line;
			line = endp;
		}
	}
	line[len] = '\0';

	printk_emit(facility, level, NULL, 0, "%s", line);
out:
	kfree(buf);
	return ret;
}

static ssize_t devkmsg_read(struct file *file, char __user *buf,
			    size_t count, loff_t *ppos)
{
	struct devkmsg_user *user = file->private_data;
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	struct printk_log *msg;
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	u64 ts_usec;
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	size_t i;
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	char cont = '-';
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	size_t len;
	ssize_t ret;

	if (!user)
		return -EBADF;

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	ret = mutex_lock_interruptible(&user->lock);
	if (ret)
		return ret;
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	raw_spin_lock_irq(&logbuf_lock);
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	while (user->seq == log_next_seq) {
		if (file->f_flags & O_NONBLOCK) {
			ret = -EAGAIN;
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			raw_spin_unlock_irq(&logbuf_lock);
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			goto out;
		}

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		raw_spin_unlock_irq(&logbuf_lock);
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		ret = wait_event_interruptible(log_wait,
					       user->seq != log_next_seq);
		if (ret)
			goto out;
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		raw_spin_lock_irq(&logbuf_lock);
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	}

	if (user->seq < log_first_seq) {
		/* our last seen message is gone, return error and reset */
		user->idx = log_first_idx;
		user->seq = log_first_seq;
		ret = -EPIPE;
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		raw_spin_unlock_irq(&logbuf_lock);
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		goto out;
	}

	msg = log_from_idx(user->idx);
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	ts_usec = msg->ts_nsec;
	do_div(ts_usec, 1000);
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	/*
	 * If we couldn't merge continuation line fragments during the print,
	 * export the stored flags to allow an optional external merge of the
	 * records. Merging the records isn't always neccessarily correct, like
	 * when we hit a race during printing. In most cases though, it produces
	 * better readable output. 'c' in the record flags mark the first
	 * fragment of a line, '+' the following.
	 */
	if (msg->flags & LOG_CONT && !(user->prev & LOG_CONT))
		cont = 'c';
	else if ((msg->flags & LOG_CONT) ||
		 ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX)))
		cont = '+';

	len = sprintf(user->buf, "%u,%llu,%llu,%c;",
		      (msg->facility << 3) | msg->level,
		      user->seq, ts_usec, cont);
	user->prev = msg->flags;
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	/* escape non-printable characters */
	for (i = 0; i < msg->text_len; i++) {
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		unsigned char c = log_text(msg)[i];
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		if (c < ' ' || c >= 127 || c == '\\')
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			len += sprintf(user->buf + len, "\\x%02x", c);
		else
			user->buf[len++] = c;
	}
	user->buf[len++] = '\n';

	if (msg->dict_len) {
		bool line = true;

		for (i = 0; i < msg->dict_len; i++) {
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			unsigned char c = log_dict(msg)[i];
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			if (line) {
				user->buf[len++] = ' ';
				line = false;
			}

			if (c == '\0') {
				user->buf[len++] = '\n';
				line = true;
				continue;
			}

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			if (c < ' ' || c >= 127 || c == '\\') {
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				len += sprintf(user->buf + len, "\\x%02x", c);
				continue;
			}

			user->buf[len++] = c;
		}
		user->buf[len++] = '\n';
	}

	user->idx = log_next(user->idx);
	user->seq++;
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	raw_spin_unlock_irq(&logbuf_lock);
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	if (len > count) {
		ret = -EINVAL;
		goto out;
	}

	if (copy_to_user(buf, user->buf, len)) {
		ret = -EFAULT;
		goto out;
	}
	ret = len;
out:
	mutex_unlock(&user->lock);
	return ret;
}

static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
{
	struct devkmsg_user *user = file->private_data;
	loff_t ret = 0;

	if (!user)
		return -EBADF;
	if (offset)
		return -ESPIPE;

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	raw_spin_lock_irq(&logbuf_lock);
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	switch (whence) {
	case SEEK_SET:
		/* the first record */
		user->idx = log_first_idx;
		user->seq = log_first_seq;
		break;
	case SEEK_DATA:
		/*
		 * The first record after the last SYSLOG_ACTION_CLEAR,
		 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
		 * changes no global state, and does not clear anything.
		 */
		user->idx = clear_idx;
		user->seq = clear_seq;
		break;
	case SEEK_END:
		/* after the last record */
		user->idx = log_next_idx;
		user->seq = log_next_seq;
		break;
	default:
		ret = -EINVAL;
	}
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	raw_spin_unlock_irq(&logbuf_lock);
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	return ret;
}

static unsigned int devkmsg_poll(struct file *file, poll_table *wait)
{
	struct devkmsg_user *user = file->private_data;
	int ret = 0;

	if (!user)
		return POLLERR|POLLNVAL;

	poll_wait(file, &log_wait, wait);

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	raw_spin_lock_irq(&logbuf_lock);
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	if (user->seq < log_next_seq) {
		/* return error when data has vanished underneath us */
		if (user->seq < log_first_seq)
			ret = POLLIN|POLLRDNORM|POLLERR|POLLPRI;
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		else
			ret = POLLIN|POLLRDNORM;
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	}
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	return ret;
}

static int devkmsg_open(struct inode *inode, struct file *file)
{
	struct devkmsg_user *user;
	int err;

	/* write-only does not need any file context */
	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
		return 0;

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	err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
				       SYSLOG_FROM_READER);
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	if (err)
		return err;

	user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
	if (!user)
		return -ENOMEM;

	mutex_init(&user->lock);

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	raw_spin_lock_irq(&logbuf_lock);
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	user->idx = log_first_idx;
	user->seq = log_first_seq;
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	raw_spin_unlock_irq(&logbuf_lock);
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	file->private_data = user;
	return 0;
}

static int devkmsg_release(struct inode *inode, struct file *file)
{
	struct devkmsg_user *user = file->private_data;

	if (!user)
		return 0;

	mutex_destroy(&user->lock);
	kfree(user);
	return 0;
}

const struct file_operations kmsg_fops = {
	.open = devkmsg_open,
	.read = devkmsg_read,
	.aio_write = devkmsg_writev,
	.llseek = devkmsg_llseek,
	.poll = devkmsg_poll,
	.release = devkmsg_release,
};

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#ifdef CONFIG_KEXEC
/*
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 * This appends the listed symbols to /proc/vmcore
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 *
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 * /proc/vmcore is used by various utilities, like crash and makedumpfile to
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 * obtain access to symbols that are otherwise very difficult to locate.  These
 * symbols are specifically used so that utilities can access and extract the
 * dmesg log from a vmcore file after a crash.
 */
void log_buf_kexec_setup(void)
{
	VMCOREINFO_SYMBOL(log_buf);
	VMCOREINFO_SYMBOL(log_buf_len);
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	VMCOREINFO_SYMBOL(log_first_idx);
	VMCOREINFO_SYMBOL(log_next_idx);
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	/*
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	 * Export struct printk_log size and field offsets. User space tools can
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	 * parse it and detect any changes to structure down the line.
	 */
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	VMCOREINFO_STRUCT_SIZE(printk_log);
	VMCOREINFO_OFFSET(printk_log, ts_nsec);
	VMCOREINFO_OFFSET(printk_log, len);
	VMCOREINFO_OFFSET(printk_log, text_len);
	VMCOREINFO_OFFSET(printk_log, dict_len);
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}
#endif

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/* requested log_buf_len from kernel cmdline */
static unsigned long __initdata new_log_buf_len;

/* save requested log_buf_len since it's too early to process it */
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static int __init log_buf_len_setup(char *str)
{
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	unsigned size = memparse(str, &str);
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	if (size)
		size = roundup_pow_of_two(size);
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	if (size > log_buf_len)
		new_log_buf_len = size;

	return 0;
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}
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early_param("log_buf_len", log_buf_len_setup);

void __init setup_log_buf(int early)
{
	unsigned long flags;
	char *new_log_buf;
	int free;

	if (!new_log_buf_len)
		return;
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	if (early) {
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		new_log_buf =
			memblock_virt_alloc(new_log_buf_len, PAGE_SIZE);
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	} else {
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		new_log_buf = memblock_virt_alloc_nopanic(new_log_buf_len, 0);
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	}

	if (unlikely(!new_log_buf)) {
		pr_err("log_buf_len: %ld bytes not available\n",
			new_log_buf_len);
		return;
	}

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	raw_spin_lock_irqsave(&logbuf_lock, flags);
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	log_buf_len = new_log_buf_len;
	log_buf = new_log_buf;
	new_log_buf_len = 0;
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	free = __LOG_BUF_LEN - log_next_idx;
	memcpy(log_buf, __log_buf, __LOG_BUF_LEN);
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	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
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	pr_info("log_buf_len: %d\n", log_buf_len);
	pr_info("early log buf free: %d(%d%%)\n",
		free, (free * 100) / __LOG_BUF_LEN);
}
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static bool __read_mostly ignore_loglevel;

static int __init ignore_loglevel_setup(char *str)
{
	ignore_loglevel = 1;
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	pr_info("debug: ignoring loglevel setting.\n");
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	return 0;
}

early_param("ignore_loglevel", ignore_loglevel_setup);
module_param(ignore_loglevel, bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(ignore_loglevel, "ignore loglevel setting, to"
	"print all kernel messages to the console.");

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#ifdef CONFIG_BOOT_PRINTK_DELAY

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static int boot_delay; /* msecs delay after each printk during bootup */
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static unsigned long long loops_per_msec;	/* based on boot_delay */
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static int __init boot_delay_setup(char *str)
{
	unsigned long lpj;

	lpj = preset_lpj ? preset_lpj : 1000000;	/* some guess */
	loops_per_msec = (unsigned long long)lpj / 1000 * HZ;

	get_option(&str, &boot_delay);
	if (boot_delay > 10 * 1000)
		boot_delay = 0;

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	pr_debug("boot_delay: %u, preset_lpj: %ld, lpj: %lu, "
		"HZ: %d, loops_per_msec: %llu\n",
		boot_delay, preset_lpj, lpj, HZ, loops_per_msec);
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	return 0;
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}
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early_param("boot_delay", boot_delay_setup);
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static void boot_delay_msec(int level)
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{
	unsigned long long k;
	unsigned long timeout;

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	if ((boot_delay == 0 || system_state != SYSTEM_BOOTING)
		|| (level >= console_loglevel && !ignore_loglevel)) {
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		return;
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	}
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	k = (unsigned long long)loops_per_msec * boot_delay;
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	timeout = jiffies + msecs_to_jiffies(boot_delay);
	while (k) {
		k--;
		cpu_relax();
		/*
		 * use (volatile) jiffies to prevent
		 * compiler reduction; loop termination via jiffies
		 * is secondary and may or may not happen.
		 */
		if (time_after(jiffies, timeout))
			break;
		touch_nmi_watchdog();
	}
}
#else
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static inline void boot_delay_msec(int level)
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{
}
#endif

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#if defined(CONFIG_PRINTK_TIME)
static bool printk_time = 1;
#else
static bool printk_time;
#endif
module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);

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static size_t print_time(u64 ts, char *buf)
{
	unsigned long rem_nsec;

	if (!printk_time)
		return 0;

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	rem_nsec = do_div(ts, 1000000000);

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	if (!buf)
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		return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
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	return sprintf(buf, "[%5lu.%06lu] ",
		       (unsigned long)ts, rem_nsec / 1000);
}

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static size_t print_prefix(const struct printk_log *msg, bool syslog, char *buf)
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{
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	size_t len = 0;
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	unsigned int prefix = (msg->facility << 3) | msg->level;
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	if (syslog) {
		if (buf) {
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			len += sprintf(buf, "<%u>", prefix);
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		} else {
			len += 3;
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			if (prefix > 999)
				len += 3;
			else if (prefix > 99)
				len += 2;
			else if (prefix > 9)
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				len++;
		}
	}
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	len += print_time(msg->ts_nsec, buf ? buf + len : NULL);
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	return len;
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}

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static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
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			     bool syslog, char *buf, size_t size)
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{
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	const char *text = log_text(msg);
	size_t text_size = msg->text_len;
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	bool prefix = true;
	bool newline = true;
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	size_t len = 0;

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	if ((prev & LOG_CONT) && !(msg->flags & LOG_PREFIX))
		prefix = false;

	if (msg->flags & LOG_CONT) {
		if ((prev & LOG_CONT) && !(prev & LOG_NEWLINE))
			prefix = false;

		if (!(msg->flags & LOG_NEWLINE))
			newline = false;
	}

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	do {
		const char *next = memchr(text, '\n', text_size);
		size_t text_len;

		if (next) {
			text_len = next - text;
			next++;
			text_size -= next - text;
		} else {
			text_len = text_size;
		}
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		if (buf) {
			if (print_prefix(msg, syslog, NULL) +
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			    text_len + 1 >= size - len)
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				break;
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			if (prefix)
				len += print_prefix(msg, syslog, buf + len);
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			memcpy(buf + len, text, text_len);
			len += text_len;
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			if (next || newline)
				buf[len++] = '\n';
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		} else {
			/* SYSLOG_ACTION_* buffer size only calculation */
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			if (prefix)
				len += print_prefix(msg, syslog, NULL);
			len += text_len;
			if (next || newline)
				len++;
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		}
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		prefix = true;
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		text = next;
	} while (text);
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	return len;
}

static int syslog_print(char __user *buf, int size)
{
	char *text;
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	int len = 0;
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	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
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	if (!text)
		return -ENOMEM;

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	while (size > 0) {
		size_t n;
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		size_t skip;
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		raw_spin_lock_irq(&logbuf_lock);
		if (syslog_seq < log_first_seq) {
			/* messages are gone, move to first one */
			syslog_seq = log_first_seq;
			syslog_idx = log_first_idx;
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			syslog_prev = 0;
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			syslog_partial = 0;
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		}
		if (syslog_seq == log_next_seq) {
			raw_spin_unlock_irq(&logbuf_lock);
			break;
		}
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		skip = syslog_partial;
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		msg = log_from_idx(syslog_idx);
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		n = msg_print_text(msg, syslog_prev, true, text,
				   LOG_LINE_MAX + PREFIX_MAX);
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		if (n - syslog_partial <= size) {
			/* message fits into buffer, move forward */
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			syslog_idx = log_next(syslog_idx);
			syslog_seq++;
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			syslog_prev = msg->flags;
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			n -= syslog_partial;
			syslog_partial = 0;
		} else if (!len){
			/* partial read(), remember position */
			n = size;
			syslog_partial += n;
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		} else
			n = 0;
		raw_spin_unlock_irq(&logbuf_lock);

		if (!n)
			break;

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		if (copy_to_user(buf, text + skip, n)) {
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			if (!len)
				len = -EFAULT;
			break;
		}
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		len += n;
		size -= n;
		buf += n;
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	}

	kfree(text);
	return len;
}

static int syslog_print_all(char __user *buf, int size, bool clear)
{
	char *text;
	int len = 0;

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	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
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	if (!text)
		return -ENOMEM;

	raw_spin_lock_irq(&logbuf_lock);
	if (buf) {
		u64 next_seq;
		u64 seq;
		u32 idx;
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		enum log_flags prev;
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		if (clear_seq < log_first_seq) {
			/* messages are gone, move to first available one */
			clear_seq = log_first_seq;
			clear_idx = log_first_idx;
		}

		/*
		 * Find first record that fits, including all following records,
		 * into the user-provided buffer for this dump.
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		 */
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		seq = clear_seq;
		idx = clear_idx;
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		prev = 0;
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		while (seq < log_next_seq) {
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			struct printk_log *msg = log_from_idx(idx);
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			len += msg_print_text(msg, prev, true, NULL, 0);
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			prev = msg->flags;
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			idx = log_next(idx);
			seq++;
		}
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		/* move first record forward until length fits into the buffer */
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		seq = clear_seq;
		idx = clear_idx;
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		prev = 0;
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		while (len > size && seq < log_next_seq) {
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			struct printk_log *msg = log_from_idx(idx);
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			len -= msg_print_text(msg, prev, true, NULL, 0);
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			prev = msg->flags;
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			idx = log_next(idx);
			seq++;
		}

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		/* last message fitting into this dump */
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		next_seq = log_next_seq;

		len = 0;
		while (len >= 0 && seq < next_seq) {
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			struct printk_log *msg = log_from_idx(idx);
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			int textlen;

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			textlen = msg_print_text(msg, prev, true, text,
						 LOG_LINE_MAX + PREFIX_MAX);
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			if (textlen < 0) {
				len = textlen;
				break;
			}
			idx = log_next(idx);
			seq++;
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			prev = msg->flags;
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			raw_spin_unlock_irq(&logbuf_lock);
			if (copy_to_user(buf + len, text, textlen))
				len = -EFAULT;
			else
				len += textlen;
			raw_spin_lock_irq(&logbuf_lock);

			if (seq < log_first_seq) {
				/* messages are gone, move to next one */
				seq = log_first_seq;
				idx = log_first_idx;
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				prev = 0;
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			}
		}
	}

	if (clear) {
		clear_seq = log_next_seq;
		clear_idx = log_next_idx;
	}
	raw_spin_unlock_irq(&logbuf_lock);

	kfree(text);
	return len;
}

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int do_syslog(int type, char __user *buf, int len, bool from_file)
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{
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	bool clear = false;
	static int saved_console_loglevel = -1;
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	int error;
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	error = check_syslog_permissions(type, from_file);
	if (error)
		goto out;
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	error = security_syslog(type);
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	if (error)
		return error;

	switch (type) {
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	case SYSLOG_ACTION_CLOSE:	/* Close log */
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		break;
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	case SYSLOG_ACTION_OPEN:	/* Open log */
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		break;
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	case SYSLOG_ACTION_READ:	/* Read from log */
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		error = -EINVAL;
		if (!buf || len < 0)
			goto out;
		error = 0;
		if (!len)
			goto out;
		if (!access_ok(VERIFY_WRITE, buf, len)) {
			error = -EFAULT;
			goto out;
		}
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		error = wait_event_interruptible(log_wait,
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						 syslog_seq != log_next_seq);
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		if (error)
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			goto out;
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		error = syslog_print(buf, len);
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		break;
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	/* Read/clear last kernel messages */
	case SYSLOG_ACTION_READ_CLEAR:
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		clear = true;
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		/* FALL THRU */
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	/* Read last kernel messages */
	case SYSLOG_ACTION_READ_ALL:
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		error = -EINVAL;
		if (!buf || len < 0)
			goto out;
		error = 0;
		if (!len)
			goto out;
		if (!access_ok(VERIFY_WRITE, buf, len)) {
			error = -EFAULT;
			goto out;
		}
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		error = syslog_print_all(buf, len, clear);
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		break;
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	/* Clear ring buffer */
	case SYSLOG_ACTION_CLEAR:
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		syslog_print_all(NULL, 0, true);
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		break;
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	/* Disable logging to console */
	case SYSLOG_ACTION_CONSOLE_OFF:
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		if (saved_console_loglevel == -1)
			saved_console_loglevel = console_loglevel;
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		console_loglevel = minimum_console_loglevel;
		break;
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	/* Enable logging to console */
	case SYSLOG_ACTION_CONSOLE_ON:
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		if (saved_console_loglevel != -1) {
			console_loglevel = saved_console_loglevel;
			saved_console_loglevel = -1;
		}
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		break;
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	/* Set level of messages printed to console */
	case SYSLOG_ACTION_CONSOLE_LEVEL:
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		error = -EINVAL;
		if (len < 1 || len > 8)
			goto out;
		if (len < minimum_console_loglevel)
			len = minimum_console_loglevel;
		console_loglevel = len;
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		/* Implicitly re-enable logging to console */
		saved_console_loglevel = -1;
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		error = 0;
		break;
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	/* Number of chars in the log buffer */
	case SYSLOG_ACTION_SIZE_UNREAD:
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		raw_spin_lock_irq(&logbuf_lock);
		if (syslog_seq < log_first_seq) {
			/* messages are gone, move to first one */
			syslog_seq = log_first_seq;
			syslog_idx = log_first_idx;
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			syslog_prev = 0;
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			syslog_partial = 0;
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		}
		if (from_file) {
			/*
			 * Short-cut for poll(/"proc/kmsg") which simply checks
			 * for pending data, not the size; return the count of
			 * records, not the length.
			 */
			error = log_next_idx - syslog_idx;
		} else {
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			u64 seq = syslog_seq;
			u32 idx = syslog_idx;
			enum log_flags prev = syslog_prev;
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			error = 0;
			while (seq < log_next_seq) {
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				struct printk_log *msg = log_from_idx(idx);
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				error += msg_print_text(msg, prev, true, NULL, 0);
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				idx = log_next(idx);
				seq++;
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				prev = msg->flags;
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			}
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			error -= syslog_partial;
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		}
		raw_spin_unlock_irq(&logbuf_lock);
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		break;
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	/* Size of the log buffer */
	case SYSLOG_ACTION_SIZE_BUFFER:
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		error = log_buf_len;
		break;
	default:
		error = -EINVAL;
		break;
	}
out:
	return error;
}

1346
SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
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{
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	return do_syslog(type, buf, len, SYSLOG_FROM_READER);