• Steven Rostedt (Red Hat)'s avatar
    ftrace/module: Hardcode ftrace_module_init() call into load_module() · a949ae56
    Steven Rostedt (Red Hat) authored
    A race exists between module loading and enabling of function tracer.
    
    	CPU 1				CPU 2
    	-----				-----
      load_module()
       module->state = MODULE_STATE_COMING
    
    				register_ftrace_function()
    				 mutex_lock(&ftrace_lock);
    				 ftrace_startup()
    				  update_ftrace_function();
    				   ftrace_arch_code_modify_prepare()
    				    set_all_module_text_rw();
    				   <enables-ftrace>
    				    ftrace_arch_code_modify_post_process()
    				     set_all_module_text_ro();
    
    				[ here all module text is set to RO,
    				  including the module that is
    				  loading!! ]
    
       blocking_notifier_call_chain(MODULE_STATE_COMING);
        ftrace_init_module()
    
         [ tries to modify code, but it's RO, and fails!
           ftrace_bug() is called]
    
    When this race happens, ftrace_bug() will produces a nasty warning and
    all of the function tracing features will be disabled until reboot.
    
    The simple solution is to treate module load the same way the core
    kernel is treated at boot. To hardcode the ftrace function modification
    of converting calls to mcount into nops. This is done in init/main.c
    there's no reason it could not be done in load_module(). This gives
    a better control of the changes and doesn't tie the state of the
    module to its notifiers as much. Ftrace is special, it needs to be
    treated as such.
    
    The reason this would work, is that the ftrace_module_init() would be
    called while the module is in MODULE_STATE_UNFORMED, which is ignored
    by the set_all_module_text_ro() call.
    
    Link: http://lkml.kernel.org/r/1395637826-3312-1-git-send-email-indou.takao@jp.fujitsu.com
    
    Reported-by: default avatarTakao Indoh <indou.takao@jp.fujitsu.com>
    Acked-by: default avatarRusty Russell <rusty@rustcorp.com.au>
    Cc: stable@vger.kernel.org # 2.6.38+
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    a949ae56