• John Reiser's avatar
    execve filename: document and export via auxiliary vector · 65191087
    John Reiser authored
    The Linux kernel puts the filename argument of execve() into the new
    address space.  Many developers are surprised to learn this.  Those who
    know and could use it, object "But it's not documented."
    
    Those who want to use it dislike the expression
      (char *)(1+ strlen(env[-1+ n_env]) + env[-1+ n_env])
    because it requires locating the last original environment variable,
    and assumes that the filename follows the characters.
    
    This patch documents the insertion of the filename, and makes it easier
    to find by adding a new tag AT_EXECFN in the ElfXX_auxv_t; see <elf.h>.
    
    In many cases readlink("/proc/self/exe",) gives the same answer.  But if
    all the original pages get unmapped, then the kernel erases the symlink
    for /proc/self/exe.  This can happen when a program decompressor does a
    good job of cleaning up after uncompressing directly to memory, so that
    the address space of the target program looks the same as if compression
    had never happened.  One example is http://upx.sourceforge.net
    
     .
    
    One notable use of the underlying concept (what path containED the
    executable) is glibc expanding $ORIGIN in DT_RUNPATH.  In practice for
    the near term, it may be a good idea for user-mode code to use both
    /proc/self/exe and AT_EXECFN as fall-back methods for each other.
    /proc/self/exe can fail due to unmapping, AT_EXECFN can fail because it
    won't be present on non-new systems.  The auxvec or {AT_EXECFN}.d_val
    also can get overwritten, although in nearly all cases this would be the
    result of a bug.
    
    The runtime cost is one NEW_AUX_ENT using two words of stack space.  The
    underlying value is maintained already as bprm->exec; setup_arg_pages()
    in fs/exec.c slides it for stack_shift, etc.
    Signed-off-by: default avatarJohn Reiser <jreiser@BitWagon.com>
    Cc: Roland McGrath <roland@redhat.com>
    Cc: Jakub Jelinek <jakub@redhat.com>
    Cc: Ulrich Drepper <drepper@redhat.com>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    65191087