6519108746
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: John 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
36 lines
1.3 KiB
C
36 lines
1.3 KiB
C
#ifndef _LINUX_AUXVEC_H
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#define _LINUX_AUXVEC_H
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#include <asm/auxvec.h>
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/* Symbolic values for the entries in the auxiliary table
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put on the initial stack */
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#define AT_NULL 0 /* end of vector */
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#define AT_IGNORE 1 /* entry should be ignored */
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#define AT_EXECFD 2 /* file descriptor of program */
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#define AT_PHDR 3 /* program headers for program */
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#define AT_PHENT 4 /* size of program header entry */
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#define AT_PHNUM 5 /* number of program headers */
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#define AT_PAGESZ 6 /* system page size */
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#define AT_BASE 7 /* base address of interpreter */
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#define AT_FLAGS 8 /* flags */
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#define AT_ENTRY 9 /* entry point of program */
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#define AT_NOTELF 10 /* program is not ELF */
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#define AT_UID 11 /* real uid */
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#define AT_EUID 12 /* effective uid */
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#define AT_GID 13 /* real gid */
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#define AT_EGID 14 /* effective gid */
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#define AT_PLATFORM 15 /* string identifying CPU for optimizations */
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#define AT_HWCAP 16 /* arch dependent hints at CPU capabilities */
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#define AT_CLKTCK 17 /* frequency at which times() increments */
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#define AT_SECURE 23 /* secure mode boolean */
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#define AT_EXECFN 31 /* filename of program */
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#ifdef __KERNEL__
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#define AT_VECTOR_SIZE_BASE 17 /* NEW_AUX_ENT entries in auxiliary table */
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/* number of "#define AT_.*" above, minus {AT_NULL, AT_IGNORE, AT_NOTELF} */
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#endif
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#endif /* _LINUX_AUXVEC_H */
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