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elf: Fix GL(dl_phdr) and GL(dl_phnum) for static builds [BZ #29864]

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The 73fc4e28b9 refactor did not add the GL(dl_phdr) and
GL(dl_phnum) for static build, relying on the __ehdr_start symbol,
which is always added by the static linker, to get the correct values.

This is problematic in some ways:

  - The segment may see its in-memory size differ from its in-file
    size (or the binary may have holes).  The Linux has fixed is to
    provide concise values for both AT_PHDR and AT_PHNUM (commit
    0da1d5002745c - "fs/binfmt_elf: Fix AT_PHDR for unusual ELF files")

  - Some archs (alpha for instance) the hidden weak reference is not
    correctly pulled by the static linker and  __ehdr_start address
    end up being 0, which makes GL(dl_phdr) and GL(dl_phnum) have both
    invalid values (and triggering a segfault later on libc.so while
    accessing TLS variables).

The safer fix is to just restore the previous behavior to setup
GL(dl_phdr) and GL(dl_phnum) for static based on kernel auxv.  The
__ehdr_start fallback can also be simplified by not assuming weak
linkage (as for PIE).

The libc-static.c auxv init logic is moved to dl-support.c, since
the later is build without SHARED and then GLRO macro is defined
to access the variables directly.

The _dl_phdr is also assumed to be always non NULL, since an invalid
NULL values does not trigger TLS initialization (which is used in
various libc systems).

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
This commit is contained in:
Adhemerval Zanella 2023-01-03 09:56:28 -03:00
parent 402853be1d
commit 7e31d16651
3 changed files with 43 additions and 47 deletions
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21
csu/libc-start.c
View File

@ -262,28 +262,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
}
# endif
_dl_aux_init (auxvec);
if (GL(dl_phdr) == NULL)
# endif
{
/* Starting from binutils-2.23, the linker will define the
magic symbol __ehdr_start to point to our own ELF header
if it is visible in a segment that also includes the phdrs.
So we can set up _dl_phdr and _dl_phnum even without any
information from auxv. */
extern const ElfW(Ehdr) __ehdr_start
# if BUILD_PIE_DEFAULT
__attribute__ ((visibility ("hidden")));
# else
__attribute__ ((weak, visibility ("hidden")));
if (&__ehdr_start != NULL)
# endif
{
assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
GL(dl_phdr) = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
GL(dl_phnum) = __ehdr_start.e_phnum;
}
}
__tunables_init (__environ);

1
csu/libc-tls.c
View File

@ -119,7 +119,6 @@ __libc_setup_tls (void)
__tls_pre_init_tp ();
/* Look through the TLS segment if there is any. */
if (_dl_phdr != NULL)
for (phdr = _dl_phdr; phdr < &_dl_phdr[_dl_phnum]; ++phdr)
if (phdr->p_type == PT_TLS)
{

20
elf/dl-support.c
View File

@ -256,6 +256,25 @@ _dl_aux_init (ElfW(auxv_t) *av)
for (int i = 0; i < array_length (auxv_values); ++i)
auxv_values[i] = 0;
_dl_parse_auxv (av, auxv_values);
_dl_phdr = (void*) auxv_values[AT_PHDR];
_dl_phnum = auxv_values[AT_PHNUM];
if (_dl_phdr == NULL)
{
/* Starting from binutils-2.23, the linker will define the
magic symbol __ehdr_start to point to our own ELF header
if it is visible in a segment that also includes the phdrs.
So we can set up _dl_phdr and _dl_phnum even without any
information from auxv. */
extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
_dl_phdr = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
_dl_phnum = __ehdr_start.e_phnum;
}
assert (_dl_phdr != NULL);
}
#endif
@ -324,7 +343,6 @@ _dl_non_dynamic_init (void)
if (_dl_platform != NULL)
_dl_platformlen = strlen (_dl_platform);
if (_dl_phdr != NULL)
for (const ElfW(Phdr) *ph = _dl_phdr; ph < &_dl_phdr[_dl_phnum]; ++ph)
switch (ph->p_type)
{