Re: [PATCH, 2.27] x86-64: Use IFUNC memcpy and mempcpy in libc.a

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Aug 2, 2017 at 11:27 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 08/02/2017 05:53 PM, H.J. Lu wrote:
>> On Sun, Jul 9, 2017 at 7:59 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>  test-ifloat128%): Force
>>>
>>> FYI, this is the patch I am submitting for glibc 2.27.
>>
>> Any comments or objections?
>
> I have not reviewed if there are any static library initialization
> issues caused by IFUNC use for string functions, but I agree in
> principle that something like this should be possible.
>

The entry point of static executable is

STATIC int
LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
int argc, char **argv,
#ifdef LIBC_START_MAIN_AUXVEC_ARG
ElfW(auxv_t) *auxvec,
#endif
__typeof (main) init,
void (*fini) (void),
void (*rtld_fini) (void), void *stack_end)
{
  /* Result of the 'main' function.  */
  int result;

  __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;

#ifndef SHARED
  char **ev = &argv[argc + 1];

  __environ = ev;

  /* Store the lowest stack address.  This is done in ld.so if this is
     the code for the DSO.  */
  __libc_stack_end = stack_end;

# ifdef HAVE_AUX_VECTOR
  /* First process the auxiliary vector since we need to find the
     program header to locate an eventually present PT_TLS entry.  */
#  ifndef LIBC_START_MAIN_AUXVEC_ARG
  ElfW(auxv_t) *auxvec;
  {
    char **evp = ev;
    while (*evp++ != NULL)
      ;
    auxvec = (ElfW(auxv_t) *) evp;
  }
#  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
__attribute__ ((weak, visibility ("hidden")));
      if (&__ehdr_start != NULL)
        {
          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;
        }
    }

  /* Initialize very early so that tunables can use it.  */
  __libc_init_secure ();

  __tunables_init (__environ);

  ARCH_INIT_CPU_FEATURES ();

  /* Perform IREL{,A} relocations.  */
  ARCH_SETUP_IREL ();

If there is no memcpy/mempcpy call before ARCH_SETUP_IREL (),
we can use IFUNC memcpy and mempcpy in static executable.
Since the code path up to ARCH_SETUP_IREL () is the same for
all static executables, the static executable tests within glibc are
sufficient to verify that it is safe to do so.


-- 
H.J.