Re: Update on the glibc segfault issue on Alpha

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 02/01/23 14:37, John Paul Adrian Glaubitz wrote:
> Hello!
> 
> On 1/2/23 12:44, John Paul Adrian Glaubitz wrote:
>> Adhemerval from glibc upstream is aware of the problem but he has not found yet a solution
>> to this issue as it needs to be debugged further. I will try to bisect which particular
>> introduced the regression and file a new upstream bug report.
>>
>> During our discussion, Adhemerval pointed out that this change [3] might be the culprit
>> but I have not been able to verify this yet.
> 
> My latest bisecting has shown this change to be the responsible change, CC'ing the author:
> 
> commit 73fc4e28b9464f0e13edc719a5372839970e7ddb (refs/bisect/bad)
> Author: Florian Weimer <fweimer@redhat.com>
> Date:   Mon Feb 28 11:50:41 2022 +0100
> 
>     Linux: Consolidate auxiliary vector parsing (redo)
>         And optimize it slightly.
>         This is commit 8c8510ab2790039e58995ef3a22309582413d3ff revised.
>         In _dl_aux_init in elf/dl-support.c, use an explicit loop
>     and -fno-tree-loop-distribute-patterns to avoid memset.
>         Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

Thanks, this commits helps narrow down the issue.  The 73fc4e28b9464f0e refactor did not
add the GL(dl_phdr) and GL(dl_phnum) for static case, relying on the __ehdr_start symbol
to get the correct values.

The issue is for some archs, alpha for instance, the hidden weak reference is not making
the static linker to define the __ehdr_start address correctly: it is being set to 0 and
thus GL(dl_phdr) and GL(dl_phnum) are set to invalid values.

And I am not sure if the hidden weak __ehdr_start does work on all architectures, so I
think it would be safer to just restore the previous behavior to setup GL(dl_phdr) and 
GL(dl_phnum) for static and we can simplify __ehdr_start fallback case to not use 
a weak ref (as for PIE). I am checking if the following patch trigger any regression,
at least for alpha it fixes the static failures:

diff --git a/csu/libc-start.c b/csu/libc-start.c
index 543560f36c..63a3eceaea 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -271,18 +271,10 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
          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;
-        }
+      extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
+      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);
diff --git a/sysdeps/unix/sysv/linux/dl-parse_auxv.h b/sysdeps/unix/sysv/linux/dl-parse_auxv.h
index bf9374371e..5913c9d6e5 100644
--- a/sysdeps/unix/sysv/linux/dl-parse_auxv.h
+++ b/sysdeps/unix/sysv/linux/dl-parse_auxv.h
@@ -56,6 +56,10 @@ void _dl_parse_auxv (ElfW(auxv_t) *av, dl_parse_auxv_t auxv_values)
   if (GLRO(dl_sysinfo_dso) != NULL)
     GLRO(dl_sysinfo) = auxv_values[AT_SYSINFO];
 #endif
+#ifndef SHARED
+  GL(dl_phdr) = (void*) auxv_values[AT_PHDR];
+  GL(dl_phnum) = auxv_values[AT_PHENT];
+#endif

   DL_PLATFORM_AUXV
 }