Bug#1012311: libc6: invalid debug symbol for optind

[Mathis MARION <mathis.marion@silabs.com>]

Package: libc6
Version: 2.31-13+deb11u3
Severity: normal

Dear Maintainer,

Here is a simple example program:


#include <unistd.h>

int main()
{
    optind = 2;
}


I compiled it with 'gcc -g -O0' and ran it through gdb:


(gdb) b main
Breakpoint 1 at 0x1129: file main.c, line 5.
(gdb) r
Starting program: /home/marionm/test/optind/a.out 

Breakpoint 1, main () at main.c:5
5		optind = 2;
(gdb) n
6	}
(gdb) p optind
$1 = 1
(gdb) p &optind
$2 = (int *) 0x7ffff7fa1344 <optind>
(gdb) disassemble 
Dump of assembler code for function main:
   0x0000555555555125 <+0>:	push   %rbp
   0x0000555555555126 <+1>:	mov    %rsp,%rbp
   0x0000555555555129 <+4>:	movl   $0x2,0x2ef5(%rip)        # 0x555555558028 <optind@GLIBC_2.2.5>
   0x0000555555555133 <+14>:	mov    $0x0,%eax
=> 0x0000555555555138 <+19>:	pop    %rbp
   0x0000555555555139 <+20>:	ret    
End of assembler dump.
(gdb) 


We can see that the address used by GDB when accessing 'optind' is not the same
as the one present in the assembly code (0x7ffff7fa1344 vs 0x555555558028).

When running this experiment on Debian with the packaged gdb version 10 and 11,
we get the unexpected behavior described above. The same test run on Fedora with
gdb version 12 results in the expected behavior of seeing the same address on
both sides.

I originally made the silly mistake of believing that 'optind' provided the
index of the current option being processed, while it actually contains the next
option to be processed. When debugging, gdb would show me a value of 1 in optind
(as I wrongly expected) instead of the real value of 3, which led to a difficult
debugging experience.

This issue might also be caused by gdb instead of libc but I don't have
a deep enough understanding of the problem to ensure one or the other.

-- System Information:
Debian Release: 11.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable-debug'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 5.16.0-0.bpo.4-amd64 (SMP w/12 CPU threads; PREEMPT)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages libc6 depends on:
ii  libcrypt1  1:4.4.18-4
ii  libgcc-s1  10.2.1-6

Versions of packages libc6 recommends:
ii  libidn2-0       2.3.0-5
pn  libnss-nis      <none>
pn  libnss-nisplus  <none>

Versions of packages libc6 suggests:
ii  debconf [debconf-2.0]  1.5.77
pn  glibc-doc              <none>
ii  libc-l10n              2.31-13+deb11u3
ii  locales                2.31-13+deb11u3

-- debconf information:
  glibc/kernel-not-supported:
  glibc/disable-screensaver:
  glibc/upgrade: true
  glibc/restart-services:
  libraries/restart-without-asking: false
  glibc/kernel-too-old:
  glibc/restart-failed: