Re: time to workaround libc/13097 in fsf gdb?

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Fri, 19 Sep 2014 16:41:34 +0200, Pedro Alves wrote:
> Here it is.  WDYT?
[...]
> --- a/gdb/solib-svr4.c
> +++ b/gdb/solib-svr4.c
[...]
> @@ -1478,6 +1479,42 @@ svr4_current_sos (void)
>    return svr4_current_sos_direct (info);
>  }
> 
> +/* Implement the "current_sos" target_so_ops method.  */
> +
> +static struct so_list *
> +svr4_current_sos (void)
> +{
> +  struct so_list *so_head = svr4_current_sos_1 ();
> +  struct objfile *objfile;
> +  struct obj_section *osect;
> +  CORE_ADDR vsyscall_addr = gdbarch_vsyscall_address (target_gdbarch ());
> +
> +  /* Filter out the vDSO module, if present.  Its symbol file would
> +     not be found on disk.  The vDSO/vsyscall's OBJFILE is instead
> +     managed by symfile-mem.c:add_vsyscall_page.  */
> +  if (vsyscall_addr != 0)
> +    {
> +      struct so_list **sop;
> +
> +      sop = &so_head;
> +      while (*sop != NULL)
> +	{
> +	  struct so_list *so = *sop;
> +
> +	  if (so->lm_info->l_addr_inferior == vsyscall_addr)

This won't work as l_addr_inferior (and also l_addr) do not necessarily
represent the real starting address of the ELF if the ELF itself is
"prelinked".  For some reason vDSOs on some kernels look like prelinked.

kernel-3.16.2-200.fc20.x86_64 appears sane, vDSO is 0-based.

But for example kernel-2.6.32-220.el6.x86_64 is "prelinked", see below.


That's the pain of solib-svr4.c which is OS-agnostic and so it cannot find out
start of the ELF file just from link map.  gdbserver can find it as it can
depend on /proc/PID/{s,}maps as its linux-low.c is Linux-specific.
That was implemented in unfinished/pending:
	[PATCH v5 0/8] Validate binary before use
	Message-ID: <20140319223004.14668.20989.stgit@host1.jankratochvil.net>


Thanks,
Jan


kernel-2.6.32-220.el6.x86_64
(gdb) p *_r_debug.r_map.l_next
$4 = {l_addr = 140737363566592, l_name = 0x3deba1ade4 "", l_ld = 0x7ffff7ffe580, l_next = 0x7ffff7ffd658, l_prev = 0x3debc21188}
(gdb) p/x *_r_debug.r_map.l_next
$5 = {l_addr = 0x7ffff88fe000, l_name = 0x3deba1ade4, l_ld = 0x7ffff7ffe580, l_next = 0x7ffff7ffd658, l_prev = 0x3debc21188}
# (gdb) p/x *new.lm_info
# $5 = {l_addr = 0x0, l_addr_inferior = 0x7ffff88fe000, l_addr_p = 0x0, lm_addr = 0x3debc21718, l_ld = 0x7ffff7ffe580, l_next = 0x7ffff7ffd658, l_prev = 0x3debc21188, l_name = 0x3deba1ade4}
(gdb) info auxv
33   AT_SYSINFO_EHDR      System-supplied DSO's ELF header 0x7ffff7ffe000
(gdb) info proc mappings
      0x7ffff7ffe000     0x7ffff7fff000     0x1000          0                           [vdso]
(gdb) dump memory vdso.bin 0x7ffff7ffe000 0x7ffff7fff000
# readelf -Wa vdso.bin
[...]
  Entry point address:               0xffffffffff700700
[...]
Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .hash             HASH            ffffffffff700120 000120 000038 04   A  2   0  8
  [ 2] .dynsym           DYNSYM          ffffffffff700158 000158 0000d8 18   A  3   2  8
[...]
  [ 9] .dynamic          DYNAMIC         ffffffffff700580 000580 0000f0 10  WA  3   0  8