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

Jan Kratochvil
2014-09-20T21:30:44Z

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