Re: [patch] solib-svr4.c - allow reading linkmap info from core without executable

Pedro Alves
2009-06-18T23:03:49Z

On Thursday 11 June 2009 14:39:18, Aleksandar Ristovski wrote:
> Pedro Alves wrote:
> >> if (info->debug_base == 0 && svr4_have_link_map_offsets ())
> >> {
> >> - if (exec_bfd != NULL
> >> + if ((exec_bfd != NULL
> >> && bfd_get_flavour (exec_bfd) == bfd_target_elf_flavour)
> >> + || (core_bfd != NULL
> >> + && bfd_get_flavour (core_bfd) == bfd_target_elf_flavour))
> >> info->debug_base = elf_locate_base ();
> >> }
> >
> > Are there live debugging cases (archs?, PIE?) (e.g., "target remote"
> > without specifying an executable) where letting elf_locate_base
> > try to read the debug base from the target's auxv (scan_dyntag_auxv)
> > will work? It would mean that removing the exec_bfd checks instead
> > of adding core_bfd checks, would still improve your case, while
> > letting other cases benefit as well.
> >
>
> Yes, in theory it should work as long as target_ops in
> question know how to read auxv (that is, as long as
> to_xfer_partial(ops, TARGET_OBJECT_AUXV,...) knows how to do
> it.)
>
> I had internally made this change:
>
> {
> - if (exec_bfd != NULL
> + if ((exec_bfd != NULL
> && bfd_get_flavour (exec_bfd) ==
> bfd_target_elf_flavour)
> + || exec_bfd == NULL)
> debug_base = elf_locate_base ();
> }
> return (debug_base);
>
>
> But wanted to be more conservative for FSF, since I do not
> quite understand under which circumstances could we end up
> in solib-svr4, and not have "bfd_target_elf_flavour".

I was thinking on pushing the elf check a bit down instead,
like the below. However, having now tested this, I see that
this doesn't work in most of the cores I have here (x86_64-linux).
In most cases I see, the segment that would contain the program
headers, as indicated by auxv info, isn't included in the
core...

(objdump -h)
Idx Name Size VMA LMA File off Algn
:
6 load1 00000000 0000000000400000 0000000000000000 000008f8 2**0
ALLOC, READONLY, CODE
:

--
Pedro Alves

---
gdb/solib-svr4.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

Index: src/gdb/solib-svr4.c
===================================================================
--- src.orig/gdb/solib-svr4.c 2009-06-18 20:55:50.000000000 +0100
+++ src/gdb/solib-svr4.c 2009-06-18 22:25:11.000000000 +0100
@@ -266,7 +266,7 @@ IGNORE_FIRST_LINK_MAP_ENTRY (struct so_l

/* Assume that everything is a library if the dynamic loader was loaded
late by a static executable. */
- if (bfd_get_section_by_name (exec_bfd, ".dynamic") == NULL)
+ if (exec_bfd && bfd_get_section_by_name (exec_bfd, ".dynamic") == NULL)
return 0;

return extract_typed_address (so->lm_info->lm + lmo->l_prev_offset,
@@ -600,9 +600,13 @@ scan_dyntag (int dyntag, bfd *abfd, CORE

if (abfd == NULL)
return 0;
+
+ if (bfd_get_flavour (abfd) != bfd_target_elf_flavour)
+ return 0;
+
arch_size = bfd_get_arch_size (abfd);
if (arch_size == -1)
- return 0;
+ return 0;

/* Find the start address of the .dynamic section. */
sect = bfd_get_section_by_name (abfd, ".dynamic");
@@ -825,11 +829,7 @@ locate_base (struct svr4_info *info)
though if we don't have some link map offsets to work with. */

if (info->debug_base == 0 && svr4_have_link_map_offsets ())
- {
- if (exec_bfd != NULL
- && bfd_get_flavour (exec_bfd) == bfd_target_elf_flavour)
- info->debug_base = elf_locate_base ();
- }
+ info->debug_base = elf_locate_base ();
return info->debug_base;
}