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

[Pedro Alves <pedro@codesourcery.com>]

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;
 }