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* [PATCH] Fix section symbols in objcopy when removing sections with  relocations from Elf objects
@ 2006-05-24 16:20 Julian Brown
  2006-05-24 17:23 ` Julian Brown
  2006-05-25 13:28 ` Alan Modra
  0 siblings, 2 replies; 6+ messages in thread
From: Julian Brown @ 2006-05-24 16:20 UTC (permalink / raw)
  To: binutils; +Cc: Richard Earnshaw, Julian Brown, Paul Brook

[-- Attachment #1: Type: text/plain, Size: 4089 bytes --]

Hi,

This patch fixes a problem where, if sections are removed during objcopy 
with Elf object files, the section symbols corresponding to the 
relocation sections for the removed sections may be left in the target 
object file.

In my test case, the file being manipulated is a "crt1.o", and the 
command-line to do the stripping is:

arm-none-linux-gnueabi-objcopy -R .debug_info [-R more debug sections] 
/path/to/crt1.o

I.e. "-R" is being used to remove various debugging sections, which in 
the source object file contain sections such as ".debug_info" along with 
a corresponding ".rel.debug_info" containing relocations. The -R switch 
removes both the section itself and its relocations.

However, the resulting file contains invalid section symbols like so:

Symbol table '.symtab' contains 34 entries:
    Num:    Value  Size Type    Bind   Vis      Ndx Name
     ...
     11: 00000000     0 SECTION LOCAL  DEFAULT   14
     12: 00000000     0 SECTION LOCAL  DEFAULT   16
     13: 00000000     0 SECTION LOCAL  DEFAULT   19
     14: 00000000     0 SECTION LOCAL  DEFAULT   10
     15: 00000000     0 SECTION LOCAL  DEFAULT   22
     16: 00000000     0 SECTION LOCAL  DEFAULT   23
     17: 00000000     0 SECTION LOCAL  DEFAULT   24

Even though the sections in the file are now only numbered up to 13. 
These symbols correspond to the relocation sections for the removed 
debug sections. Internally, bfd knows little about these sections -- 
they are folded into the bfd section containing information on the 
relocatee during Elf parsing.

Fixing this cleanly isn't particularly easy, because there isn't any one 
point at which all the right information seems to be readily available. 
The obvious point to remove the extraneous symbols is in 
objcopy.c:filter_symbols(), though that function is supposed to be 
target-independent, so this involves an Elf-specific hack.

Other choices might have been to remove the bad symbols whilst parsing 
in or writing out the Elf file, though since the right information isn't 
really available in either of those places either, that would probably 
involve similar or worse hacks (elf.c:bfd_section_from_shdr knows 
nothing about symbols, and elf.c:swap_out_symbols knows nothing about 
the input bfd).

Once the bad symbol sections are removed, another problem becomes 
apparent, which is that higher-numbered section symbols (for .shstrtab, 
.symtab, .strtab) are not rewritten to point to their now lower-numbered 
sections correctly. I think there's existing logic which is supposed to 
handle this, e.g. elf.c:swap_out_syms has a comment:

     /* This symbol is in a real ELF section which we did
        not create as a BFD section.  Undo the mapping done
        by copy_private_symbol_data.  */

With code following to handle several specific sections, including the 
ones which are now numbered wrongly in this case. Unfortunately, during 
an objcopy invocation, copy_private_symbol_data isn't actually ever 
called to set up the mapping before this reverse mapping takes place, so 
I've added a call to it in filter_symbols. I then discovered that the 
necessary private fields to make this to work aren't set when reading in 
the source file, so I've added those (to bfd_section_from_shdr).

I've tested the patch against the proprietary linker which originally 
barfed on the bad crt1.o file, and against hand-written tests (based on 
tweaked compiler output with -ffunction-sections turned on), and I 
believe it DTRT, at least in those cases. It's also tested with "make 
check" for --enable-targets=all.

The patch is currently against the CSL branch. OK to apply there? On 
mainline? Does this need a new testcase?

Cheers,

Julian

ChangeLog:

bfd/

     * elf.c (bfd_section_from_shdr): Set symtab_shndx_section,
     shstrtab_section, strtab_section in ELF private data.

binutils/

     * objcopy.c (filter_symbols): Filter out section symbols for
     sections which are being stripped and their associated relocation
     sections. Call bfd_copy_private_symbol_data for each symbol which
     passes the filter.

[-- Attachment #2: section-stripping-6 --]
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Index: bfd/elf.c
===================================================================
RCS file: /cvs/src/src/bfd/elf.c,v
retrieving revision 1.329
diff -c -p -r1.329 elf.c
*** bfd/elf.c	16 Mar 2006 12:20:15 -0000	1.329
--- bfd/elf.c	24 May 2006 12:52:58 -0000
*************** bfd_section_from_shdr (bfd *abfd, unsign
*** 1935,1940 ****
--- 1935,1941 ----
        BFD_ASSERT (elf_symtab_shndx (abfd) == 0);
        elf_symtab_shndx (abfd) = shindex;
        elf_tdata (abfd)->symtab_shndx_hdr = *hdr;
+       elf_tdata (abfd)->symtab_shndx_section = shindex;
        elf_elfsections (abfd)[shindex] = &elf_tdata (abfd)->symtab_shndx_hdr;
        return TRUE;
  
*************** bfd_section_from_shdr (bfd *abfd, unsign
*** 1944,1949 ****
--- 1945,1951 ----
        if (ehdr->e_shstrndx == shindex)
  	{
  	  elf_tdata (abfd)->shstrtab_hdr = *hdr;
+           elf_tdata (abfd)->shstrtab_section = shindex;
  	  elf_elfsections (abfd)[shindex] = &elf_tdata (abfd)->shstrtab_hdr;
  	  return TRUE;
  	}
*************** bfd_section_from_shdr (bfd *abfd, unsign
*** 1951,1956 ****
--- 1953,1959 ----
  	{
  	symtab_strtab:
  	  elf_tdata (abfd)->strtab_hdr = *hdr;
+           elf_tdata (abfd)->strtab_section = shindex;
  	  elf_elfsections (abfd)[shindex] = &elf_tdata (abfd)->strtab_hdr;
  	  return TRUE;
  	}
Index: binutils/objcopy.c
===================================================================
RCS file: /cvs/src/src/binutils/objcopy.c,v
retrieving revision 1.95
diff -c -p -r1.95 objcopy.c
*** binutils/objcopy.c	28 Feb 2006 16:09:01 -0000	1.95
--- binutils/objcopy.c	24 May 2006 12:52:58 -0000
*************** filter_symbols (bfd *abfd, bfd *obfd, as
*** 915,920 ****
--- 915,959 ----
  	       || undefined
  	       || bfd_is_com_section (bfd_get_section (sym)))
  	keep = strip_symbols != STRIP_UNNEEDED;
+       else if ((flags & BSF_SECTION_SYM) != 0)  /* Section symbol.  */
+         {
+           struct bfd_section *section = sym->section;
+ 
+           keep = 1;
+ 
+           if (!bfd_is_abs_section (section)
+               && is_strip_section (abfd, section))
+ 	    keep = 0;
+           else if (bfd_get_flavour (abfd) == bfd_target_elf_flavour)
+             {
+               Elf_Internal_Shdr **elfsections = elf_elfsections (abfd);
+               unsigned int sec;
+               
+               for (sec = 0; sec < elf_numsections (abfd); sec++)
+                 {
+                   Elf_Internal_Shdr *elfhdr = elfsections[sec];
+                   if ((elfhdr->sh_type == SHT_REL
+                        || elfhdr->sh_type == SHT_RELA))
+                     {
+                       char *secname =
+                         bfd_elf_string_from_elf_section (abfd,
+                           elf_elfheader (abfd)->e_shstrndx,
+                           elfhdr->sh_name);
+ 
+                       if (strcmp (secname, name) == 0)
+                         {
+                           struct bfd_section *targsec =
+                             bfd_section_from_elf_index (abfd, elfhdr->sh_info);
+                           if (targsec && is_strip_section (abfd, targsec))
+                             {
+                               keep = 0;
+                               break;
+                             }
+                         }
+                     }
+                 }
+             }
+         }
        else if ((flags & BSF_DEBUGGING) != 0)	/* Debugging symbol.  */
  	keep = (strip_symbols != STRIP_DEBUG
  		&& strip_symbols != STRIP_UNNEEDED
*************** filter_symbols (bfd *abfd, bfd *obfd, as
*** 969,975 ****
  	      sym->flags |= BSF_GLOBAL;
  	    }
  
! 	  to[dst_count++] = sym;
  	}
      }
  
--- 1008,1016 ----
  	      sym->flags |= BSF_GLOBAL;
  	    }
  
! 	  to[dst_count] = sym;
!           bfd_copy_private_symbol_data (abfd, sym, obfd, to[dst_count]);
!           dst_count++;
  	}
      }
  

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2006-05-25 16:53 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2006-05-24 16:20 [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects Julian Brown
2006-05-24 17:23 ` Julian Brown
2006-05-25  0:58   ` H. J. Lu
2006-05-25 13:28 ` Alan Modra
2006-05-25 21:44   ` Alan Modra
2006-05-25 23:10     ` Julian Brown

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