[PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[Julian Brown]

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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.

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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++;
  	}
      }
  

Re: [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[Julian Brown]

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Julian Brown wrote:
> 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.

This is a slightly-smarter version (thanks to offlist discussion with 
Dan) which knows about recoving Elf symbols from BFD symbols, hence 
avoids some unnecessary work.

Changelog as before.

Cheers,

Julian

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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 14:21:21 -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 14:21:21 -0000
*************** filter_symbols (bfd *abfd, bfd *obfd, as
*** 915,920 ****
--- 915,945 ----
  	       || 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);
+               elf_symbol_type *elfsym = elf_symbol_from (abfd, sym);
+               unsigned int sec = elfsym->internal_elf_sym.st_shndx;
+               Elf_Internal_Shdr *elfhdr = elfsections[sec];
+               
+               if (elfhdr->sh_type == SHT_REL || elfhdr->sh_type == SHT_RELA)
+                 {
+                   struct bfd_section *targsec =
+                     bfd_section_from_elf_index (abfd, elfhdr->sh_info);
+                   if (targsec && is_strip_section (abfd, targsec))
+                     keep = 0;
+                 }
+             }
+         }
        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;
  	}
      }
  
--- 994,1002 ----
  	      sym->flags |= BSF_GLOBAL;
  	    }
  
! 	  to[dst_count] = sym;
!           bfd_copy_private_symbol_data (abfd, sym, obfd, to[dst_count]);
!           dst_count++;
  	}
      }
  

Re: [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[H. J. Lu]

On Wed, May 24, 2006 at 03:29:59PM +0100, Julian Brown wrote:
> Julian Brown wrote:
> >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.
> 
> This is a slightly-smarter version (thanks to offlist discussion with 
> Dan) which knows about recoving Elf symbols from BFD symbols, hence 
> avoids some unnecessary work.
> 

Can you add a testcase so that it won't be broken by accident?


H.J.

Re: [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[Alan Modra]

On Wed, May 24, 2006 at 02:35:14PM +0100, Julian Brown wrote:
>     * 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.

I think this should be fixed in elf.c:elf_map_symbols by throwing away
any bad section syms.  That way we keep (more) elf specific code out of
objcopy.c.  I'll take a look at it.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

Re: [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[Alan Modra]

On Thu, May 25, 2006 at 02:31:07PM +0930, Alan Modra wrote:
> On Wed, May 24, 2006 at 02:35:14PM +0100, Julian Brown wrote:
> >     * 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.
> 
> I think this should be fixed in elf.c:elf_map_symbols by throwing away
> any bad section syms.  That way we keep (more) elf specific code out of
> objcopy.c.  I'll take a look at it.

Like this.  Tested with "make all check" on practically all elf
targets.  I wouldn't rush to put a change like this one on the 2.17
branch, not that I think there's anything wrong with it, but we seem to
have code in elf.c that goes to great lengths to support objcopy of
symbols in unlikely sections like .strtab.  This was no doubt because
the assembler at one stage did output section syms for all these special
sections, and you could argue that objcopy ought to make minimal changes
for "objcopy foo.o new.o".  Nowadays the assembler doesn't output these
rather silly symbols, so new object files should pass thru objcopy
relatively unchanged.  However, "objcopy old_object.o new.o" will strip
out some section symbols with this patch.  I can't see this being a
problem, but it's probably best not to make this change on a release
branch.

	* elf.c (sym_is_global): Return a bfd_boolean.
	(ignore_section_sym): New function.
	(elf_map_symbols): Use ignore_section_sym to discard some syms.
	(_bfd_elf_symbol_from_bfd_symbol): Ensure section belongs to
	bfd before using elf_section_syms.

Index: bfd/elf.c
===================================================================
RCS file: /cvs/src/src/bfd/elf.c,v
retrieving revision 1.338
diff -u -p -r1.338 elf.c
--- bfd/elf.c	19 May 2006 00:51:28 -0000	1.338
+++ bfd/elf.c	25 May 2006 12:51:45 -0000
@@ -3280,7 +3280,7 @@ assign_section_numbers (bfd *abfd, struc
 /* Map symbol from it's internal number to the external number, moving
    all local symbols to be at the head of the list.  */
 
-static int
+static bfd_boolean
 sym_is_global (bfd *abfd, asymbol *sym)
 {
   /* If the backend has a special mapping, use it.  */
@@ -3293,6 +3293,20 @@ sym_is_global (bfd *abfd, asymbol *sym)
 	  || bfd_is_com_section (bfd_get_section (sym)));
 }
 
+/* Don't output section symbols for sections that are not going to be
+   output.  Also, don't output section symbols for reloc and other
+   special sections.  */
+
+static bfd_boolean
+ignore_section_sym (bfd *abfd, asymbol *sym)
+{
+  return ((sym->flags & BSF_SECTION_SYM) != 0
+	  && (sym->value != 0
+	      || (sym->section->owner != abfd
+		  && (sym->section->output_section->owner != abfd
+		      || sym->section->output_offset != 0))));
+}
+
 static bfd_boolean
 elf_map_symbols (bfd *abfd)
 {
@@ -3333,51 +3347,29 @@ elf_map_symbols (bfd *abfd)
       asymbol *sym = syms[idx];
 
       if ((sym->flags & BSF_SECTION_SYM) != 0
-	  && sym->value == 0)
+	  && !ignore_section_sym (abfd, sym))
 	{
-	  asection *sec;
-
-	  sec = sym->section;
+	  asection *sec = sym->section;
 
-	  if (sec->owner != NULL)
-	    {
-	      if (sec->owner != abfd)
-		{
-		  if (sec->output_offset != 0)
-		    continue;
-
-		  sec = sec->output_section;
+	  if (sec->owner != abfd)
+	    sec = sec->output_section;
 
-		  /* Empty sections in the input files may have had a
-		     section symbol created for them.  (See the comment
-		     near the end of _bfd_generic_link_output_symbols in
-		     linker.c).  If the linker script discards such
-		     sections then we will reach this point.  Since we know
-		     that we cannot avoid this case, we detect it and skip
-		     the abort and the assignment to the sect_syms array.
-		     To reproduce this particular case try running the
-		     linker testsuite test ld-scripts/weak.exp for an ELF
-		     port that uses the generic linker.  */
-		  if (sec->owner == NULL)
-		    continue;
-
-		  BFD_ASSERT (sec->owner == abfd);
-		}
-	      sect_syms[sec->index] = syms[idx];
-	    }
+	  sect_syms[sec->index] = syms[idx];
 	}
     }
 
   /* Classify all of the symbols.  */
   for (idx = 0; idx < symcount; idx++)
     {
+      if (ignore_section_sym (abfd, syms[idx]))
+	continue;
       if (!sym_is_global (abfd, syms[idx]))
 	num_locals++;
       else
 	num_globals++;
     }
 
-  /* We will be adding a section symbol for each BFD section.  Most normal
+  /* We will be adding a section symbol for each normal BFD section.  Most
      sections will already have a section symbol in outsymbols, but
      eg. SHT_GROUP sections will not, and we need the section symbol mapped
      at least in that case.  */
@@ -3403,6 +3395,8 @@ elf_map_symbols (bfd *abfd)
       asymbol *sym = syms[idx];
       unsigned int i;
 
+      if (ignore_section_sym (abfd, sym))
+	continue;
       if (!sym_is_global (abfd, sym))
 	i = num_locals2++;
       else
@@ -5130,13 +5124,14 @@ _bfd_elf_symbol_from_bfd_symbol (bfd *ab
       && (flags & BSF_SECTION_SYM)
       && asym_ptr->section)
     {
+      asection *sec;
       int indx;
 
-      if (asym_ptr->section->output_section != NULL)
-	indx = asym_ptr->section->output_section->index;
-      else
-	indx = asym_ptr->section->index;
-      if (indx < elf_num_section_syms (abfd)
+      sec = asym_ptr->section;
+      if (sec->owner != abfd && sec->output_section != NULL)
+	sec = sec->output_section;
+      if (sec->owner == abfd
+	  && (indx = sec->index) < elf_num_section_syms (abfd)
 	  && elf_section_syms (abfd)[indx] != NULL)
 	asym_ptr->udata.i = elf_section_syms (abfd)[indx]->udata.i;
     }


-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

Re: [PATCH] Fix section symbols in objcopy when removing sections with relocations from Elf objects

[Julian Brown]

Hi,

Alan Modra wrote:
>	* elf.c (sym_is_global): Return a bfd_boolean.
> 	(ignore_section_sym): New function.
> 	(elf_map_symbols): Use ignore_section_sym to discard some syms.
> 	(_bfd_elf_symbol_from_bfd_symbol): Ensure section belongs to
> 	bfd before using elf_section_syms.

Thanks very much for looking into this! It's far preferable that the fix 
is in Elf-specific code rather than the generic code, I agree. FYI, this 
patch fixes the original problem just fine, and I've committed it on our 
binutils-csl-2_17-branch.

Cheers,

Julian