Fix courgette ELF x86 dissembler

courgette/disassembler_elf_32.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "courgette/disassembler_elf_32.h"
 
 #include <algorithm>
 #include <string>
 #include <vector>
 
@@ skipping 244 matching lines @@
 
   std::vector<size_t>::iterator end_abs_offset = abs_offsets.end();
   ScopedVector<TypedRVA>::iterator end_rel = rel32_locations_.end();
 
   for (int section_id = 0;
        section_id < SectionHeaderCount();
        section_id++) {
 
     const Elf32_Shdr *section_header = SectionHeader(section_id);
 
+    if (section_header->sh_type == SHT_NOBITS)

[dgarrett, 2014/10/20 18:15:44]

This seems like the right thing to do... I'm surprised the original code was
working.

Did I have no NOBITS sections in the files it worked with?

There was substantial testing using a wide variety of binaries, but I have no
memory of how common NOBITS sections are.

[Will Harris, 2014/10/20 19:01:44]

NOBITS are in almost every ELF file as the .bss is always NOBITS as it's just
going to be some memory that the dynamic linker will zero out with enough space
for the globals.

In all the files I can compile, the .bss is near the end of the file (typically
before the comment or debug section) but the next section is either at the same
file offset or higher, which means the bug won't trigger.

e.g. a file I compile just now has

[24] .data             PROGBITS        0804a00c 00100c 000008 00  WA  0   0  4
[25] .bss              NOBITS          0804a014 001014 000008 00  WA  0   0  4
[26] .comment          PROGBITS        00000000 001014 00002a 01  MS  0   0  1

The issue with the ARC files was that I think they were manually shifting the
.bss beyond the end of the file, perhaps they are using a custom linker script
for this?  There is more info in the bug on why this causes a segfault.  I can't
find the makefile for libdvm.so to check what's special about it.

+      continue;
+
     if (!ParseSimpleRegion(file_offset,
                            section_header->sh_offset,
                            program))
       return false;
     file_offset = section_header->sh_offset;
 
     switch (section_header->sh_type) {
       case SHT_REL:
         if (!ParseRelocationSection(section_header, program))
           return false;
         file_offset = section_header->sh_offset + section_header->sh_size;
         break;
       case SHT_PROGBITS:
         if (!ParseProgbitsSection(section_header,
                                   &current_abs_offset, end_abs_offset,
                                   &current_rel, end_rel,
                                   program))
           return false;
         file_offset = section_header->sh_offset + section_header->sh_size;
         break;
-      case SHT_NOBITS:
-        // Fall through
       case SHT_INIT_ARRAY:
         // Fall through
       case SHT_FINI_ARRAY:
         while (current_abs_offset != end_abs_offset &&
                *current_abs_offset >= section_header->sh_offset &&
                *current_abs_offset <
                (section_header->sh_offset + section_header->sh_size)) {
           // Skip any abs_offsets appear in the unsupported INIT_ARRAY section
           VLOG(1) << "Skipping relocation entry for unsupported section: " <<
             section_header->sh_type;
@@ skipping 196 matching lines @@
       return false;
   }
 
   std::sort(rel32_locations_.begin(),
             rel32_locations_.end(),
             TypedRVA::IsLessThan);
   return true;
 }
 
 }  // namespace courgette