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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [obj2yaml][yaml2obj] Cannot round-trip clang-produced object file"
href="https://bugs.llvm.org/show_bug.cgi?id=50506">50506</a>
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<th>Summary</th>
<td>[obj2yaml][yaml2obj] Cannot round-trip clang-produced object file
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jh7370.2008@my.bristol.ac.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>If you take a trivial clang object file, run it through obj2yaml and then the
output through yaml2obj, the latter tool reports an error:
C:\Work\TempWork> touch test.cpp
C:\Work\TempWork> C:\llvm\build\Debug\bin\clang.exe -c test.cpp
C:\Work\TempWork> C:\llvm\build\Debug\bin\obj2yaml.exe test.o -o test.yaml
C:\Work\TempWork> C:\llvm\build\Debug\bin\yaml2obj test.yaml -o test2.o
yaml2obj: error: section '.shstrtab' should be present in the 'Sections' or
'Excluded' lists
The intermediate YAML file looks like this:
--- !ELF
FileHeader:
Class: ELFCLASS64
Data: ELFDATA2LSB
Type: ET_REL
Machine: EM_X86_64
Sections:
- Name: .text
Type: SHT_PROGBITS
Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
AddressAlign: 0x4
- Name: .comment
Type: SHT_PROGBITS
Flags: [ SHF_MERGE, SHF_STRINGS ]
AddressAlign: 0x1
EntSize: 0x1
Content: 00636C616E672076657273696F6E2031332E302E3000
- Name: .note.GNU-stack
Type: SHT_PROGBITS
AddressAlign: 0x1
- Name: .llvm_addrsig
Type: SHT_LLVM_ADDRSIG
Flags: [ SHF_EXCLUDE ]
Link: .symtab
AddressAlign: 0x1
Offset: 0x88
Symbols: [ ]
- Type: SectionHeaderTable
Sections:
- Name: .strtab
- Name: .text
- Name: .comment
- Name: .note.GNU-stack
- Name: .llvm_addrsig
- Name: .symtab
Symbols:
- Name: test.cpp
Type: STT_FILE
Index: SHN_ABS
...
The problem is the presence of the SectionHeaderTable key. Deleting it causes
the error to go away. The SectionHeaderTable key however is necessary to
faithfully represent the input object file: the section data in that object
file is not in the same order as the section header table. Additionally, clang
quite reasonably produces a single combined string table for the symbol and
section names (the .strtab) section. This single section is represented in the
SectionHeaderTable. yaml2obj implicitly generates separate sections for these
strings however(i.e .strtab and .shstrtab), and when a SectionHeaderTable
element is present, it expects all sections to be listed (either in the
Sections list, or the Exclude list). Since .shstrtab wasn't present in the
object file, it doesn't appear in the YAML, which causes yaml2obj to reject it.
I can think of a couple of different possible fixes:
1) Add yaml2obj implicitly created sections to the end of the output object
file, if not mentioned in the SectionHeaderTable.
2) Add a key to obj2yaml/yaml2obj which indicates which section to use for the
section header table and/or symbol string table, or which indicates the two
tables should be merged. yaml2obj would then generate one or two string tables
as requesteed.
3) Change the LLVM code to emit sections in the section header table in the
order they appear in the output. That would mean obj2yaml doesn't need to
generate a SectionHeaderKey in this instance.
I think 2) most faithfully represents the input object, and should therefore be
our preferred approach.</pre>
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