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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - llvm-objcopy outputs broken ELF image on large object files built with -ffunction-sections"
href="https://bugs.llvm.org/show_bug.cgi?id=41461">41461</a>
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<th>Summary</th>
<td>llvm-objcopy outputs broken ELF image on large object files built with -ffunction-sections
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<th>Product</th>
<td>tools
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>release blocker
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>llvm-objcopy/strip
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>eleviant@accesssoftek.com
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<th>CC</th>
<td>alexander.v.shaposhnikov@gmail.com, jake.h.ehrlich@gmail.com, jh7370.2008@my.bristol.ac.uk, llvm-bugs@lists.llvm.org, rupprecht@google.com
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<pre>Steps to reproduce:
1. Generate source file with this bash script:
#!/bin/bash
echo "extern int baz();"
count=0
while [ $count -le 65280 ]; do
echo "int fun_$count() { return baz(); }"
((count++))
done
This will generate C source file with large number of functions (65281),
forcing some of symbol section indexes to become SHN_XINDEX.
2. Compile source
clang <file> -c -ffunction-sections -o output.o
This will generate object file with each of 'fun_[0-9]+' residing in it's own
section followed by relocation section '.rela.fun_[0-9]+'. It's important that
sections in output.o are not sorted by offset, so llvm-objcopy will reorder
them in sortSection changing indexes.
3. Run llvm-objcopy
llvm-objcopy output.o output-bad.o
Now the bug can be observed using readelf:
readelf -sW output-bad.o | grep 'fun_65278'
126706: 0000000000000000 13 FUNC GLOBAL DEFAULT UND fun_65278
readelf -sW output-bad.o | grep '65280:'
65280: 0000000000000000 0 SECTION LOCAL DEFAULT UND
====================
Explanation:
The problem lies in .symtab_shndx section being filled too early (in
prepareForLayout) before indexes are assigned to the sections in
layoutSections. Those indexes may not match original indexes because
llvm-objcopy sorts sections by OriginalOffset. This causes symbol section
indexes in result image to not match sections those symbols are defined in
(when section index exceeds SHN_LORESERVE).</pre>
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