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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - PIE creates incorrect relocations for TLS: R_X86_64_TPOFF32 gets translated to R_X86_64_RELATIVE"
href="https://llvm.org/bugs/show_bug.cgi?id=27174">27174</a>
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<th>Summary</th>
<td>PIE creates incorrect relocations for TLS: R_X86_64_TPOFF32 gets translated to R_X86_64_RELATIVE
</td>
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<th>Product</th>
<td>lld
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>ed@80386.nl
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>This bug applies to LLVM trunk (well, r265054 to be exact).
Consider the following made-up program:
_Thread_local int i;
void _start(void) { i = 3; }
If compiled to an object file, it contains the following machine code (x86-64):
$ objdump -d exec.o
Disassembly of section .text._start:
_start:
0: 55 pushq %rbp
1: 48 89 e5 movq %rsp, %rbp
4: 64 48 8b 04 25 00 00 00 00 movq %fs:0, %rax
d: c7 80 00 00 00 00 03 00 00 00 movl $3, (%rax)
17: 5d popq %rbp
18: c3 retq
The relocation needed to get this code to work:
$ readelf -r exec.o
Relocation section '.rela.text._start' at offset 0x148 contains 1 entries:
Offset Info Type Sym. Value Sym. Name +
Addend
00000000000f 000500000017 R_X86_64_TPOFF32 0000000000000000 i + 0
If I now use GNU ld 2.26 (BFD, not Gold) to link this executable with "-pie", I
correctly end up with an executable that does not contain any relocations. The
reason for this is that we're using TLS with a fixed layout of the TLS area.
There is no dynamic TLS.
If I use the latest version of LLD, I see that the following relocation is
generated:
$ readelf -r exec
Relocation section '.rela.dyn' at offset 0x2a0 contains 1 entries:
Offset Info Type Sym. Value Sym. Name +
Addend
00000000100f 000000000008 R_X86_64_RELATIVE
0000000000000000
This is incorrect for two reasons:
- R_X86_64_RELATIVE applies a relocation relative to the base address of the
executable. The location of a TLS variable should not depend on this.
- This relocation points to a location inside of the .text segment, which is
read-only. Applying this relocation at runtime will cause a segmentation fault.
$ readelf -S exec.o
[ 7] .text PROGBITS 0000000000001000 00001000
0000000000000019 0000000000000000 AX 0 0 16
Apart from that, I'd say that LLD is doing a flawless job. Arguably even better
than GNU ld. It generates executables that only depend on R_X86_64_RELATIVE
relocations, which are easier to apply than the R_X86_64_64 relocations that
GNU ld depends on.</pre>
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