https://llvm.org/bugs/show_bug.cgi?id=27174
Bug ID: 27174
Summary: PIE creates incorrect relocations for TLS:
R_X86_64_TPOFF32 gets translated to R_X86_64_RELATIVE
Product: lld
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: unassignedb...@nondot.org
Reporter: e...@80386.nl
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
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.
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