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Hi,</div>
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I was debugging rustc segfaults and traced them to GOTPCRELX handling</div>
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in older ld.</div>
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Since commit 11c2852449825 (binutils 2.45), gas emits</div>
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R_X86_64_GOTPCRELX for "push foo@GOTPCREL(%rip)". Older ld's</div>
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elf_x86_64_convert_load_reloc() only checks modrm 0x25 (jmp) in the</div>
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0xff case and treats everything else as call, so a push (modrm 0x35)</div>
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gets silently rewritten to "addr32 call foo". In our case foo is a</div>
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read-only LLVM table statically linked into librustc_driver.so, so</div>
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rustc calls into .rodata and crashes.</div>
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Reproducer (two installs on purpose - that mismatch is the bug):</div>
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<i> $ cat repro.s</i></div>
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<i> .text</i></div>
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<i> .globl func</i></div>
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<i> func:</i></div>
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<i> push tbl@GOTPCREL(%rip)</i></div>
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<i> add $8, %rsp</i></div>
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<i> ret</i></div>
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<i> .section .rodata</i></div>
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<i> .globl tbl</i></div>
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<i> .hidden tbl</i></div>
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<i> tbl:</i></div>
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<i> .zero 576</i></div>
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<i><br>
</i></div>
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<i> $ as --version | head -1 </i></div>
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<i> GNU assembler (GNU Binutils) 2.46.1</i></div>
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<i> $ as -o repro.o repro.s</i></div>
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<i> $ readelf -rW repro.o | grep tbl</i></div>
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<i> 0000000000000002 0000000300000029 R_X86_64_GOTPCRELX 0000000000000000 tbl - 4</i></div>
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<i><br>
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<i> $ ld --version | head -1</i></div>
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<i> GNU ld (GNU Binutils for Ubuntu) 2.38</i></div>
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<i> $ ld -shared -o bad.so repro.o</i></div>
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<i> $ objdump -d bad.so | grep -A1 '<func>:'</i></div>
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<i> 0000000000001000 <func>:</i></div>
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<i> 1000: 67 e8 fa 0f 00 00 addr32 call 2000 <tbl></i></div>
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ld 2.46.1 on the same repro.o keeps "ff 35" (push through GOT).</div>
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gas 2.38 emits plain GOTPCREL for this source, so only new-gas</div>
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objects with old ld are affected.</div>
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Old branches are closed, I know - but some of our build hosts must</div>
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stay on distro binutils for a while, so we plan to carry this locally</div>
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(skip conversion for anything that isn't call/jmp, matching what newer</div>
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ld does for push in a shared library):</div>
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<i> --- a/bfd/elf64-x86-64.c</i></div>
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<i> +++ b/bfd/elf64-x86-64.c</i></div>
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<i> @@ -1659,6 +1659,8 @@ elf_x86_64_convert_load_reloc (bfd *abfd,</i></div>
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<i> modrm = bfd_get_8 (abfd, contents + roff - 1);</i></div>
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<i> </i><b><i> + if (modrm != 0x15 && modrm != 0x25)</i></b></div>
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<b><i> + return true;</i></b></div>
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<i> if (modrm == 0x25)</i></div>
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Tested: repro links correctly with the patched ld, and the rust build</div>
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that segfaulted before now passes.</div>
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<b>Two things I'd like to confirm:</b></div>
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1. Is this guard safe, or can modrm be something other than</div>
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0x15/0x25/0x35 here that needs different handling?</div>
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2. Worth documenting this incompatibility (PR or release notes)?</div>
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Every ld before 2.45 silently produces bad code from such objects,</div>
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and tracing the crash back to the linker was not easy.</div>
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Thanks,</div>
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Sunil Dora</div>
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