https://sourceware.org/bugzilla/show_bug.cgi?id=28858
Bug ID: 28858
Summary: Some relative relocations are missing when linking
with -z pack-relative-relocs
Product: binutils
Version: 2.38
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: dani at danielbertalan dot dev
Target Milestone: ---
I'm working on adding RELR relocation support to an x86_64 kernel (we have
dynamic relocations in the kernel because of UBSAN and vtables). While it boots
correctly when I use the LLD linker with --pack-dyn-relocs=relr, the kernel
immediately triple faults when I link it with ld's pack-relative-relocs option.
The issue seems to be that we're trying to read from an address that needs to
be relocated but has not been.
Without pack-relative-relocs, the kernel binary has 6012 R_X86_64_RELATIVE
relocations:
$ LD=path/to/ld.bfd ./good.sh
$ llvm-readelf --relocs good
Relocation section '.rela.dyn' at offset 0x44b3f0 contains 6012 entries:
00000000007b0000 0000000000000008 R_X86_64_RELATIVE 74a020
00000000007b0008 0000000000000008 R_X86_64_RELATIVE 431990
00000000007b0010 0000000000000008 R_X86_64_RELATIVE 47c380
...
When I enable pack-relative-relocs, the total number of relocations stays the
same, but instead of the expected R_X86_64_RELATIVE, the entries in .rela.dyn
contain R_X86_64_NONE. This might explain the un-relocated address I was
seeing.
$ LD=path/to/ld.bfd ./bad.sh
$ llvm-readelf --relocs bad
Relocation section '.rela.dyn' at offset 0x44b3c0 contains 895 entries:
0000000000000000 0000000000000000 R_X86_64_NONE 0
0000000000000000 0000000000000000 R_X86_64_NONE 0
0000000000000000 0000000000000000 R_X86_64_NONE 0
0000000000000000 0000000000000000 R_X86_64_NONE 0
...
Relocation section '.relr.dyn' at offset 0x4507a8 contains 5117 entries:
00000000007b0000 0000000000000008 R_X86_64_RELATIVE
00000000007b0008 0000000000000008 R_X86_64_RELATIVE
....
The attached tarball contains all inputs to the linker and the command line
arguments. I'm using ld.bfd built from commit
2405fc4016feadea33cb747d5654514f62b74ff4 (2.38.50.20220129).
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