diff options
| author | Fangrui Song <maskray@google.com> | 2023-09-19 17:17:28 -0700 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2023-09-22 11:41:24 +0200 |
| commit | b8ec60e1186cdcfce41e7db4c827cb107e459002 (patch) | |
| tree | 46254a8042c0a4ddcf4c57d51d98f7fa046c198b /arch/x86/include/asm/alternative.h | |
| parent | fef44ebaf61b57a71ab818058926a3f9a0ac81e6 (diff) | |
x86/speculation, objtool: Use absolute relocations for annotations
.discard.retpoline_safe sections do not have the SHF_ALLOC flag. These
sections referencing text sections' STT_SECTION symbols with PC-relative
relocations like R_386_PC32 [0] is conceptually not suitable. Newer
LLD will report warnings for REL relocations even for relocatable links [1]:
ld.lld: warning: vmlinux.a(drivers/i2c/busses/i2c-i801.o):(.discard.retpoline_safe+0x120): has non-ABS relocation R_386_PC32 against symbol ''
Switch to absolute relocations instead, which indicate link-time
addresses. In a relocatable link, these addresses are also output
section offsets, used by checks in tools/objtool/check.c. When linking
vmlinux, these .discard.* sections will be discarded, therefore it is
not a problem that R_X86_64_32 cannot represent a kernel address.
Alternatively, we could set the SHF_ALLOC flag for .discard.* sections,
but I think non-SHF_ALLOC for sections to be discarded makes more sense.
Note: if we decide to never support REL architectures (e.g. arm, i386),
we can utilize R_*_NONE relocations (.reloc ., BFD_RELOC_NONE, sym),
making .discard.* sections zero-sized. That said, the section content
waste is 4 bytes per entry, much smaller than sizeof(Elf{32,64}_Rel).
[0] commit 1c0c1faf5692 ("objtool: Use relative pointers for annotations")
[1] https://github.com/ClangBuiltLinux/linux/issues/1937
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20230920001728.1439947-1-maskray@google.com
Diffstat (limited to 'arch/x86/include/asm/alternative.h')
| -rw-r--r-- | arch/x86/include/asm/alternative.h | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h index 9c4da699e11a..65f79092c9d9 100644 --- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -58,7 +58,7 @@ #define ANNOTATE_IGNORE_ALTERNATIVE \ "999:\n\t" \ ".pushsection .discard.ignore_alts\n\t" \ - ".long 999b - .\n\t" \ + ".long 999b\n\t" \ ".popsection\n\t" /* @@ -352,7 +352,7 @@ static inline int alternatives_text_reserved(void *start, void *end) .macro ANNOTATE_IGNORE_ALTERNATIVE .Lannotate_\@: .pushsection .discard.ignore_alts - .long .Lannotate_\@ - . + .long .Lannotate_\@ .popsection .endm |