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x86/speculation, objtool: Use absolute relocations for annotations

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.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 1c0c1faf56 ("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
This commit is contained in:
Fangrui Song 2023-09-19 17:17:28 -07:00 committed by Ingo Molnar
parent fef44ebaf6
commit b8ec60e118
3 changed files with 9 additions and 9 deletions
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4
arch/x86/include/asm/alternative.h
View file

@ -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

4
arch/x86/include/asm/nospec-branch.h
View file

@ -196,7 +196,7 @@
.macro ANNOTATE_RETPOLINE_SAFE
.Lhere_\@:
.pushsection .discard.retpoline_safe
.long .Lhere_\@ - .
.long .Lhere_\@
.popsection
.endm
@ -334,7 +334,7 @@
#define ANNOTATE_RETPOLINE_SAFE \
"999:\n\t" \
".pushsection .discard.retpoline_safe\n\t" \
".long 999b - .\n\t" \
".long 999b\n\t" \
".popsection\n\t"
typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];

10
include/linux/objtool.h
View file

@ -48,13 +48,13 @@
#define ANNOTATE_NOENDBR \
"986: \n\t" \
".pushsection .discard.noendbr\n\t" \
".long 986b - .\n\t" \
".long 986b\n\t" \
".popsection\n\t"
#define ASM_REACHABLE \
"998:\n\t" \
".pushsection .discard.reachable\n\t" \
".long 998b - .\n\t" \
".long 998b\n\t" \
".popsection\n\t"
#else /* __ASSEMBLY__ */
@ -66,7 +66,7 @@
#define ANNOTATE_INTRA_FUNCTION_CALL \
999: \
.pushsection .discard.intra_function_calls; \
.long 999b - .; \
.long 999b; \
.popsection;
/*
@ -118,7 +118,7 @@
.macro ANNOTATE_NOENDBR
.Lhere_\@:
.pushsection .discard.noendbr
.long .Lhere_\@ - .
.long .Lhere_\@
.popsection
.endm
@ -141,7 +141,7 @@
.macro REACHABLE
.Lhere_\@:
.pushsection .discard.reachable
.long .Lhere_\@ - .
.long .Lhere_\@
.popsection
.endm