linux-kernel - [PATCH] x86/speculation, objtool: Use absolute relocations for annotations

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Date:   Tue, 19 Sep 2023 17:17:28 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>
Subject: [PATCH] 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")

Link: https://github.com/ClangBuiltLinux/linux/issues/1937 [1]
Signed-off-by: Fangrui Song <maskray@...gle.com>
---
 arch/x86/include/asm/alternative.h   |  4 ++--
 arch/x86/include/asm/nospec-branch.h |  4 ++--
 include/linux/objtool.h              | 10 +++++-----
 3 files changed, 9 insertions(+), 9 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
 
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index c55cc243592e..4952b73d944e 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -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];
diff --git a/include/linux/objtool.h b/include/linux/objtool.h
index 03f82c2c2ebf..6f6da95fe7f9 100644
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h
@@ -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
 
-- 
2.42.0.459.ge4e396fd5e-goog