x86/speculation, objtool: Use absolute relocations for annotations

commit b8ec60e1186cdcfce41e7db4c827cb107e459002 upstream.

.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
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 9c4da69..65f7909 100644 (file)
--- 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 197ff4f..0396458 100644 (file)
--- 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 b5440e7..33212e9 100644 (file)
--- 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
 
@@ -142,7 +142,7 @@
 .macro REACHABLE
 .Lhere_\@:
        .pushsection .discard.reachable
-       .long   .Lhere_\@ - .
+       .long   .Lhere_\@
        .popsection
 .endm