linux-kernel - [PATCH] compiler.h: Specify correct attribute for .rodata..c_jump

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Message-ID: <20240924062710.1243-1-yangtiezhu@loongson.cn>
Date: Tue, 24 Sep 2024 14:27:10 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH] compiler.h: Specify correct attribute for .rodata..c_jump_table

Currently, there is an assembler message when generating kernel/bpf/core.o
under CONFIG_OBJTOOL with LoongArch compiler toolchain:

  Warning: setting incorrect section attributes for .rodata..c_jump_table

This is because the section ".rodata..c_jump_table" should be readonly,
but there is a "W" (writable) part of the flags:

  $ readelf -S kernel/bpf/core.o | grep -A 1 "rodata..c"
  [34] .rodata..c_j[...] PROGBITS         0000000000000000  0000d2e0
       0000000000000800  0000000000000000  WA       0     0     8

There is no above issue on x86 due to the generated section flag is only
"A" (allocatable). In order to silence the warning on LoongArch, specify
the attribute like ".rodata..c_jump_table,\"a\",@progbits #" explicitly,
then the section attribute of ".rodata..c_jump_table" must be readonly
in the kernel/bpf/core.o file.

Before:

  $ objdump -h kernel/bpf/core.o | grep -A 1 "rodata..c"
   21 .rodata..c_jump_table 00000800  0000000000000000  0000000000000000  0000d2e0  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, DATA

After:

  $ objdump -h kernel/bpf/core.o | grep -A 1 "rodata..c"
   21 .rodata..c_jump_table 00000800  0000000000000000  0000000000000000  0000d2e0  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

By the way, AFAICT, maybe the root cause is related with the different
compiler behavior of various archs, so to some extent this change is a
workaround for LoongArch, and also there is no effect for x86 which is
the only port supported by objtool before LoongArch with this patch.

Cc: stable@...r.kernel.org # 6.9+
Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
---
 include/linux/compiler.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ec55bcce4146..4d4e23b6e3e7 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -133,7 +133,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #define annotate_unreachable() __annotate_unreachable(__COUNTER__)

 /* Annotate a C jump table to allow objtool to follow the code flow */
-#define __annotate_jump_table __section(".rodata..c_jump_table")
+#define __annotate_jump_table __section(".rodata..c_jump_table,\"a\",@progbits #")

 #else /* !CONFIG_OBJTOOL */
 #define annotate_reachable()
-- 
2.42.0