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

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Message-ID: <20250212175023.rsxsw7pno57gsxps@jpoimboe>
Date: Wed, 12 Feb 2025 09:50:23 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>,
	llvm@...ts.linux.dev
Subject: Re: [PATCH] compiler.h: Specify correct attribute for
 .rodata..c_jump_table

On Tue, Sep 24, 2024 at 02:27:10PM +0800, Tiezhu Yang wrote:
> 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.

Right, this sounds like a bug in the GNU assembler.  It should default
to the same section flags regardless of arch.

> 
> 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 #")

This caused a regression, this hack apparently doesn't work with Clang:

  $ readelf -WS kernel/bpf/core.o | grep c_jump_table
    [43] .rodata..c_jump_table,"a",@progbits # PROGBITS        0000000000000000 00d610 000800 00   A  0   0 16

Notice the section name is literally:

  .rodata..c_jump_table,"a",@progbits #

-- 
Josh