linux-kernel - Re: [PATCH] kallsyms: ignore all local labels prefixed by '.L'

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Date:   Tue, 1 Feb 2022 09:33:43 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Changbin Du <changbin.du@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] kallsyms: ignore all local labels prefixed by '.L'

On Tue, Feb 01, 2022 at 09:32:57AM +0800, Changbin Du wrote:
> The llvm compiler can generate lots of local labels ('.LBB', '.Ltmpxxx',
> '.L__unnamed_xx', etc.). These symbols usually are useless for debugging.
> And they might overlap with handwritten symbols.
> 
> Before this change, a dumpstack shows a local symbol for epc:
> [    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
> [    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
> [    0.040452][    T0]  ra : restore_all+0x12/0x6e
> 
> The simple solution is that we can ignore all local labels prefixed by '.L'.
> For handwritten symbols which need to be preserved should drop the '.L'
> prefix.
> 
> After this change, the C defined symbol is shown so we can locate the
> problematical code immediately:
> [    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
> [    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
> [    0.036567][    T0]  ra : restore_all+0x12/0x6e
> 
> Signed-off-by: Changbin Du <changbin.du@...il.com>

Does not seem too unreasonable to me.

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  scripts/kallsyms.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index 54ad86d13784..8caabddf817c 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -108,7 +108,7 @@ static bool is_ignored_symbol(const char *name, char type)
>  	/* Symbol names that begin with the following are ignored.*/
>  	static const char * const ignored_prefixes[] = {
>  		"$",			/* local symbols for ARM, MIPS, etc. */
> -		".LASANPC",		/* s390 kasan local symbols */
> +		".L",			/* local labels, .LBB,.Ltmpxxx,.L__unnamed_xx,.LASANPC, etc. */
>  		"__crc_",		/* modversions */
>  		"__efistub_",		/* arm64 EFI stub namespace */
>  		"__kvm_nvhe_",		/* arm64 non-VHE KVM namespace */
> -- 
> 2.32.0
> 
>