linux-kernel - Re: [PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

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Message-ID: <e5008073-f779-492b-807f-d14c39ec0ba3@linux.ibm.com>
Date: Fri, 24 Jan 2025 17:08:57 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
        Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, Weinan Liu <wnliu@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

On 22.01.2025 03:30, Josh Poimboeuf wrote:
> It was decided years ago that .cfi_* annotations aren't maintainable in
> the kernel.  They were replaced by objtool unwind hints.  For the kernel
> proper, ensure the CFI_* macros don't do anything.
> 
> On the other hand the VDSO library *does* use them, so user space can
> unwind through it.
> 
> Make sure these macros only work for VDSO.  They aren't actually being
> used outside of VDSO anyway, so there's no functional change.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>

> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h

> -#ifndef BUILD_VDSO
> -	/*
> -	 * Emit CFI data in .debug_frame sections, not .eh_frame sections.
> -	 * The latter we currently just discard since we don't do DWARF
> -	 * unwinding at runtime.  So only the offline DWARF information is
> -	 * useful to anyone.  Note we should not use this directive if we
> -	 * ever decide to enable DWARF unwinding at runtime.
> -	 */
> -	.cfi_sections .debug_frame
> -#else
> -	 /*
> -	  * For the vDSO, emit both runtime unwind information and debug
> -	  * symbols for the .dbg file.
> -	  */
> -	.cfi_sections .eh_frame, .debug_frame
> -#endif
> +#else /* !BUILD_VDSO */
> +

Did you remove ".cfi_sections .debug_frame" on purpose from the
!BUILD_VDSO path compared to V3? Presumably to not only not emit
DWARF CFI from assembler, but any source?

> +/*
> + * On x86, these macros aren't used outside VDSO.  As well they shouldn't be:
> + * they're fragile and very difficult to maintain.
> + */

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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