linux-kernel - Re: [PATCH v3 06/19] x86/vdso: Enable sframe generation in VDSO

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Message-ID: <b89bcb68-d010-4041-aacf-15b934675705@linux.ibm.com>
Date: Wed, 30 Oct 2024 19:20:08 +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.kerne.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v3 06/19] x86/vdso: Enable sframe generation in VDSO

On 28.10.2024 22:47, Josh Poimboeuf wrote:
> Enable sframe generation in the VDSO library so kernel and user space
> can unwind through it.

...

Applying similar changes to s390 and using the current binutils release without SFrame support on s390 results in build errors.

AFAIK the kernel has a minimum binutils requirement of 2.25 [1] and the assembler option "--gsframe was introduced with 2.40.

> diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
> index c9216ac4fb1e..75ae9e093a2d 100644
> --- a/arch/x86/entry/vdso/Makefile
> +++ b/arch/x86/entry/vdso/Makefile
> @@ -47,13 +47,15 @@ quiet_cmd_vdso2c = VDSO2C  $@
>   $(obj)/vdso-image-%.c: $(obj)/vdso%.so.dbg $(obj)/vdso%.so $(obj)/vdso2c FORCE
>   	$(call if_changed,vdso2c)
>   
> +SFRAME_CFLAGS := $(call as-option,-Wa$(comma)-gsframe,)

I have the impression this test might not work as expected. On s390 the assembler accepts option --gsframe and only generates an error when the assembler code actually contains any CFI directives:

$ cat test-nocfi.a
         la      %r1,42
$ as --gsframe test-nocfi.a
$ echo $?
0

$ cat test-cfi.a
         .cfi_startproc
         .cfi_endproc
$ as --gsframe test-cfi.a
test-cfi.a: Assembler messages:
test-cfi.a: Error: .sframe not supported for target
$ echo $?
1

But the following assembler code triggers an error:

$ cat test-sframe.a
         .cfi_sections .sframe
         .cfi_startproc
         .cfi_endproc
$ as test-sframe.a
test-sframe.a: Assembler messages:
test-sframe.a: Error: .sframe not supported for target
$ echo $?
1

Maybe the following would be an alternative test in the Makefile?

SFRAME_CFLAGS := $(call as-instr,.cfi_sections .sframe\n.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_SFRAME=1)
ifneq ($(SFRAME_CFLAGS),)
        SFRAME_CFLAGS += -Wa,--gsframe
endif

> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
> index b1aa3fcd5bca..1a49492817a1 100644
> --- a/arch/x86/include/asm/dwarf2.h
> +++ b/arch/x86/include/asm/dwarf2.h
> @@ -12,8 +12,11 @@
>   	 * For the vDSO, emit both runtime unwind information and debug
>   	 * symbols for the .dbg file.
>   	 */
> -
> +#ifdef __x86_64__

#if defined(__x86_64__) && defined(CONFIG_AS_SFRAME)

> +	.cfi_sections .eh_frame, .debug_frame, .sframe
> +#else
>   	.cfi_sections .eh_frame, .debug_frame
> +#endif
>   
>   #define CFI_STARTPROC		.cfi_startproc
>   #define CFI_ENDPROC		.cfi_endproc

[1]: https://docs.kernel.org/process/changes.html

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303) and z/VSE Support
+49-7031-16-1128 Office
jremus@...ibm.com

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