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

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Message-ID: <20241030191731.tqdw4nmd7htvfrua@treble.attlocal.net>
Date: Wed, 30 Oct 2024 12:17:31 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: x86@...nel.org, 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 Wed, Oct 30, 2024 at 07:20:08PM +0100, Jens Remus wrote:
> > 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.

I suspect it works fine on x86-64, since that was the first arch
supported, so --gsframe being supported also means x86-64 is supported.

But yeah, other arches (and x86-32) are a different story.

> 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

Looks good, though the ifneq isn't needed:

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


Though, if multiple arches are going to be using that, maybe it should
just be a config option:


diff --git a/arch/Kconfig b/arch/Kconfig
index 33449485eafd..676dd45a7255 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -446,6 +446,9 @@ config HAVE_UNWIND_USER_SFRAME
 	bool
 	select UNWIND_USER
 
+config AS_SFRAME
+	def_bool $(as-instr,.cfi_sections .sframe\n.cfi_startproc\n.cfi_endproc)
+
 config HAVE_PERF_CALLCHAIN_DEFERRED
 	bool