linux-kernel - Re: [PATCH v8 6/6] x86/vdso: Enable sframe generation in VDSO

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Message-ID: <15f2af3b-be33-46fc-b972-6b8e7e0aa52e@linux.ibm.com>
Date: Tue, 10 Feb 2026 17:46:33 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Steven Rostedt <rostedt@...nel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Indu Bhagat <indu.bhagat@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Florian Weimer <fweimer@...hat.com>, Kees Cook <kees@...nel.org>,
        "Carlos O'Donell" <codonell@...hat.com>, Sam James <sam@...too.org>,
        Dylan Hatch <dylanbhatch@...gle.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Michal Hocko
 <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>
Subject: Re: [PATCH v8 6/6] x86/vdso: Enable sframe generation in VDSO

On 2/6/2026 8:36 PM, Jens Remus wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Enable sframe generation in the VDSO library so kernel and user space
> can unwind through it.
> 
> SFrame isn't supported for x32 or x86-32.  Discard .sframe sections for
> those VDSOs.
> 
> [ Jens Remus: Add support for SFrame V3.  Prevent GNU_SFRAME program
> table entry to empty .sframe section. ]
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Signed-off-by: Jens Remus <jremus@...ux.ibm.com>

> diff --git a/arch/Kconfig b/arch/Kconfig

> @@ -479,6 +479,13 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
>  	  It uses the same command line parameters, and sysctl interface,
>  	  as the generic hardlockup detectors.
>  
> +config AS_SFRAME
> +	bool
> +
> +config AS_SFRAME3
> +	def_bool $(as-instr,.cfi_startproc\n.cfi_endproc,-Wa$(comma)--gsframe-3)

Above tests whether the assembler supports --gsframe-3 to generate
.sframe in SFrame V3 format.  GNU assembler only supports to generate
SFrame for x86-64.  If the assembler is built for i386 (x86-32) the test
will fail unless option --64 is specified.  I wonder whether that could
happen in (the rather hypothetical?) case, when the kernel is cross built
for x86-64 on i386?  Should this therefore be changed to the following?

config AS_SFRAME_64
	def_bool $(as-instr,.cfi_startproc\n.cfi_endproc,$(m64-flag) -Wa$(comma)--gsframe)

An alternative would be to enable as-instr64 to accept an optional flag
operand, similar to as-instr, and use as-instr64 instead of as-instr.

Or is this not required, as in a cross build for 64-bit on a 32-bit
platform (or inverse) a respective cross toolchain is a prerequisite and
the m64-flag (or m32-flag) is not used?

I tried to simulate this using docker and an i386 image and then using
the i386 tool in the container to switch "uname -m" to "i686".  But both
of my following attempts to build for x86-64 on i386 fail with fixdep
not found:

$ docker run -it --rm -v "$HOME/linux:$HOME/linux" --platform i386 public.ecr.aws/docker/library/debian:13.3
# sed -i -s 's/^Types: deb/& deb-src/' /etc/apt/sources.list.d/debian.sources
# apt update
# apt -y build-dep linux

# make mrproper
# i386 make ARCH=x86_64 defconfig
# i386 make ARCH=x86_64 arch/x86/entry/vdso/

# make mrproper
# apt -y install gcc-x86-64-linux-gnu
# i386 make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- defconfig
# i386 make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- arch/x86/entry/vdso/

> +	select AS_SFRAME
> +
>  config UNWIND_USER
>  	bool

> diff --git a/arch/x86/entry/vdso/vdso64/Makefile b/arch/x86/entry/vdso/vdso64/Makefile

> @@ -14,6 +14,7 @@ vobjs-$(CONFIG_X86_SGX)		+= vsgx.o
>  
>  # Compilation flags
>  flags-y				:= -DBUILD_VDSO64 -m64 -mcmodel=small
> +flags-$(CONFIG_AS_SFRAME3)	+= -Wa,--gsframe-3

flags-$(CONFIG_AS_SFRAME3_64)	+= -Wa,--gsframe-3

>  
>  # The location of this include matters!
>  include $(src)/../common/Makefile.include
Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@...ibm.com / jremus@...ux.ibm.com

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