linux-kernel - Re: [PATCH 2/2] vdso: Reject absolute relocations during build

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Message-ID: <20250430095411-349b13bf-a3c7-4968-9f97-551a1ded1696@linutronix.de>
Date: Wed, 30 Apr 2025 09:57:02 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Jan Stancek <jstancek@...hat.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Nam Cao <namcao@...utronix.de>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, Andy Lutomirski <luto@...nel.org>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vdso: Reject absolute relocations during build

On Wed, Apr 30, 2025 at 09:32:27AM +0200, Thomas Gleixner wrote:
> On Tue, Apr 29 2025 at 17:17, Jan Stancek wrote:
> > On Tue, Apr 29, 2025 at 2:56 PM Thomas Weißschuh
> > <thomas.weissschuh@...utronix.de> wrote:
> >>
> >> +# Also validate that no absolute relocations are present in the object files themselves.
> >>  quiet_cmd_vdso_check = VDSOCHK $@
> >>        cmd_vdso_check = if $(READELF) -rW $@ | grep -v _NONE | grep -q " R_\w*_"; \
> >>                        then (echo >&2 "$@: dynamic relocations are not supported"; \
> >> +                            rm -f $@; /bin/false); fi && \
> >> +                      if $(READELF) -rW $(filter %.o, $(real-prereqs)) | grep -q " R_\w*_ABS"; \
> >> +                      then (echo >&2 "$@: absolute relocations are not supported"; \
> >>                              rm -f $@; /bin/false); fi
> >
> > Should this check only some sections? I'm getting lot of matches on
> > debuginfo related sections:
> 
> Hmm. All architecture VDSO Makefiles have -fPIC in CFLAGS except for
> arm64, which only adds it in arm64/kernel/vdso32/Makefile but not in
> arm64/kernel/vdso/Makefile. Confused.

Unfortunately -fPIC does not help. It generates code that is sufficiently
position independent for regular DSOs, but not the vDSO.

See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120002#c5