Re: [PATCH] x86/purgatory: Make sure we fail the build if purgatory.ro has missing symbols

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Hans de Goede <hdegoede@redhat.com>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH] x86/purgatory: Make sure we fail the build if purgatory.ro has missing symbols
Date: Mon, 7 Oct 2019 17:52:13 -0400	[thread overview]
Message-ID: <20191007215213.GA405660@rani.riverdale.lan> (raw)
In-Reply-To: <c24d8bef-ad76-4986-0c16-268e7d09bf7c@redhat.com>

On Mon, Oct 07, 2019 at 10:31:49PM +0200, Hans de Goede wrote:
> HI,
> 
> On 07-10-2019 22:05, Nathan Chancellor wrote:
> > On Mon, Oct 07, 2019 at 07:55:46PM +0200, Hans de Goede wrote:
> >> Since we link purgatory.ro with -r aka we enable "incremental linking"
> >> no checks for unresolved symbols is done while linking purgatory.ro.
> >>
> >> Changes to the sha256 code has caused the purgatory in 5.4-rc1 to have
> >> a missing symbol on memzero_explicit, yet things still happily build.
> >>
> >> This commit adds an extra check for unresolved symbols by calling ld
> >> without -r before running bin2c to generate kexec-purgatory.c.
> >>
> >> This causes a build of 5.4-rc1 with this patch added to fail as it should:
> >>
> >>    CHK     arch/x86/purgatory/purgatory.ro
> >> ld: arch/x86/purgatory/purgatory.ro: in function `sha256_transform':
> >> sha256.c:(.text+0x1c0c): undefined reference to `memzero_explicit'
> >> make[2]: *** [arch/x86/purgatory/Makefile:72:
> >>      arch/x86/purgatory/kexec-purgatory.c] Error 1
> >> make[1]: *** [scripts/Makefile.build:509: arch/x86/purgatory] Error 2
> >> make: *** [Makefile:1650: arch/x86] Error 2
> >>
> >> This will help us catch missing symbols in the purgatory sooner.
> >>
> >> Note this commit also removes --no-undefined from LDFLAGS_purgatory.ro
> >> as that has no effect.
> >>
> >> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> >> ---
> >>   arch/x86/purgatory/Makefile | 8 +++++++-
> >>   1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> >> index fb4ee5444379..0da0794ef1f0 100644
> >> --- a/arch/x86/purgatory/Makefile
> >> +++ b/arch/x86/purgatory/Makefile
> >> @@ -14,7 +14,7 @@ $(obj)/sha256.o: $(srctree)/lib/crypto/sha256.c FORCE
> >>   
> >>   CFLAGS_sha256.o := -D__DISABLE_EXPORTS
> >>   
> >> -LDFLAGS_purgatory.ro := -e purgatory_start -r --no-undefined -nostdlib -z nodefaultlib
> >> +LDFLAGS_purgatory.ro := -e purgatory_start -r -nostdlib -z nodefaultlib
> >>   targets += purgatory.ro
> >>   
> >>   KASAN_SANITIZE	:= n
> >> @@ -60,10 +60,16 @@ $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
> >>   
> >>   targets += kexec-purgatory.c
> >>   
> >> +# Since we link purgatory.ro with -r unresolved symbols are not checked,
> >> +# so we check this before generating kexec-purgatory.c instead
> >> +quiet_cmd_check_purgatory = CHK     $<
> >> +      cmd_check_purgatory = ld -e purgatory_start $<
> > 
> > I think this should be $(LD) -e ... so that using a cross compile prefix
> > (like x86_64-linux-) or an alternative linker like ld.lld works properly.
> 
> Good point, also the ld command is actually outputting an a.out file
> which is also something which we do not want.
> 
> I will prepare a new version fixing both.
> 
> Regards,
> 
> Hans

We could just use $(NM) -u, right?