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

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Message-ID: <c24d8bef-ad76-4986-0c16-268e7d09bf7c@redhat.com>
Date:   Mon, 7 Oct 2019 22:31:49 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Arvind Sankar <nivedita@...m.mit.edu>, x86@...nel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86/purgatory: Make sure we fail the build if
 purgatory.ro has missing symbols

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@...hat.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