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

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Date:   Tue, 8 Oct 2019 07:57:45 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, 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 23:52, Arvind Sankar wrote:
> 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@...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
> 
> We could just use $(NM) -u, right?

That would require wrapping it in some shell code like this (untested):

SHOULD_BE_EMPTY=$(nm -u purgatory.ro); if [ "$SHOULD_BE_EMPTY" == "" ]; then true; else false; fi

And then escaping the $ in there and in injecting $(NM) into it, IMHO
it is easier to just do:

$(LD) $PURGATORY_LDFLAGS) purgatory.ro -o /dev/null

Also it seems better to actually use the linker for this test then usig an other
tool which may have subtly different semantics.

Regards,

Hans