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

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Date:   Mon, 7 Oct 2019 13:05:29 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Hans de Goede <hdegoede@...hat.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

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.

> +
>  quiet_cmd_bin2c = BIN2C   $@
>        cmd_bin2c = $(objtree)/scripts/bin2c kexec_purgatory < $< > $@
>  
>  $(obj)/kexec-purgatory.c: $(obj)/purgatory.ro FORCE
> +	$(call if_changed,check_purgatory)
>  	$(call if_changed,bin2c)
>  
>  obj-$(CONFIG_KEXEC_FILE)	+= kexec-purgatory.o
> -- 
> 2.23.0
> 

Cheers,
Nathan