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Subject: Re: x86/purgatory: undefined symbol __stack_chk_fail
To:     Andreas Smas <andreas@lonelycoder.com>,
        linux-kernel@vger.kernel.org
References: <CAObFT-SqdcM2Xo7P3FqgwTABao5uoWrb+A3bXy9vKt5rBffSwA@mail.gmail.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <fc900727-b79f-b22a-4ae5-8774c2feab84@rasmusvillemoes.dk>
Date:   Sat, 28 Sep 2019 19:17:55 +0200
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On 03/09/2019 17.50, Andreas Smas wrote:
> Hi,
> 
> For me, kernels built including this commit
> b059f801a937 (x86/purgatory: Use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS)
> 
> results in kexec() failing to load the kernel:
> 
> kexec: Undefined symbol: __stack_chk_fail
> kexec-bzImage64: Loading purgatory failed
> 
> Can be seen:
> 
> $ readelf -a arch/x86/purgatory/purgatory.ro | grep UND
>      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>     51: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __stack_chk_fail
> 
> Using: gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)

Ubuntu's gcc has -fstack-protector enabled by default, so this happens
if one doesn't pass -fno-stack-protector (which I guess is implied by
-ffreestanding) explicitly.

> Adding -ffreestanding or -fno-stack-protector to ccflags-y in
> arch/x86/purgatory/Makefile
> fixes the problem. Not sure which would be preferred.

Probably -fno-stack-protector, guarded by
CONFIG_CC_HAS_STACKPROTECTOR_NONE (because not all gccs understand
-fno-stack-protector), so

ifdef CONFIG_CC_HAS_STACKPROTECTOR_NONE
ccflags-y += -fno-stack-protector
endif

Rasmus