linux-kernel - Re: [PATCH 2/2] x86/purgatory: do not use __builtin_memcpy and __builtin

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Date:   Thu, 18 Jul 2019 09:47:15 +0900
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Vaibhav Rustagi <vaibhavrustagi@...gle.com>
Cc:     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, Vivek Goyal <vgoyal@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        stable@...r.kernel.org, Manoj Gupta <manojgupta@...gle.com>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH 2/2] x86/purgatory: do not use __builtin_memcpy and
 __builtin_memset.

On Wed, Jul 17, 2019 at 05:02:06PM -0700, Vaibhav Rustagi wrote:
> From: Nick Desaulniers <ndesaulniers@...gle.com>
> 
> Implementing memcpy and memset in terms of __builtin_memcpy and
> __builtin_memset is problematic.
> 
> GCC at -O2 will replace calls to the builtins with calls to memcpy and
> memset (but will generate an inline implementation at -Os).  Clang will
> replace the builtins with these calls regardless of optimization level.
> 
> $ llvm-objdump -dr arch/x86/purgatory/string.o | tail
> 
> 0000000000000339 memcpy:
>      339: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
>                 000000000000033b:  R_X86_64_64  memcpy
>      343: ff e0                         jmpq    *%rax
> 
> 0000000000000345 memset:
>      345: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
>                 0000000000000347:  R_X86_64_64  memset
>      34f: ff e0
> 
> Such code results in infinite recursion at runtime. This is observed
> when doing kexec.
> 
> Instead, reuse an implementation from arch/x86/boot/compressed/string.c
> if we define warn as a symbol.
> 
> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056
> Reported-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> Tested-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> Debugged-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> Debugged-by: Manoj Gupta <manojgupta@...gle.com>
> Suggested-by: Alistair Delva <adelva@...gle.com>
> Signed-off-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  arch/x86/purgatory/Makefile    |  3 +++
>  arch/x86/purgatory/purgatory.c |  6 ++++++
>  arch/x86/purgatory/string.c    | 23 -----------------------
>  3 files changed, 9 insertions(+), 23 deletions(-)
>  delete mode 100644 arch/x86/purgatory/string.c

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>