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Subject: Re: [PATCH V2 02/41] x86/traps: Remove stack-protector from traps.c
To:     Borislav Petkov <bp@alien8.de>
Cc:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Joerg Roedel <jroedel@suse.de>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Juergen Gross <jgross@suse.com>, Chester Lin <clin@suse.com>
References: <20210926150838.197719-1-jiangshanlai@gmail.com>
 <20210926150838.197719-3-jiangshanlai@gmail.com> <YVGalJLOyK235XvG@zn.tnic>
 <d4e653e7-1f23-eec9-4db7-964298fc1dc5@linux.alibaba.com>
 <YVGkhlaDBwASW7dZ@zn.tnic>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <fb9b6013-8281-9864-d48f-e23ca651a20d@linux.alibaba.com>
Date:   Mon, 27 Sep 2021 22:38:34 +0800
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On 2021/9/27 19:01, Borislav Petkov wrote:
> On Mon, Sep 27, 2021 at 06:49:16PM +0800, Lai Jiangshan wrote:
>> Using only "-fno-stack-protector" is enough to disable stack protector with
>> my .config, I'm not so sure about other configuration.
> 
> What does the gcc manpage say about it?
>

In gcc's code, all the -f[no-]stack-protector* argument overwrites the
same flag_stack_protect variable, so the last one takes effect.

 > fstack-protector
 > Common Var(flag_stack_protect, 1) Init(-1) Optimization
 > Use propolice as a stack protection method.
 >
 > fstack-protector-all
 > Common RejectNegative Var(flag_stack_protect, 2) Init(-1) Optimization
 > Use a stack protection method for every function.
 >
 > fstack-protector-strong
 > Common RejectNegative Var(flag_stack_protect, 3) Init(-1) Optimization
 > Use a smart stack protection method for certain functions.
 >
 > fstack-protector-explicit
 > Common RejectNegative Var(flag_stack_protect, 4) Optimization
 > Use stack protection method only for functions with the stack_protect attribute.

In linux kernel's scripts/Makefile.lib, CFLAGS_traps.o is the last flags for
gcc invocation, so only "CFLAGS_traps.o += -fno-stack-protector" must be enough.

 > _c_flags       = $(filter-out $(CFLAGS_REMOVE_$(target-stem).o), \
 >                     $(filter-out $(ccflags-remove-y), \
 >                         $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) $(ccflags-y)) \
 >                     $(CFLAGS_$(target-stem).o))