linux-kernel - Re: [PATCH] tools: bpf: Disable stack protector

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Date:   Tue, 17 Jan 2023 09:14:38 -0800
From:   Yonghong Song <yhs@...a.com>
To:     "Jose E. Marchesi" <jose.marchesi@...cle.com>
Cc:     Peter Foley <pefoley2@...oley.com>,
        Eduard Zingerman <eddyz87@...il.com>,
        Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        david.faust@...cle.com, elena.zannoni@...cle.com
Subject: Re: [PATCH] tools: bpf: Disable stack protector



On 1/17/23 8:31 AM, Jose E. Marchesi wrote:
> 
>>> On 1/16/23 2:49 PM, Peter Foley wrote:
>>>> On Mon, Jan 16, 2023 at 4:59 AM Eduard Zingerman <eddyz87@...il.com> wrote:
>>>>>
>>>>> A bit tangential, but since BPF LLVM backend does not support the
>>>>> stack protector (should it?) there is also an option to adjust LLVM
>>>>> to avoid this instrumentation, WDYT?
>>>>>
>>>> That would probably be worth doing, yes.
>>>> But given that won't help already released versions of clang, it
>>>> should probably happen in addition to this patch.
>>>
>>> Peter,
>>>
>>> If I understand correctly (by inspecting clang code), the stack
>>> protector is off by default. Do you have link to Gentoo build
>>> page to show how they enable stack protector? cmake config or
>>> a private patch?
>>>
>>> Jose,
>>>
>>> How gcc-bpf handle stack protector? The compiler just disables
>>> stack protector for bpf target?
>>
>> It doesn't.  -fstack-protector is disabled by default in GCC.  When you
>> use it you get something like:
>>
>>    $ echo 'int foo() { char s[256]; return s[3]; }' | bpf-unknown-none-gcc \
>>      -fstack-protector -S -o foo.s -O2 -xc -
>>    $ cat foo.s
>>    	.file	"<stdin>"
>>    	.text
>>    	.align	3
>>    	.global	foo
>>    	.type	foo, @function
>>    foo:
>>    	lddw	%r1,__stack_chk_guard
>>    	ldxdw	%r0,[%r1+0]
>>    	stxdw	[%fp+-8],%r0
>>    	ldxb	%r0,[%fp+-261]
>>    	lsh	%r0,56
>>    	arsh	%r0,56
>>    	ldxdw	%r2,[%fp+-8]
>>    	ldxdw	%r3,[%r1+0]
>>   	jne	%r2,%r3,.L4
>>    	exit
>>    .L4:
>>    	call	__stack_chk_fail
>>    	.size	foo, .-foo
>>    	.ident	"GCC: (GNU) 12.0.0 20211206 (experimental)"
>>
>> i.e. it pushes a stack canary and checks it upon function exit, calling
>> __stack_chk_fail.
>>
>> If clang has -fstack-protector ON by default and you change the BPF
>> backend in order to ignore the flag, I think we should do the same in
>> GCC.
> 
> I went ahead and pushed the patch below to GCC master.  If
> -fstack-protector is ever considered useful in the architecture, we can
> always stop disabling it.
> 
> I would recommend to change the default for -fstack-protector in clang
> to be off by default when targetting BPF targets, and to emit the same
> or similar note to the user when the option is enabled explicitly with
> -fstack-protector:
> 
>    note: ‘-fstack-protector’ does not work  on this architecture
> 
> WDYT?
> 
>  From 3b81f5c4d8e0d79cbd6927d004185707c14e54b2 Mon Sep 17 00:00:00 2001
> Date: Tue, 17 Jan 2023 17:16:32 +0100
> Subject: [COMMITTED] bpf: disable -fstack-protector in BPF
> 
> The stack protector is not supported in BPF.  This patch disables
> -fstack-protector in bpf-* targets, along with the emission of a note
> indicating that the feature is not supported in this platform.
> 
> Regtested in bpf-unknown-none.
> 
> gcc/ChangeLog:
> 
> 	* config/bpf/bpf.cc (bpf_option_override): Disable
> 	-fstack-protector.
> ---
>   gcc/config/bpf/bpf.cc | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/gcc/config/bpf/bpf.cc b/gcc/config/bpf/bpf.cc
> index 576a1fe8eab..b268801d00c 100644
> --- a/gcc/config/bpf/bpf.cc
> +++ b/gcc/config/bpf/bpf.cc
> @@ -253,6 +253,14 @@ bpf_option_override (void)
>     if (bpf_has_jmp32 == -1)
>       bpf_has_jmp32 = (bpf_isa >= ISA_V3);
>   
> +  /* Disable -fstack-protector as it is not supported in BPF.  */
> +  if (flag_stack_protect)
> +    {
> +      inform (input_location,
> +              "%<-fstack-protector%> does not work "
> +              " on this architecture");
> +      flag_stack_protect = 0;
> +    }
>   }

Thanks, just replied based on a previous email
communication a while back. Yes, clang could
do similar things.

>   
>   #undef TARGET_OPTION_OVERRIDE