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SubjectRe: [PATCH] tools: bpf: Disable stack protector
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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@gmail.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

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