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SubjectRe: [PATCH] tools: bpf: Disable stack protector
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On 1/17/23 5:23 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.

clang itself does not have -fstack-protector on by default. It is
hardened gentoo distribution unconditionally added -fstack-protector
to its clang distribution.

In clang/lib/Driver/ToolChains/Clang.cpp, we have
...
// NVPTX doesn't support stack protectors; from the compiler's
perspective, it
// doesn't even have a stack!
if (EffectiveTriple.isNVPTX())
return;

and -fstack-protector is not effective for NVPTX. I guess we
could make it noop for BPF target as well.

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