> On 1/16/23 2:49 PM, Peter Foley wrote:
>> On Mon, Jan 16, 2023 at 4:59 AM Eduard Zingerman <eddyz87@xxxxxxxxx> 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.
