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

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Date:   Tue, 17 Jan 2023 14:23:51 +0100
From:   "Jose E. Marchesi" <jose.marchesi@...cle.com>
To:     Yonghong Song <yhs@...a.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 1/16/23 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.