Re: [PATCH] tools: bpf: Disable stack protector

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

>> 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;
+    }
 }
 
 #undef TARGET_OPTION_OVERRIDE
-- 
2.30.2