Re: [PATCH] tools: bpf: Disable stack protector
From: Yonghong Song
Date: Tue Jan 17 2023 - 12:16:50 EST
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@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.
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