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Marchesi" To: Yonghong Song Cc: Peter Foley , Eduard Zingerman , Quentin Monnet , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Nathan Chancellor , Nick Desaulniers , Tom Rix , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, david.faust@oracle.com, elena.zannoni@oracle.com Subject: Re: [PATCH] tools: bpf: Disable stack protector References: <20230114-bpf-v1-1-f836695a8b62@pefoley.com> <194f38f2dc7d521375e5a660baaf1be31536be9a.camel@gmail.com> <1135125e-6b8a-7b75-5f0b-3208f6b6e8ae@meta.com> <87bkmx9tw8.fsf@oracle.com> Date: Tue, 17 Jan 2023 17:31:15 +0100 In-Reply-To: <87bkmx9tw8.fsf@oracle.com> (Jose E. 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Rv3rHZMeLYC+/8f1ywEUxplc/iBL9k0W6/jm6o6fyXwj1N4oxassmO4SiHXHcymOo9zAR2YcP14RnRFe51zKaAE7izidR0CELIeaNw+khOg= X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS0PR10MB6248 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.219,Aquarius:18.0.923,Hydra:6.0.562,FMLib:17.11.122.1 definitions=2023-01-17_08,2023-01-17_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 phishscore=0 mlxlogscore=999 adultscore=0 malwarescore=0 suspectscore=0 mlxscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2212070000 definitions=main-2301170132 X-Proofpoint-GUID: fnHDBs40URzioc8_X0Te6pj-aMiz8LLh X-Proofpoint-ORIG-GUID: fnHDBs40URzioc8_X0Te6pj-aMiz8LLh Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >> On 1/16/23 2:49 PM, Peter Foley wrote: >>> On Mon, Jan 16, 2023 at 4:59 AM Eduard Zingerman wr= ote: >>>> >>>> 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 "" > .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: =E2=80=98-fstack-protector=E2=80=99 does not work on this architec= ture 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 =3D=3D -1) bpf_has_jmp32 =3D (bpf_isa >=3D ISA_V3); =20 + /* 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 =3D 0; + } } =20 #undef TARGET_OPTION_OVERRIDE --=20 2.30.2