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Mon, 16 Dec 2024 10:27:04 -0800 (PST) Message-ID: <49407656def0054fb62c47907c2338bfc36df47e.camel@gmail.com> Subject: Re: [PATCH] bpf: do not inline bpf_get_smp_processor_id() with CONFIG_SMP disabled From: Eduard Zingerman To: Andrea Righi , Daniel Borkmann Cc: Alexei Starovoitov , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Song Liu , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , bpf@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 16 Dec 2024 10:26:59 -0800 In-Reply-To: References: <20241216104615.503706-1-arighi@nvidia.com> <5e7c4b07-f5f0-400f-a84f-36699f867a4a@iogearbox.net> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.54.1 (3.54.1-1.fc41) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2024-12-16 at 18:24 +0100, Andrea Righi wrote: > On Mon, Dec 16, 2024 at 05:16:33PM +0100, Daniel Borkmann wrote: > > On 12/16/24 11:46 AM, Andrea Righi wrote: > > > Calling bpf_get_smp_processor_id() in a kernel with CONFIG_SMP disabl= ed > > > can trigger the following bug, as pcpu_hot is unavailable: > > >=20 > > > [ 8.471774] BUG: unable to handle page fault for address: 00000000= 936a290c > > > [ 8.471849] #PF: supervisor read access in kernel mode > > > [ 8.471881] #PF: error_code(0x0000) - not-present page > > >=20 > > > Fix by preventing the inlining of bpf_get_smp_processor_id() when > > > CONFIG_SMP disabled. > > >=20 > > > Fixes: 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper") > > > Signed-off-by: Andrea Righi > >=20 > > lgtm, but can't we instead do sth like this : > >=20 > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index f7f892a52a37..761c70899754 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -21281,11 +21281,15 @@ static int do_misc_fixups(struct bpf_verifier= _env *env) > > * changed in some incompatible and hard to support > > * way, it's fine to back out this inlining logic > > */ > > +#ifdef CONFIG_SMP > > insn_buf[0] =3D BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_= hot.cpu_number); > > insn_buf[1] =3D BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0); > > insn_buf[2] =3D BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0); > > cnt =3D 3; > > - > > +#else > > + BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0), > > + cnt =3D 1; > > +#endif > > new_prog =3D bpf_patch_insn_data(env, i + delta, insn_buf, cnt); > > if (!new_prog) > > return -ENOMEM; >=20 > That works as well (just tested) and it's probably better since we're > basically inlining the return 0. Do you want me to send a v2 with this? I think both Andrea's and Daniel's versions of the fix are good. Note, however, that I missed one more configuration variable when making bpf_get_smp_processor_id() inlinable: CONFIG_DEBUG_PREEMPT. Helper body: BPF_CALL_0(bpf_get_smp_processor_id) { return smp_processor_id(); } smp_processor_id definition: #ifdef CONFIG_DEBUG_PREEMPT extern unsigned int debug_smp_processor_id(void); # define smp_processor_id() debug_smp_processor_id() #else # define smp_processor_id() __smp_processor_id() #endif Thanks, Eduard.