linux-kernel - Re: [PATCH] bpf: do not inline bpf_get_smp_processor_id() with CONFIG

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Message-ID: <5e7c4b07-f5f0-400f-a84f-36699f867a4a@iogearbox.net>
Date: Mon, 16 Dec 2024 17:16:33 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Andrea Righi <arighi@...dia.com>, Alexei Starovoitov <ast@...nel.org>
Cc: John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: do not inline bpf_get_smp_processor_id() with
 CONFIG_SMP disabled

On 12/16/24 11:46 AM, Andrea Righi wrote:
> Calling bpf_get_smp_processor_id() in a kernel with CONFIG_SMP disabled
> can trigger the following bug, as pcpu_hot is unavailable:
> 
> [    8.471774] BUG: unable to handle page fault for address: 00000000936a290c
> [    8.471849] #PF: supervisor read access in kernel mode
> [    8.471881] #PF: error_code(0x0000) - not-present page
> 
> Fix by preventing the inlining of bpf_get_smp_processor_id() when
> CONFIG_SMP disabled.
> 
> Fixes: 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper")
> Signed-off-by: Andrea Righi <arighi@...dia.com>

lgtm, but can't we instead do sth like this :

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] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
  			insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
  			insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
  			cnt = 3;
-
+#else
+			BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0),
+			cnt = 1;
+#endif
  			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
  			if (!new_prog)
  				return -ENOMEM;

Thanks,
Daniel