Re: [PATCH] bpf: do not inline bpf_get_smp_processor_id() with CONFIG_SMP disabled

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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@xxxxxxxxxx>

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




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