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

[Andrea Righi <arighi@nvidia.com>]

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 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@nvidia.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;

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?

Thanks,
-Andrea