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

[Daniel Borkmann <daniel@iogearbox.net>]

On 12/16/24 6:24 PM, 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 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?

Yes, pls send v2, I think this is better than explicitly calling the
helper under !CONFIG_SMP.

Thanks,
Daniel