Re: [PATCHv2 bpf-next 05/11] bpf: Disable preemption in bpf_get_stackid

[Leon Hwang <leon.hwang@linux.dev>]

On 29/7/26 16:38, Jiri Olsa wrote:
> The get_perf_callchain call needs disabled preemption plus we need
> it disabled as long as we access its returned trace entries buffer.
> 
> Note the bpf_get_stackid_pe function is executed already with
> preemption disabled.
> 
> Cc: stable@vger.kernel.org
> Fixes: d5a3b1f69186 ("bpf: introduce BPF_MAP_TYPE_STACK_TRACE")
> Reported-by: Tao Chen <chen.dylane@linux.dev>
> Closes: https://lore.kernel.org/bpf/20260206090653.1336687-2-chen.dylane@linux.dev/
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/bpf/stackmap.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 43b1e5436047..e48ddb4edd47 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -624,30 +624,37 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>  	struct perf_callchain_entry *trace;
>  	struct stackid stackid;
>  	bool kernel = !user;
> +	int err = -EFAULT;
>  	u32 max_depth;
> -	int err;
>  
>  	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
>  			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
>  		return -EINVAL;
>  
>  	max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> +
> +	preempt_disable();

Better to use the guard-style guard(preempt)() in this series.

Thanks,
Leon

>  	trace = get_perf_callchain(regs, kernel, user, max_depth,
>  				   false, false, 0);
>  
>  	if (unlikely(!trace))
>  		/* couldn't fetch the stack trace */
> -		return -EFAULT;
> +		goto out;
>  
>  	err = stackid_fastpath(&stackid, map, trace, flags);
>  	if (err != -ENOENT)
> -		return err;
> +		goto out;
>  
>  	new_bucket = stackid_new_bucket(&stackid, map);
> -	if (!new_bucket)
> -		return -ENOMEM;
> +	if (new_bucket) {
> +		preempt_enable();
> +		return stackid_install(&stackid, map, new_bucket, flags);
> +	}
> +	err = -ENOMEM;
>  
> -	return stackid_install(&stackid, map, new_bucket, flags);
> +out:
> +	preempt_enable();
> +	return err;
>  }
>  
>  const struct bpf_func_proto bpf_get_stackid_proto = {