Re: [PATCH bpf v2] cpumap: Avoid warning when CONFIG_DEBUG_PER_CPU_MAPS is enabled

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Thu, 16 Apr 2020 10:31:20 +0200
Toke Høiland-Jørgensen <toke@redhat.com> wrote:

> When the kernel is built with CONFIG_DEBUG_PER_CPU_MAPS, the cpumap code
> can trigger a spurious warning if CONFIG_CPUMASK_OFFSTACK is also set. This
> happens because in this configuration, NR_CPUS can be larger than
> nr_cpumask_bits, so the initial check in cpu_map_alloc() is not sufficient
> to guard against hitting the warning in cpumask_check().
> 
> Fix this by explicitly checking the supplied key against the
> nr_cpumask_bits variable before calling cpu_possible().
> 
> Fixes: 6710e1126934 ("bpf: introduce new bpf cpu map type BPF_MAP_TYPE_CPUMAP")
> Cc: Jesper Dangaard Brouer <brouer@redhat.com>
> Reported-by: Xiumei Mu <xmu@redhat.com>
> Tested-by: Xiumei Mu <xmu@redhat.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>

> v2:
>   - Move check to cpu_map_update_elem() to not affect max size of map
> 
>  kernel/bpf/cpumap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 70f71b154fa5..3fe0b006d2d2 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -469,7 +469,7 @@ static int cpu_map_update_elem(struct bpf_map *map, void *key, void *value,
>  		return -EOVERFLOW;
>  
>  	/* Make sure CPU is a valid possible cpu */
> -	if (!cpu_possible(key_cpu))
> +	if (key_cpu >= nr_cpumask_bits || !cpu_possible(key_cpu))

Toke use 'nr_cpumask_bits' here, because cpumask_check() also uses it,
which is the warning we are trying to avoid.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer