Re: [PATCH bpf-next v3 1/2] bpf: Require a BPF cpumask for bpf_cpumask_populate()

On Thu Jul 9, 2026 at 2:27 PM EDT, Nicholas Dudar wrote:
> bpf_cpumask_populate() writes to its destination with bitmap_copy(), but
> the destination is typed as struct cpumask *. That allows the verifier to
> accept borrowed cpumask pointers returned by read-only kfuncs, such as
> scx_bpf_get_online_cpumask(), as a writable destination.
>
> Make the destination a struct bpf_cpumask * so populate follows the same
> ownership rule as the other mutating cpumask kfuncs. Query kfuncs continue
> to accept const struct cpumask * inputs.
>
> Fixes: 950ad93df2fc ("bpf: add kfunc for populating cpumask bits")
> Signed-off-by: Nicholas Dudar <[email protected]>
> Assisted-by: Claude:claude-opus-4-8

Reviewed-by: Emil Tsalapatis <[email protected]>

> ---
>  kernel/bpf/cpumask.c                     | 6 +++---
>  tools/sched_ext/include/scx/compat.bpf.h | 2 +-
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> index b8c805b4b06a..1336a4efa755 100644
> --- a/kernel/bpf/cpumask.c
> +++ b/kernel/bpf/cpumask.c
> @@ -449,12 +449,12 @@ __bpf_kfunc u32 bpf_cpumask_weight(const struct cpumask *cpumask)
>   * @src__sz: Length of the BPF memory region in bytes.
>   *
>   * Return:
> - * * 0 if the struct cpumask * instance was populated successfully.
> + * * 0 if the struct bpf_cpumask * instance was populated successfully.
>   * * -EACCES if the memory region is too small to populate the cpumask.
>   * * -EINVAL if the memory region is not aligned to the size of a long
>   *   and the architecture does not support efficient unaligned accesses.
>   */
> -__bpf_kfunc int bpf_cpumask_populate(struct cpumask *cpumask, void *src, size_t src__sz)
> +__bpf_kfunc int bpf_cpumask_populate(struct bpf_cpumask *cpumask, void *src, size_t src__sz)
>  {
>  	unsigned long source = (unsigned long)src;
>  
> @@ -467,7 +467,7 @@ __bpf_kfunc int bpf_cpumask_populate(struct cpumask *cpumask, void *src, size_t
>  		!IS_ALIGNED(source, sizeof(long)))
>  		return -EINVAL;
>  
> -	bitmap_copy(cpumask_bits(cpumask), src, nr_cpu_ids);
> +	bitmap_copy(cpumask_bits(&cpumask->cpumask), src, nr_cpu_ids);
>  
>  	return 0;
>  }
> diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
> index 87f15f296234..3f74d522f7e7 100644
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -84,7 +84,7 @@ bool scx_bpf_dispatch_vtime_from_dsq___old(struct bpf_iter_scx_dsq *it__iter, st
>   *
>   * Compat macro will be dropped on v6.19 release.
>   */
> -int bpf_cpumask_populate(struct cpumask *dst, void *src, size_t src__sz) __ksym __weak;
> +int bpf_cpumask_populate(struct bpf_cpumask *dst, void *src, size_t src__sz) __ksym __weak;
>  
>  #define __COMPAT_bpf_cpumask_populate(cpumask, src, size__sz)		\
>  	(bpf_ksym_exists(bpf_cpumask_populate) ?			\