Re: [PATCH bpf-next v3 2/2] selftests/bpf: test bpf_cpumask_populate() rejects a borrowed cpumask

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Thu Jul 9, 2026 at 2:28 PM EDT, Nicholas Dudar wrote:
> bpf_cpumask_populate() now takes a struct bpf_cpumask *, so update the
> kfunc declaration and drop the struct cpumask * casts in the existing
> populate tests. Add test_populate_borrowed_destination, which passes a
> borrowed task->cpus_ptr and asserts the verifier rejects it as a writable
> destination.
>
> Signed-off-by: Nicholas Dudar <main.kalliope@gmail.com>
> Assisted-by: Claude:claude-opus-4-8

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

One thing that'd be nice is an explicit test that checks modifying the
cpumask being returned by scx_bpf_get_idle_cpumask, since this was the
motivation in the first place. But we don't have a sched_ext test in
selftests/bpf, so it's not worth the complexity. Since what we're really
testing is that bpf_cpumask -> cpumask conversions are acceptable but
not the opposite, I think the tp test suffices.

> ---
>  .../selftests/bpf/progs/cpumask_common.h      |  2 +-
>  .../selftests/bpf/progs/cpumask_failure.c     | 23 +++++++++++++++++--
>  .../selftests/bpf/progs/cpumask_success.c     |  6 ++---
>  3 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/cpumask_common.h b/tools/testing/selftests/bpf/progs/cpumask_common.h
> index 86085b79f5ca..8fe01308d210 100644
> --- a/tools/testing/selftests/bpf/progs/cpumask_common.h
> +++ b/tools/testing/selftests/bpf/progs/cpumask_common.h
> @@ -61,7 +61,7 @@ u32 bpf_cpumask_any_distribute(const struct cpumask *src) __ksym __weak;
>  u32 bpf_cpumask_any_and_distribute(const struct cpumask *src1,
>  				   const struct cpumask *src2) __ksym __weak;
>  u32 bpf_cpumask_weight(const struct cpumask *cpumask) __ksym __weak;
> -int bpf_cpumask_populate(struct cpumask *cpumask, void *src, size_t src__sz) __ksym __weak;
> +int bpf_cpumask_populate(struct bpf_cpumask *cpumask, void *src, size_t src__sz) __ksym __weak;
>  
>  void bpf_rcu_read_lock(void) __ksym __weak;
>  void bpf_rcu_read_unlock(void) __ksym __weak;
> diff --git a/tools/testing/selftests/bpf/progs/cpumask_failure.c b/tools/testing/selftests/bpf/progs/cpumask_failure.c
> index 4c45346fe6f7..74b4cd4bcdbb 100644
> --- a/tools/testing/selftests/bpf/progs/cpumask_failure.c
> +++ b/tools/testing/selftests/bpf/progs/cpumask_failure.c
> @@ -231,7 +231,7 @@ int BPF_PROG(test_populate_invalid_destination, struct task_struct *task, u64 cl
>  	u64 bits;
>  	int ret;
>  
> -	ret = bpf_cpumask_populate((struct cpumask *)invalid, &bits, sizeof(bits));
> +	ret = bpf_cpumask_populate(invalid, &bits, sizeof(bits));
>  	if (!ret)
>  		err = 2;
>  
> @@ -252,7 +252,7 @@ int BPF_PROG(test_populate_invalid_source, struct task_struct *task, u64 clone_f
>  		return 0;
>  	}
>  
> -	ret = bpf_cpumask_populate((struct cpumask *)local, garbage, 8);
> +	ret = bpf_cpumask_populate(local, garbage, 8);
>  	if (!ret)
>  		err = 2;
>  
> @@ -260,3 +260,22 @@ int BPF_PROG(test_populate_invalid_source, struct task_struct *task, u64 clone_f
>  
>  	return 0;
>  }
> +
> +SEC("tp_btf/task_newtask")
> +__failure __msg("expected pointer to STRUCT bpf_cpumask but R1 has a pointer to STRUCT cpumask")
> +int BPF_PROG(test_populate_borrowed_destination, struct task_struct *task, u64 clone_flags)
> +{
> +	u64 bits;
> +	int ret;
> +
> +	/*
> +	 * task->cpus_ptr is a borrowed, read-only struct cpumask *, not an
> +	 * owned struct bpf_cpumask *. The verifier must reject it as a
> +	 * writable destination for bpf_cpumask_populate().
> +	 */
> +	ret = bpf_cpumask_populate((struct bpf_cpumask *)task->cpus_ptr, &bits, sizeof(bits));
> +	if (!ret)
> +		err = 2;
> +
> +	return 0;
> +}
> diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
> index 774706e7b058..36f77b9732d4 100644
> --- a/tools/testing/selftests/bpf/progs/cpumask_success.c
> +++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
> @@ -785,7 +785,7 @@ int BPF_PROG(test_populate_reject_small_mask, struct task_struct *task, u64 clon
>  		return 0;
>  
>  	/* The kfunc should prevent this operation */
> -	ret = bpf_cpumask_populate((struct cpumask *)local, &toofewbits, sizeof(toofewbits));
> +	ret = bpf_cpumask_populate(local, &toofewbits, sizeof(toofewbits));
>  	if (ret != -EACCES)
>  		err = 2;
>  
> @@ -824,7 +824,7 @@ int BPF_PROG(test_populate_reject_unaligned, struct task_struct *task, u64 clone
>  	/* Misalign the source array by a byte. */
>  	src = &((char *)bits)[1];
>  
> -	ret = bpf_cpumask_populate((struct cpumask *)mask, src, CPUMASK_TEST_MASKLEN);
> +	ret = bpf_cpumask_populate(mask, src, CPUMASK_TEST_MASKLEN);
>  	if (ret != -EINVAL)
>  		err = 2;
>  
> @@ -855,7 +855,7 @@ int BPF_PROG(test_populate, struct task_struct *task, u64 clone_flags)
>  	}
>  
>  	/* Pass the entire bits array, the kfunc will only copy the valid bits. */
> -	ret = bpf_cpumask_populate((struct cpumask *)mask, bits, CPUMASK_TEST_MASKLEN);
> +	ret = bpf_cpumask_populate(mask, bits, CPUMASK_TEST_MASKLEN);
>  	if (ret) {
>  		err = 2;
>  		goto out;