selftests/bpf: Test bpf_cpumask_populate() rejects a borrowed cpumask

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>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260709182800.2037938-3-main.kalliope@gmail.com
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

1 files changed, 21 insertions, 2 deletions

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;
+}