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

[Nicholas Dudar <main.kalliope@gmail.com>]

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 <main.kalliope@gmail.com>
Assisted-by: Claude:claude-opus-4-8
---
 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) ?			\
-- 
2.34.1