[PATCH bpf-next] bpf: Build-time assert that cpumask offset is zero

[David Vernet]

The first element of a struct bpf_cpumask is a cpumask_t. This is done
to allow struct bpf_cpumask to be cast to a struct cpumask. If this
element were ever moved to another field, any BPF program passing a
struct bpf_cpumask * to a kfunc expecting a const struct cpumask * would
immediately fail to load. Add a build-time assertion so this is
assumption is captured and verified.

Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
---
 kernel/bpf/cpumask.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index 25355a0a367a..6bbb67dfc998 100644
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -52,6 +52,9 @@ struct bpf_cpumask *bpf_cpumask_create(void)
 {
 	struct bpf_cpumask *cpumask;
 
+	/* cpumask must be the first element so struct bpf_cpumask be cast to struct cpumask. */
+	BUILD_BUG_ON(offsetof(struct bpf_cpumask, cpumask) != 0);
+
 	cpumask = bpf_mem_alloc(&bpf_cpumask_ma, sizeof(*cpumask));
 	if (!cpumask)
 		return NULL;
-- 
2.39.0