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@manifault.com>
---
 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

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sat, 28 Jan 2023 08:15:37 -0600 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [bpf-next] bpf: Build-time assert that cpumask offset is zero
    https://git.kernel.org/bpf/bpf-next/c/cb4a21ea592f

You are awesome, thank you!
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