[PATCH][next] bpf: Avoid thousands of -Wflex-array-members-not-at-end warnings

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

struct bpf_prog_array is a flexible structure, this is a structure that
contains a flexible-array member (struct bpf_prog_array_item items[];).

We create the new struct bpf_prog_array_hdr type, and use it to replace
the object type causing trouble in struct bpf_empty_prog_array, namely
struct bpf_prog_array hdr; 

Also, once -fms-extensions is enabled, we can use transparent struct
members in struct bpf_prog_array.

Notice that the newly created type does not contain the flex-array
member `items`, which is the object causing the -Wfamnae warnings
in struct bpf_empty_prog_array.

With these changes, fix the following warnings:
    
7659 ./include/linux/bpf.h:2369:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/bpf-cgroup.h | 2 +-
 include/linux/bpf.h        | 8 ++++++--
 kernel/bpf/core.c          | 6 +++---
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 2f535331f926..e7d266600ac7 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -184,7 +184,7 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
 	struct bpf_prog_array *array;
 
 	array = rcu_access_pointer(cgrp->bpf.effective[type]);
-	return array != &bpf_empty_prog_array.hdr;
+	return (void *)array != (void *)&bpf_empty_prog_array.hdr;
 }
 
 /* Wrappers for __cgroup_bpf_run_filter_skb() guarded by cgroup_bpf_enabled. */
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 05b34a6355b0..488de065466e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2360,13 +2360,17 @@ struct bpf_prog_array_item {
 	};
 };
 
-struct bpf_prog_array {
+struct bpf_prog_array_hdr {
 	struct rcu_head rcu;
+};
+
+struct bpf_prog_array {
+	struct bpf_prog_array_hdr;
 	struct bpf_prog_array_item items[];
 };
 
 struct bpf_empty_prog_array {
-	struct bpf_prog_array hdr;
+	struct bpf_prog_array_hdr hdr;
 	struct bpf_prog *null_prog;
 };
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 229c74f3d6ae..ac15ab8b7d3c 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2598,14 +2598,14 @@ struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
 	if (prog_cnt)
 		p = kzalloc_flex(*p, items, prog_cnt + 1, flags);
 	else
-		p = &bpf_empty_prog_array.hdr;
+		p = (void *)&bpf_empty_prog_array.hdr;
 
 	return p;
 }
 
 void bpf_prog_array_free(struct bpf_prog_array *progs)
 {
-	if (!progs || progs == &bpf_empty_prog_array.hdr)
+	if (!progs || (void *)progs == (void *)&bpf_empty_prog_array.hdr)
 		return;
 	kfree_rcu(progs, rcu);
 }
@@ -2626,7 +2626,7 @@ static void __bpf_prog_array_free_sleepable_cb(struct rcu_head *rcu)
 
 void bpf_prog_array_free_sleepable(struct bpf_prog_array *progs)
 {
-	if (!progs || progs == &bpf_empty_prog_array.hdr)
+	if (!progs || (void *)progs == (void *)&bpf_empty_prog_array.hdr)
 		return;
 	call_rcu_tasks_trace(&progs->rcu, __bpf_prog_array_free_sleepable_cb);
 }
-- 
2.43.0