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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 3/4/26 01:30, Kees Cook wrote:
> 
> 
> On March 2, 2026 9:04:24 PM PST, "Gustavo A. R. Silva" <gustavoars@kernel.org> wrote:
>> -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
> 
> Typo: "since" instead of "once".

Ah yes, thanks!

> 
>> 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;
>> };
> 
> AFAICT, this struct exists entirely to populate a single element of "items" in a global variable. (I only see "null_prog" used during the initializer.) None of this is needed; globals will be correctly sized with an array initializer of a FAM. Totally untested:
> 
> struct bpf_prog_array bpf_empty_prog_array = {
>      .items = { NULL, },
> };

Okay, in this case the patch would look as follows:

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 2f535331f926..b2e79c2b41d5 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 array != &bpf_empty_prog_array;
  }

  /* 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..4f5b9e85a20c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2365,18 +2365,13 @@ struct bpf_prog_array {
  	struct bpf_prog_array_item items[];
  };

-struct bpf_empty_prog_array {
-	struct bpf_prog_array hdr;
-	struct bpf_prog *null_prog;
-};
-
  /* to avoid allocating empty bpf_prog_array for cgroups that
   * don't have bpf program attached use one global 'bpf_empty_prog_array'
   * It will not be modified the caller of bpf_prog_array_alloc()
   * (since caller requested prog_cnt == 0)
   * that pointer should be 'freed' by bpf_prog_array_free()
   */
-extern struct bpf_empty_prog_array bpf_empty_prog_array;
+extern struct bpf_prog_array bpf_empty_prog_array;

  struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags);
  void bpf_prog_array_free(struct bpf_prog_array *progs);
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 67eb12b637a5..ca39d2e690b9 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2613,8 +2613,10 @@ static struct bpf_prog_dummy {
  	},
  };

-struct bpf_empty_prog_array bpf_empty_prog_array = {
-	.null_prog = NULL,
+struct bpf_prog_array bpf_empty_prog_array = {
+	.items = {
+		{ .prog = NULL },
+	},
  };
  EXPORT_SYMBOL(bpf_empty_prog_array);

@@ -2625,14 +2627,14 @@ 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 = &bpf_empty_prog_array;

  	return p;
  }

  void bpf_prog_array_free(struct bpf_prog_array *progs)
  {
-	if (!progs || progs == &bpf_empty_prog_array.hdr)
+	if (!progs || progs == &bpf_empty_prog_array)
  		return;
  	kfree_rcu(progs, rcu);
  }
@@ -2653,7 +2655,7 @@ 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 || progs == &bpf_empty_prog_array)
  		return;
  	call_rcu_tasks_trace(&progs->rcu, __bpf_prog_array_free_sleepable_cb);
  }

Could maintainers give us feedback or comment on this, please? :)

Thanks!
-Gustavo