On 3/4/26 01:30, Kees Cook wrote:
On March 2, 2026 9:04:24 PM PST, "Gustavo A. R. Silva" <[email protected]>
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 <[email protected]>
---
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
