bpf: Add a few bpf_cgroup_array_* helper functions

In the upcoming patch, the array can store a struct_ops map.
The array could have a cfi_stubs acting as a dummy instead of
the dummy_bpf_prog. The array logic will need to skip the cfi_stubs
also in order to support storing struct_ops map in the array.

bpf_cgroup_array_length(), bpf_cgroup_array_copy_to_user(), and
bpf_cgroup_array_delete_safe_at() are added as a preparation work
to allow skipping the cfi_stubs in the upcoming patch. This patch
only skips the dummy_bpf_prog which is the same as the existing behavior.
The current bpf_prog_array_*() callers are changed to call the new
bpf_cgroup_array_*(). This is a no-op change.

Unlike bpf_prog_array_copy_to_user(), bpf_cgroup_array_copy_to_user()
does not need a temporary buffer. The cgroup caller already holds
cgroup_mutex and dereferences the effective array with
rcu_dereference_protected(), so it does not copy to userspace
from an RCU read-side critical section. Details in commit 0911287ce3.

Another addition is the bpf_cgroup_array_free(). This prepares
the array to have a different rcu gp for the struct_ops use case,
for example, a struct_ops could have mix of sleepable ops and
non-sleepable ops. In this patch, bpf_cgroup_array_free() only
goes through the regular rcu gp. This is a no-op change also.

bpf_prog_dummy() is also added to return the global dummy_bpf_prog.

bpf_cgroup_array_dummy() is added to decide the sentinel based on atype.
It now always returns bpf_prog_dummy(). In the upcoming patch,
it can return a cfi_stubs if the atype belongs to a struct_ops.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260917200542.3689605-9-ameryhung@gmail.com

include/linux/bpf.h

@@ -2598,6 +2598,7 @@ int bpf_prog_array_copy(struct bpf_prog_array *old_array,
 			struct bpf_prog *include_prog,
 			u64 bpf_cookie,
 			struct bpf_prog_array **new_array);
+struct bpf_prog *bpf_prog_dummy(void);
 
 struct bpf_run_ctx {};
 

kernel/bpf/cgroup.c

@@ -319,6 +319,68 @@ static void bpf_cgroup_link_auto_detach(struct bpf_cgroup_link *link)
 	link->cgroup = NULL;
 }
 
+static void bpf_cgroup_array_free(struct bpf_prog_array *array)
+{
+	if (!array || array == &bpf_empty_prog_array)
+		return;
+	kfree_rcu(array, rcu);
+}
+
+static void *bpf_cgroup_array_dummy(enum cgroup_bpf_attach_type atype)
+{
+	return bpf_prog_dummy();
+}
+
+static int bpf_cgroup_array_length(struct bpf_prog_array *array,
+				   enum cgroup_bpf_attach_type atype)
+{
+	struct bpf_prog_array_item *item;
+	int cnt = 0;
+
+	for (item = array->items; item->prog; item++) {
+		if (item->prog != bpf_cgroup_array_dummy(atype))
+			cnt++;
+	}
+
+	return cnt;
+}
+
+static int bpf_cgroup_array_copy_to_user(struct bpf_prog_array *array,
+					 __u32 __user *prog_ids, int cnt,
+					 enum cgroup_bpf_attach_type atype)
+{
+	struct bpf_prog_array_item *item;
+	int i = 0;
+	u32 id;
+
+	for (item = array->items; item->prog && i < cnt; item++) {
+		if (item->prog == bpf_cgroup_array_dummy(atype))
+			continue;
+		id = item->prog->aux->id;
+		if (copy_to_user(prog_ids + i, &id, sizeof(id)))
+			return -EFAULT;
+		i++;
+	}
+	return item->prog ? -ENOSPC : 0;
+}
+
+static int bpf_cgroup_array_delete_safe_at(struct bpf_prog_array *array,
+					   int index, enum cgroup_bpf_attach_type atype)
+{
+	struct bpf_prog_array_item *item;
+
+	for (item = array->items; item->prog; item++) {
+		if (item->prog == bpf_cgroup_array_dummy(atype))
+			continue;
+		if (!index) {
+			WRITE_ONCE(item->prog, bpf_cgroup_array_dummy(atype));
+			return 0;
+		}
+		index--;
+	}
+	return -ENOENT;
+}
+
 /**
  * cgroup_bpf_release() - put references of all bpf programs and
  *                        release all cgroup bpf data
@@ -356,7 +418,7 @@ static void cgroup_bpf_release(struct work_struct *work)
 		old_array = rcu_dereference_protected(
 				cgrp->bpf.effective[atype],
 				lockdep_is_held(&cgroup_mutex));
-		bpf_prog_array_free(old_array);
+		bpf_cgroup_array_free(old_array);
 	}
 
 	list_for_each_entry_safe(storage, stmp, storages, list_cg) {
@@ -530,7 +592,7 @@ static void activate_effective_progs(struct cgroup *cgrp,
 	/* free prog array after grace period, since __cgroup_bpf_run_*()
 	 * might be still walking the array
 	 */
-	bpf_prog_array_free(old_array);
+	bpf_cgroup_array_free(old_array);
 }
 
 /**
@@ -570,7 +632,7 @@ static int cgroup_bpf_inherit(struct cgroup *cgrp)
 	return 0;
 cleanup:
 	for (i = 0; i < NR; i++)
-		bpf_prog_array_free(arrays[i]);
+		bpf_cgroup_array_free(arrays[i]);
 
 	for (p = cgroup_parent(cgrp); p; p = cgroup_parent(p))
 		cgroup_bpf_put(p);
@@ -625,7 +687,7 @@ static int update_effective_progs(struct cgroup *cgrp,
 
 		if (percpu_ref_is_zero(&desc->bpf.refcnt)) {
 			if (unlikely(desc->bpf.inactive)) {
-				bpf_prog_array_free(desc->bpf.inactive);
+				bpf_cgroup_array_free(desc->bpf.inactive);
 				desc->bpf.inactive = NULL;
 			}
 			continue;
@@ -644,7 +706,7 @@ static int update_effective_progs(struct cgroup *cgrp,
 	css_for_each_descendant_pre(css, &cgrp->self) {
 		struct cgroup *desc = container_of(css, struct cgroup, self);
 
-		bpf_prog_array_free(desc->bpf.inactive);
+		bpf_cgroup_array_free(desc->bpf.inactive);
 		desc->bpf.inactive = NULL;
 	}
 
@@ -1191,7 +1253,7 @@ static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog_list *pl,
 				lockdep_is_held(&cgroup_mutex));
 
 		/* Remove the program from the array */
-		WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos),
+		WARN_ONCE(bpf_cgroup_array_delete_safe_at(progs, pos, atype),
 			  "Failed to purge a prog from array at index %d", pos);
 	}
 }
@@ -1321,7 +1383,7 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 		if (effective_query) {
 			effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
 							      lockdep_is_held(&cgroup_mutex));
-			total_cnt += bpf_prog_array_length(effective);
+			total_cnt += bpf_cgroup_array_length(effective, atype);
 		} else {
 			total_cnt += prog_list_length(&cgrp->bpf.progs[atype], NULL);
 		}
@@ -1351,8 +1413,8 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 		if (effective_query) {
 			effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
 							      lockdep_is_held(&cgroup_mutex));
-			cnt = min_t(int, bpf_prog_array_length(effective), total_cnt);
-			ret = bpf_prog_array_copy_to_user(effective, prog_ids, cnt);
+			cnt = min_t(int, bpf_cgroup_array_length(effective, atype), total_cnt);
+			ret = bpf_cgroup_array_copy_to_user(effective, prog_ids, cnt, atype);
 		} else {
 			struct hlist_head *progs;
 			struct bpf_prog_list *pl;

kernel/bpf/core.c

@@ -2784,6 +2784,11 @@ void bpf_prog_array_free_sleepable(struct bpf_prog_array *progs)
 	call_rcu_tasks_trace(&progs->rcu, __bpf_prog_array_free_sleepable_cb);
 }
 
+struct bpf_prog *bpf_prog_dummy(void)
+{
+	return &dummy_bpf_prog.prog;
+}
+
 int bpf_prog_array_length(struct bpf_prog_array *array)
 {
 	struct bpf_prog_array_item *item;