[PATCH bpf-next v5 0/4] bpf: Add bpf_call_rcu() and bpf_call_rcu_tasks_trace()

[PATCH bpf-next v5 0/4] bpf: Add bpf_call_rcu() and bpf_call_rcu_tasks_trace()

[Puranjay Mohan]

Changelog:
v4: https://lore.kernel.org/all/20260915154248.3612028-1-puranjay@kernel.org/
Changes in v5:
- Rename the "hash_map" subtest to "bad_map" so that it matches its
  helper test_call_rcu_bad_map() (bpf-ci)
- Bump the callback counter after chain_err in the selftest callback.
  Userspace polls that counter and then reads chain_err, so it could
  still see the initial value before the re-arm had stored one, which
  let the chain subtest's assertion pass without checking anything
- Say in patch 1 why embedding struct rcu_head in a uapi struct is
  acceptable here (Mykyta, Paul, Alexei)
- Rebase on bpf-next/master
v3: https://lore.kernel.org/all/20260915143640.36292-1-puranjay@kernel.org/
Changes in v4:
- Drop an unrelated hunk in bpf_async_update_prog_callback() that turned
  PTR_ERR(prog) into -EBADF.  That is the shared bpf_timer/bpf_wq path
  and bpf_prog_inc_not_zero() returns -ENOENT, so it would have changed
  the errno bpf_timer_set_callback() and bpf_wq_set_callback() report to
  userspace (Sashiko).  No other changes from v3
v2: https://lore.kernel.org/all/20260915114240.3269184-1-puranjay@kernel.org/
Changes in v3:
- Move the bpf_call_rcu_tasks_trace() verifier bits from patch 1 to
  patch 3; patch 1 alone emitted "resolve_btfids: unresolved symbol
  bpf_call_rcu_tasks_trace" (Sashiko)
- Poll the callback counter with an acquire load (Sashiko)
- teardown: v2 only checked that the program was eventually freed, which
  passes even if nothing was ever armed.  Also assert that the chain ran,
  and read the -EPERM back through an independent .bss fd
- Use kern_sync_rcu() instead of open coding the grace-period wait
- Return -EBADF rather than -ENOENT when the calling program is going
  away, matching bpf_task_work_schedule()
- mismatch_map now pins the bpf_rcu_head label the new code emits; it
  passed with that branch removed.  Add a two_heads test, and wait for
  each grace period separately in the chain test
- Commit messages: correct the -EPERM parity claim, explain the inline
  callback state and the struct size, motivate the tasks trace flavour
v1: https://lore.kernel.org/all/20260907134552.1772405-1-puranjay@kernel.org/
Changes in v2:
- Rebase on bpf-next/master
- Use rcu_read_lock_dont_migrate() over open coding (Alexei)
- Improve re-arming selftest to detect failure (Sashiko)

BPF programs that manage their own objects have no way to run their own
logic once an RCU grace period has elapsed.  bpf_obj_drop() defers a
free, but returning an index to an allocator or unpinning a resource
once readers are done has no equivalent.  sched_ext's BPF library works
around this today by pushing freed nodes onto a list and having a
userspace thread call membarrier(MEMBARRIER_CMD_GLOBAL) and then run a
BPF program to reclaim them; it is the first intended user.

Add:

	int bpf_call_rcu(struct bpf_rcu_head *rh, void *map,
			 int (*callback)(struct bpf_map *map, void *key,
					 void *value));

and bpf_call_rcu_tasks_trace(), same signature, which also waits for
sleepable programs.

@rh is a struct bpf_rcu_head embedded in a value of @map, so the
callback runs as callback(map, key, value) for the element it lives in
and needs no cookie.  The field is only accepted in BPF_MAP_TYPE_ARRAY,
and arming holds a reference on the calling program until the callback
has run.  Patch 1 covers the lifetime rules.

This needs

  https://lore.kernel.org/all/20260810122758.183765-1-puranjay@kernel.org/

for call_rcu() and call_srcu() to be safe from the contexts a BPF
program can be called in.

Puranjay Mohan (4):
  bpf: Add bpf_call_rcu() kfunc
  selftests/bpf: Add tests for bpf_call_rcu()
  bpf: Add bpf_call_rcu_tasks_trace() kfunc
  selftests/bpf: Add a test for bpf_call_rcu_tasks_trace()

 include/linux/bpf.h                           |  10 +
 include/uapi/linux/bpf.h                      |   4 +
 kernel/bpf/btf.c                              |   7 +
 kernel/bpf/helpers.c                          | 102 +++++++
 kernel/bpf/map_in_map.c                       |   4 +
 kernel/bpf/map_iter.c                         |   6 +
 kernel/bpf/syscall.c                          |  11 +-
 kernel/bpf/verifier.c                         |  84 +++++-
 tools/include/uapi/linux/bpf.h                |   4 +
 .../selftests/bpf/prog_tests/call_rcu.c       | 275 ++++++++++++++++++
 tools/testing/selftests/bpf/progs/call_rcu.c  | 110 +++++++
 .../selftests/bpf/progs/call_rcu_fail.c       | 114 ++++++++
 12 files changed, 728 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/call_rcu.c
 create mode 100644 tools/testing/selftests/bpf/progs/call_rcu.c
 create mode 100644 tools/testing/selftests/bpf/progs/call_rcu_fail.c


base-commit: 15972a29ac91dbbf4d33246962b7fcc7b1dea7ed
-- 
2.53.0-Meta

[PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Puranjay Mohan]

BPF programs that manage their own objects have no way to run their own
logic once an RCU grace period has elapsed.  bpf_obj_drop() defers a
free, but returning an index to an allocator or unpinning a resource
once readers are done has no equivalent.  sched_ext's BPF library works
around this today by pushing freed nodes onto a list and having a
userspace thread call membarrier(MEMBARRIER_CMD_GLOBAL) and then run a
BPF program to reclaim them.

Add:

	int bpf_call_rcu(struct bpf_rcu_head *rh, void *map,
			 int (*callback)(struct bpf_map *map, void *key,
					 void *value));

@rh is a struct bpf_rcu_head embedded in a value of @map, so the
callback runs as callback(map, key, value) for the element it lives in
and needs no cookie.  A head can only be armed once, which bounds
outstanding work by the number of elements.

struct bpf_rcu_head holds the callback state inline rather than a
pointer to it, as bpf_timer, bpf_wq and bpf_task_work do, because there
is nothing to cancel and so nothing that has to outlive the map value.
That avoids an allocation and a state machine on the arming path at the
cost of 64 bytes per element, 48 of which are used today.  Embedding
struct rcu_head ties part of a uapi struct to a definition outside of
BPF, which is acceptable here only because it is two pointers, a
callback and its argument, with no room to grow.

An RCU callback cannot be cancelled, so everything it touches has to
stay alive until it runs:

  - The callback is the program's text, so arming takes a program
    reference as bpf_timer, bpf_wq and bpf_task_work do, dropped once
    the callback returns.  bpf_prog_inc_not_zero() also fails the arm
    with -EBADF once the program is dying.

  - The map is held by that reference through used_maps.  An inner map
    is not, so bpf_rcu_head is rejected in one.

  - The field is only accepted in BPF_MAP_TYPE_ARRAY, whose elements
    are never freed individually.  A hash element can be deleted and
    recycled while a callback is queued on it.

  - The head is disarmed before the callback runs so it can be armed
    again from there, which takes a new program reference before the
    running callback drops its own.  Arming therefore fails with -EPERM
    once the map is held by neither a process nor bpffs, which is what
    bpf_timer and bpf_wq do at init time; bpf_task_work uses -EBUSY and
    additionally cancels, which is not possible here.

bpf_iter hands a program a writable pointer to the live element, which
would let it overwrite a queued head, so bpf_iter_attach_map() rejects
maps carrying one.

The callback is verified non-sleepable even when the caller is
sleepable, and RCU invokes it with BH disabled.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 include/linux/bpf.h            | 10 +++++
 include/uapi/linux/bpf.h       |  4 ++
 kernel/bpf/btf.c               |  7 +++
 kernel/bpf/helpers.c           | 75 +++++++++++++++++++++++++++++++
 kernel/bpf/map_in_map.c        |  4 ++
 kernel/bpf/map_iter.c          |  6 +++
 kernel/bpf/syscall.c           | 11 ++++-
 kernel/bpf/verifier.c          | 81 +++++++++++++++++++++++++++++++++-
 tools/include/uapi/linux/bpf.h |  4 ++
 9 files changed, 199 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index fd22db8bc6c50..e7c5e203edddb 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -215,6 +215,7 @@ enum btf_field_type {
 	BPF_UPTR       = (1 << 11),
 	BPF_RES_SPIN_LOCK = (1 << 12),
 	BPF_TASK_WORK  = (1 << 13),
+	BPF_RCU_HEAD   = (1 << 14),
 };
 
 enum bpf_cgroup_storage_type {
@@ -269,6 +270,7 @@ struct btf_record {
 	int wq_off;
 	int refcount_off;
 	int task_work_off;
+	int rcu_head_off;
 	struct btf_field fields[];
 };
 
@@ -374,6 +376,8 @@ static inline const char *btf_field_type_name(enum btf_field_type type)
 		return "bpf_refcount";
 	case BPF_TASK_WORK:
 		return "bpf_task_work";
+	case BPF_RCU_HEAD:
+		return "bpf_rcu_head";
 	default:
 		WARN_ON_ONCE(1);
 		return "unknown";
@@ -414,6 +418,8 @@ static inline u32 btf_field_type_size(enum btf_field_type type)
 		return sizeof(struct bpf_refcount);
 	case BPF_TASK_WORK:
 		return sizeof(struct bpf_task_work);
+	case BPF_RCU_HEAD:
+		return sizeof(struct bpf_rcu_head);
 	default:
 		WARN_ON_ONCE(1);
 		return 0;
@@ -448,6 +454,8 @@ static inline u32 btf_field_type_align(enum btf_field_type type)
 		return __alignof__(struct bpf_refcount);
 	case BPF_TASK_WORK:
 		return __alignof__(struct bpf_task_work);
+	case BPF_RCU_HEAD:
+		return __alignof__(struct bpf_rcu_head);
 	default:
 		WARN_ON_ONCE(1);
 		return 0;
@@ -480,6 +488,7 @@ static inline void bpf_obj_init_field(const struct btf_field *field, void *addr)
 	case BPF_KPTR_PERCPU:
 	case BPF_UPTR:
 	case BPF_TASK_WORK:
+	case BPF_RCU_HEAD:
 		break;
 	default:
 		WARN_ON_ONCE(1);
@@ -925,6 +934,7 @@ enum bpf_arg_type {
 	ARG_PTR_TO_RB_NODE,	/* pointer to bpf_rb_node */
 	ARG_PTR_TO_WORKQUEUE,	/* pointer to bpf_wq */
 	ARG_PTR_TO_TASK_WORK,	/* pointer to bpf_task_work */
+	ARG_PTR_TO_RCU_HEAD,	/* pointer to bpf_rcu_head */
 	ARG_PTR_TO_IRQ_FLAG,	/* pointer to saved IRQ flags on the stack */
 	ARG_PTR_TO_RES_SPIN_LOCK,	/* pointer to bpf_res_spin_lock */
 	ARG_PTR_TO_CTX_OUT,	/* hook output argument passed through from ctx */
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 6330b7d745c57..eafeba23f5da7 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7611,6 +7611,10 @@ struct bpf_task_work {
 	__u64 __opaque;
 } __attribute__((aligned(8)));
 
+struct bpf_rcu_head {
+	__u64 __opaque[8];
+} __attribute__((aligned(8)));
+
 struct bpf_wq {
 	__u64 __opaque[2];
 } __attribute__((aligned(8)));
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 314ecb0e593b0..4a1fa4fbdf4e8 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3696,6 +3696,7 @@ static int btf_get_field_type(const struct btf *btf, const struct btf_type *var_
 		{ BPF_TIMER, "bpf_timer", true },
 		{ BPF_WORKQUEUE, "bpf_wq", true },
 		{ BPF_TASK_WORK, "bpf_task_work", true },
+		{ BPF_RCU_HEAD, "bpf_rcu_head", true },
 		{ BPF_LIST_HEAD, "bpf_list_head", false },
 		{ BPF_LIST_NODE, "bpf_list_node", false },
 		{ BPF_RB_ROOT, "bpf_rb_root", false },
@@ -3881,6 +3882,7 @@ static int btf_find_field_one(const struct btf *btf,
 	case BPF_RB_NODE:
 	case BPF_REFCOUNT:
 	case BPF_TASK_WORK:
+	case BPF_RCU_HEAD:
 		ret = btf_find_struct(btf, var_type, off, sz, field_type,
 				      info_cnt ? &info[0] : &tmp);
 		if (ret < 0)
@@ -4176,6 +4178,7 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
 	rec->wq_off = -EINVAL;
 	rec->refcount_off = -EINVAL;
 	rec->task_work_off = -EINVAL;
+	rec->rcu_head_off = -EINVAL;
 	for (i = 0; i < cnt; i++) {
 		field_type_size = btf_field_type_size(info_arr[i].type);
 		if (info_arr[i].off + field_type_size > value_size) {
@@ -4219,6 +4222,10 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
 			WARN_ON_ONCE(rec->task_work_off >= 0);
 			rec->task_work_off = rec->fields[i].offset;
 			break;
+		case BPF_RCU_HEAD:
+			WARN_ON_ONCE(rec->rcu_head_off >= 0);
+			rec->rcu_head_off = rec->fields[i].offset;
+			break;
 		case BPF_REFCOUNT:
 			WARN_ON_ONCE(rec->refcount_off >= 0);
 			/* Cache offset for faster lookup at runtime */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 501c7ce35cba9..8a01dd4058a03 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4805,6 +4805,80 @@ __bpf_kfunc int bpf_task_work_schedule_resume(struct task_struct *task, struct b
 	return bpf_task_work_schedule(task, tw, map__const_map, callback, aux, TWA_RESUME);
 }
 
+typedef int (*bpf_rcu_callback_t)(struct bpf_map *map, void *key, void *value);
+
+/* Actual type for struct bpf_rcu_head */
+struct bpf_rcu_head_kern {
+	struct rcu_head rcu;
+	bpf_callback_t callback_fn;
+	struct bpf_map *map;
+	struct bpf_prog *prog;
+	u32 armed;
+} __aligned(8);
+
+static void bpf_rcu_run_callback(struct rcu_head *rcu)
+{
+	struct bpf_rcu_head_kern *rh = container_of(rcu, struct bpf_rcu_head_kern, rcu);
+	bpf_callback_t callback_fn = rh->callback_fn;
+	struct bpf_prog *prog = rh->prog;
+	struct bpf_map *map = rh->map;
+	void *value, *key;
+	u32 idx;
+
+	value = (void *)rh - map->record->rcu_head_off;
+	key = map_key_from_value(map, value, &idx);
+
+	/* Pairs with the arming cmpxchg(): rh may be re-armed as soon as this store lands. */
+	smp_store_release(&rh->armed, 0);
+
+	rcu_read_lock_dont_migrate();
+	callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
+	rcu_read_unlock_migrate();
+
+	bpf_prog_put(prog);
+}
+
+/**
+ * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
+ * @rh: struct bpf_rcu_head in a BPF map value
+ * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
+ * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
+ * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
+ *
+ * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
+ * nor bpffs, or -EBADF if the calling program is going away.
+ */
+__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
+			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
+{
+	struct bpf_rcu_head_kern *rhk = (void *)rh;
+	struct bpf_map *map = map__const_map;
+	struct bpf_prog *prog;
+
+	BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));
+	BUILD_BUG_ON(__alignof__(struct bpf_rcu_head_kern) != __alignof__(struct bpf_rcu_head));
+	BTF_TYPE_EMIT(struct bpf_rcu_head);
+
+	/* A queued callback cannot be cancelled, so a self-rearming one would pin prog and map. */
+	if (!atomic64_read(&map->usercnt))
+		return -EPERM;
+
+	if (cmpxchg(&rhk->armed, 0, 1))
+		return -EBUSY;
+
+	prog = bpf_prog_inc_not_zero(aux->prog);
+	if (IS_ERR(prog)) {
+		WRITE_ONCE(rhk->armed, 0);
+		return -EBADF;
+	}
+
+	rhk->callback_fn = (bpf_callback_t)(void *)callback;
+	rhk->map = map;
+	rhk->prog = prog;
+	call_rcu(&rhk->rcu, bpf_rcu_run_callback);
+	return 0;
+}
+
 static int make_file_dynptr(struct file *file, u32 flags, bool may_sleep,
 			    struct bpf_dynptr_kern *ptr)
 {
@@ -5100,6 +5174,7 @@ BTF_ID_FLAGS(func, bpf_stream_vprintk, KF_IMPLICIT_ARGS | KF_SPINLOCK_SAFE)
 BTF_ID_FLAGS(func, bpf_stream_print_stack, KF_IMPLICIT_ARGS | KF_SPINLOCK_SAFE)
 BTF_ID_FLAGS(func, bpf_task_work_schedule_signal, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_task_work_schedule_resume, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, bpf_call_rcu, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_dynptr_from_file)
 BTF_ID_FLAGS(func, bpf_dynptr_file_discard, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_timer_cancel_async)
diff --git a/kernel/bpf/map_in_map.c b/kernel/bpf/map_in_map.c
index d2cbab4bdf644..5ee8aae7435ba 100644
--- a/kernel/bpf/map_in_map.c
+++ b/kernel/bpf/map_in_map.c
@@ -25,6 +25,10 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
 	if (!inner_map->ops->map_meta_equal)
 		return ERR_PTR(-ENOTSUPP);
 
+	/* An inner map has no used_maps reference to hold it under a queued callback. */
+	if (btf_record_has_field(inner_map->record, BPF_RCU_HEAD))
+		return ERR_PTR(-EOPNOTSUPP);
+
 	inner_map_meta_size = sizeof(*inner_map_meta);
 	/* In some cases verifier needs to access beyond just base map. */
 	if (inner_map->ops == &array_map_ops || inner_map->ops == &percpu_array_map_ops)
diff --git a/kernel/bpf/map_iter.c b/kernel/bpf/map_iter.c
index c19b360bad9ea..2077d46d167c7 100644
--- a/kernel/bpf/map_iter.c
+++ b/kernel/bpf/map_iter.c
@@ -117,6 +117,12 @@ static int bpf_iter_attach_map(struct bpf_prog *prog,
 		goto put_map;
 	}
 
+	/* The value ctx arg is writable and aliases the live element. */
+	if (btf_record_has_field(map->record, BPF_RCU_HEAD)) {
+		err = -EOPNOTSUPP;
+		goto put_map;
+	}
+
 	if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
 	    map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH ||
 	    map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 113486b15d29f..74496fd716d3b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -687,6 +687,7 @@ void btf_record_free(struct btf_record *rec)
 		case BPF_REFCOUNT:
 		case BPF_WORKQUEUE:
 		case BPF_TASK_WORK:
+		case BPF_RCU_HEAD:
 			/* Nothing to release */
 			break;
 		default:
@@ -741,6 +742,7 @@ struct btf_record *btf_record_dup(const struct btf_record *rec)
 		case BPF_REFCOUNT:
 		case BPF_WORKQUEUE:
 		case BPF_TASK_WORK:
+		case BPF_RCU_HEAD:
 			/* Nothing to acquire */
 			break;
 		default:
@@ -874,6 +876,7 @@ void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
 		case BPF_LIST_NODE:
 		case BPF_RB_NODE:
 		case BPF_REFCOUNT:
+		case BPF_RCU_HEAD:
 			break;
 		default:
 			WARN_ON_ONCE(1);
@@ -1280,7 +1283,7 @@ static int map_check_btf(struct bpf_map *map, struct bpf_token *token,
 	map->record = btf_parse_fields(btf, value_type,
 				       BPF_SPIN_LOCK | BPF_RES_SPIN_LOCK | BPF_TIMER | BPF_KPTR | BPF_LIST_HEAD |
 				       BPF_RB_ROOT | BPF_REFCOUNT | BPF_WORKQUEUE | BPF_UPTR |
-				       BPF_TASK_WORK,
+				       BPF_TASK_WORK | BPF_RCU_HEAD,
 				       map->value_size);
 	if (!IS_ERR_OR_NULL(map->record)) {
 		int i;
@@ -1322,6 +1325,12 @@ static int map_check_btf(struct bpf_map *map, struct bpf_token *token,
 					goto free_map_tab;
 				}
 				break;
+			case BPF_RCU_HEAD:
+				if (map->map_type != BPF_MAP_TYPE_ARRAY) {
+					ret = -EOPNOTSUPP;
+					goto free_map_tab;
+				}
+				break;
 			case BPF_KPTR_UNREF:
 			case BPF_KPTR_REF:
 			case BPF_KPTR_PERCPU:
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d62c0f74cff5e..64db47964ff9f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -535,6 +535,7 @@ static bool is_ptr_cast_function(enum bpf_func_id func_id)
 
 static bool is_sync_callback_calling_kfunc(u32 btf_id);
 static bool is_async_callback_calling_kfunc(u32 btf_id);
+static bool is_call_rcu_kfunc(u32 btf_id);
 static bool is_callback_calling_kfunc(u32 btf_id);
 
 static bool is_bpf_wq_set_callback_kfunc(u32 btf_id);
@@ -577,6 +578,10 @@ static bool is_async_cb_sleepable(struct bpf_verifier_env *env, struct bpf_insn
 	if (bpf_helper_call(insn) && insn->imm == BPF_FUNC_timer_set_callback)
 		return false;
 
+	/* bpf_call_rcu callbacks are never sleepable. */
+	if (bpf_pseudo_kfunc_call(insn) && insn->off == 0 && is_call_rcu_kfunc(insn->imm))
+		return false;
+
 	/* bpf_wq and bpf_task_work callbacks are always sleepable. */
 	if (bpf_pseudo_kfunc_call(insn) && insn->off == 0 &&
 	    (is_bpf_wq_set_callback_kfunc(insn->imm) || is_task_work_add_kfunc(insn->imm)))
@@ -7590,6 +7595,9 @@ static int check_map_field_pointer(struct bpf_verifier_env *env, struct bpf_reg_
 	case BPF_TASK_WORK:
 		field_off = map->record->task_work_off;
 		break;
+	case BPF_RCU_HEAD:
+		field_off = map->record->rcu_head_off;
+		break;
 	case BPF_WORKQUEUE:
 		field_off = map->record->wq_off;
 		break;
@@ -8448,6 +8456,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
 	[ARG_PTR_TO_RES_SPIN_LOCK]	= &map_value_or_alloc_obj_types,
 	[ARG_PTR_TO_WORKQUEUE]		= &map_value_types,
 	[ARG_PTR_TO_TASK_WORK]		= &map_value_types,
+	[ARG_PTR_TO_RCU_HEAD]		= &map_value_types,
 	[ARG_PTR_TO_IRQ_FLAG]		= &stack_ptr_types,
 	[ARG_PTR_TO_ARENA]		= &arena_types,
 	[ARG_PTR_TO_CTX_OUT]		= &ctx_out_types,
@@ -8894,6 +8903,8 @@ static int process_map_ptr_arg(struct bpf_verifier_env *env, struct bpf_reg_stat
 			obj_name = "timer";
 		else if (rec->task_work_off >= 0)
 			obj_name = "bpf_task_work";
+		else if (rec->rcu_head_off >= 0)
+			obj_name = "bpf_rcu_head";
 
 		verbose(env, "%s pointer in %s map_uid=%d ",
 			obj_name, reg_arg_name(env, obj_argno), meta->map.uid);
@@ -9442,6 +9453,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg, u32 slot, u32 p
 		if (err < 0)
 			return err;
 		break;
+	case ARG_PTR_TO_RCU_HEAD:
+		err = check_map_field_pointer(env, reg, argno, BPF_RCU_HEAD, &meta->map);
+		if (err < 0)
+			return err;
+		break;
 	case ARG_PTR_TO_IRQ_FLAG:
 		err = process_irq_flag(env, reg, argno, meta);
 		if (err < 0)
@@ -10955,6 +10971,40 @@ static int set_task_work_schedule_callback_state(struct bpf_verifier_env *env,
 	return 0;
 }
 
+static int set_rcu_callback_state(struct bpf_verifier_env *env,
+				  struct bpf_func_state *caller,
+				  struct bpf_func_state *callee,
+				  int insn_idx)
+{
+	struct bpf_map *map_ptr = caller->regs[BPF_REG_2].map_ptr;
+	u32 map_uid = caller->regs[BPF_REG_2].map_uid;
+
+	/*
+	 * callback_fn(struct bpf_map *map, void *key, void *value);
+	 */
+	callee->regs[BPF_REG_1].type = CONST_PTR_TO_MAP;
+	__mark_reg_known_zero(&callee->regs[BPF_REG_1]);
+	callee->regs[BPF_REG_1].map_ptr = map_ptr;
+	callee->regs[BPF_REG_1].map_uid = map_uid;
+
+	callee->regs[BPF_REG_2].type = PTR_TO_MAP_KEY;
+	__mark_reg_known_zero(&callee->regs[BPF_REG_2]);
+	callee->regs[BPF_REG_2].map_ptr = map_ptr;
+	callee->regs[BPF_REG_2].map_uid = map_uid;
+
+	callee->regs[BPF_REG_3].type = PTR_TO_MAP_VALUE;
+	__mark_reg_known_zero(&callee->regs[BPF_REG_3]);
+	callee->regs[BPF_REG_3].map_ptr = map_ptr;
+	callee->regs[BPF_REG_3].map_uid = map_uid;
+
+	/* unused */
+	bpf_mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
+	bpf_mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
+	callee->in_async_callback_fn = true;
+	callee->callback_ret_range = retval_range(S32_MIN, S32_MAX);
+	return 0;
+}
+
 static bool is_rbtree_lock_required_kfunc(u32 btf_id);
 
 static void account_processed_insn(struct bpf_verifier_env *env)
@@ -12216,7 +12266,8 @@ enum {
 	KF_ARG_RES_SPIN_LOCK_ID,
 	KF_ARG_TASK_WORK_ID,
 	KF_ARG_PROG_AUX_ID,
-	KF_ARG_TIMER_ID
+	KF_ARG_TIMER_ID,
+	KF_ARG_RCU_HEAD_ID
 };
 
 BTF_ID_LIST(kf_arg_btf_ids)
@@ -12230,6 +12281,7 @@ BTF_ID(struct, bpf_res_spin_lock)
 BTF_ID(struct, bpf_task_work)
 BTF_ID(struct, bpf_prog_aux)
 BTF_ID(struct, bpf_timer)
+BTF_ID(struct, bpf_rcu_head)
 
 static bool __is_kfunc_ptr_arg_type(const struct btf *btf,
 				    const struct btf_param *arg, int type)
@@ -12288,6 +12340,11 @@ static bool is_kfunc_arg_task_work(const struct btf *btf, const struct btf_param
 	return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_TASK_WORK_ID);
 }
 
+static bool is_kfunc_arg_rcu_head(const struct btf *btf, const struct btf_param *arg)
+{
+	return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_RCU_HEAD_ID);
+}
+
 static bool is_kfunc_arg_res_spin_lock(const struct btf *btf, const struct btf_param *arg)
 {
 	return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_RES_SPIN_LOCK_ID);
@@ -12571,6 +12628,7 @@ enum special_kfunc_type {
 	KF___bpf_trap,
 	KF_bpf_task_work_schedule_signal,
 	KF_bpf_task_work_schedule_resume,
+	KF_bpf_call_rcu,
 	KF_bpf_arena_alloc_pages,
 	KF_bpf_arena_free_pages,
 	KF_bpf_arena_reserve_pages,
@@ -12664,6 +12722,7 @@ BTF_ID(func, bpf_dynptr_file_discard)
 BTF_ID(func, __bpf_trap)
 BTF_ID(func, bpf_task_work_schedule_signal)
 BTF_ID(func, bpf_task_work_schedule_resume)
+BTF_ID(func, bpf_call_rcu)
 BTF_ID(func, bpf_arena_alloc_pages)
 BTF_ID(func, bpf_arena_free_pages)
 BTF_ID(func, bpf_arena_reserve_pages)
@@ -12735,6 +12794,11 @@ static bool is_bpf_rbtree_add_kfunc(u32 func_id)
 	       func_id == special_kfunc_list[KF_bpf_rbtree_add_impl];
 }
 
+static bool is_call_rcu_kfunc(u32 func_id)
+{
+	return func_id == special_kfunc_list[KF_bpf_call_rcu];
+}
+
 static bool is_task_work_add_kfunc(u32 func_id)
 {
 	return func_id == special_kfunc_list[KF_bpf_task_work_schedule_signal] ||
@@ -12941,6 +13005,8 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 		arg_type = ARG_PTR_TO_TIMER;
 	else if (is_kfunc_arg_task_work(meta->btf, &args[arg]))
 		arg_type = ARG_PTR_TO_TASK_WORK;
+	else if (is_kfunc_arg_rcu_head(meta->btf, &args[arg]))
+		arg_type = ARG_PTR_TO_RCU_HEAD;
 	else if (is_kfunc_arg_irq_flag(meta->btf, &args[arg]))
 		arg_type = ARG_PTR_TO_IRQ_FLAG;
 	else if (is_kfunc_arg_res_spin_lock(meta->btf, &args[arg]))
@@ -13442,7 +13508,8 @@ static bool is_sync_callback_calling_kfunc(u32 btf_id)
 static bool is_async_callback_calling_kfunc(u32 btf_id)
 {
 	return is_bpf_wq_set_callback_kfunc(btf_id) ||
-	       is_task_work_add_kfunc(btf_id);
+	       is_task_work_add_kfunc(btf_id) ||
+	       is_call_rcu_kfunc(btf_id);
 }
 
 bool bpf_is_throw_kfunc(struct bpf_insn *insn)
@@ -14259,6 +14326,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		}
 	}
 
+	if (is_call_rcu_kfunc(meta.func_id)) {
+		err = push_callback_call(env, insn, insn_idx, meta.subprogno,
+					 set_rcu_callback_state);
+		if (err) {
+			verbose(env, "kfunc %s#%d failed callback verification\n",
+				func_name, meta.func_id);
+			return err;
+		}
+	}
+
 	rcu_lock = is_kfunc_bpf_rcu_read_lock(&meta);
 	rcu_unlock = is_kfunc_bpf_rcu_read_unlock(&meta);
 
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 6330b7d745c57..eafeba23f5da7 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7611,6 +7611,10 @@ struct bpf_task_work {
 	__u64 __opaque;
 } __attribute__((aligned(8)));
 
+struct bpf_rcu_head {
+	__u64 __opaque[8];
+} __attribute__((aligned(8)));
+
 struct bpf_wq {
 	__u64 __opaque[2];
 } __attribute__((aligned(8)));
-- 
2.53.0-Meta

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] bpf: bpf_call_rcu() causes deadlock when called from NMI context
--

commit 088862a20bc8f57242ca8bd55ca1e35a37a5380f
Author: Puranjay Mohan <puranjay@kernel.org>

bpf: Add bpf_call_rcu() kfunc

This commit adds a bpf_call_rcu() kfunc, allowing BPF programs to
queue custom callbacks to run after an RCU grace period. It embeds
a struct bpf_rcu_head inside a BPF map value to hold the callback state.

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 501c7ce35cba9..8a01dd4058a03 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c

[ ... ]

> @@ -4805,6 +4805,80 @@ __bpf_kfunc int bpf_task_work_schedule_resume(struct task_struct *task, struct b

[ ... ]

> +__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> +			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +{
> +	struct bpf_rcu_head_kern *rhk = (void *)rh;
> +	struct bpf_map *map = map__const_map;
> +	struct bpf_prog *prog;

[ ... ]

> +	rhk->callback_fn = (bpf_callback_t)(void *)callback;
> +	rhk->map = map;
> +	rhk->prog = prog;
> +	call_rcu(&rhk->rcu, bpf_rcu_run_callback);

[Severity: High]
Does this code trigger a deadlock if bpf_call_rcu() is called from an NMI
context? 

The call_rcu() function acquires raw spinlocks internally. If a BPF program
executing in NMI context (such as a kprobe or BPF_PROG_TYPE_PERF_EVENT) calls
this while interrupting a thread that already holds the same RCU lock, the CPU
will try to acquire the non-reentrant raw spinlock again and hard deadlock.

Should there be an in_nmi() check here or deferral via irq_work_queue() when
called from an NMI context?

> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260921191407.1742386-1-puranjay@kernel.org?part=1

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 3692 bytes --]

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 501c7ce35cba9..8a01dd4058a03 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c

[ ... ]

> @@ -4805,6 +4805,80 @@ __bpf_kfunc int bpf_task_work_schedule_resume(struct task_struct *task, struct b
>  	return bpf_task_work_schedule(task, tw, map__const_map, callback, aux, TWA_RESUME);
>  }
>
> +typedef int (*bpf_rcu_callback_t)(struct bpf_map *map, void *key, void *value);
> +
> +/* Actual type for struct bpf_rcu_head */
> +struct bpf_rcu_head_kern {
> +	struct rcu_head rcu;
> +	bpf_callback_t callback_fn;
> +	struct bpf_map *map;
> +	struct bpf_prog *prog;
> +	u32 armed;
> +} __aligned(8);

In v4, Mykyta Yatsenko raised a concern about embedding struct rcu_head
inline: "I tried to fully embed structure back when implementing
bpf_task_work, Alexei rejected that approach, because some of the
structures (irq_work) I used are not in our control and might change."

On Sept 18 you replied: "Yeah, you are right. I will change it to be
heap-allocated like bpf_task_work." But v5 still embeds the structure
inline.

The commit message justifies this: "Embedding struct rcu_head ties part of
a uapi struct to a definition outside of BPF, which is acceptable here only
because it is two pointers, a callback and its argument, with no room to
grow."

Could you clarify the resolution? struct rcu_head is still not under BPF
control, and if it expands in the future, the 64-byte uapi allocation
becomes too small and breaks userspace ABI.

Reference: https://lore.kernel.org/bpf/210f1bfa-e6ae-4d1e-a920-a6c8c9b4269f@gmail.com/

[ ... ]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index d62c0f74cff5e..64db47964ff9f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c

[ ... ]

> @@ -10955,6 +10971,40 @@ static int set_task_work_schedule_callback_state(struct bpf_verifier_env *env,
>  	return 0;
>  }
>
> +static int set_rcu_callback_state(struct bpf_verifier_env *env,
> +				  struct bpf_func_state *caller,
> +				  struct bpf_func_state *callee,
> +				  int insn_idx)
> +{
> +	struct bpf_map *map_ptr = caller->regs[BPF_REG_2].map_ptr;
> +	u32 map_uid = caller->regs[BPF_REG_2].map_uid;
> +
> +	/*
> +	 * callback_fn(struct bpf_map *map, void *key, void *value);
> +	 */
> +	callee->regs[BPF_REG_1].type = CONST_PTR_TO_MAP;
> +	__mark_reg_known_zero(&callee->regs[BPF_REG_1]);
> +	callee->regs[BPF_REG_1].map_ptr = map_ptr;
> +	callee->regs[BPF_REG_1].map_uid = map_uid;
> +
> +	callee->regs[BPF_REG_2].type = PTR_TO_MAP_KEY;
> +	__mark_reg_known_zero(&callee->regs[BPF_REG_2]);
> +	callee->regs[BPF_REG_2].map_ptr = map_ptr;
> +	callee->regs[BPF_REG_2].map_uid = map_uid;
> +
> +	callee->regs[BPF_REG_3].type = PTR_TO_MAP_VALUE;
> +	__mark_reg_known_zero(&callee->regs[BPF_REG_3]);
> +	callee->regs[BPF_REG_3].map_ptr = map_ptr;
> +	callee->regs[BPF_REG_3].map_uid = map_uid;
> +
> +	/* unused */
> +	bpf_mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
> +	bpf_mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
> +	callee->in_async_callback_fn = true;
> +	callee->callback_ret_range = retval_range(S32_MIN, S32_MAX);
> +	return 0;
> +}
> +

This isn't a bug, but set_rcu_callback_state() differs from
set_task_work_schedule_callback_state() only in reading the map from
BPF_REG_2 instead of BPF_REG_3 - would a shared helper parameterised by
the map register reduce the maintenance burden of these ~30 nearly
identical lines?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/35644498619

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Alexei Starovoitov]

On Mon Sep 21, 2026 at 7:14 PM UTC, Puranjay Mohan wrote:
> BPF programs that manage their own objects have no way to run their own
> logic once an RCU grace period has elapsed.  bpf_obj_drop() defers a
> free, but returning an index to an allocator or unpinning a resource
> once readers are done has no equivalent.  sched_ext's BPF library works
> around this today by pushing freed nodes onto a list and having a
> userspace thread call membarrier(MEMBARRIER_CMD_GLOBAL) and then run a
> BPF program to reclaim them.
>
> Add:
>
> 	int bpf_call_rcu(struct bpf_rcu_head *rh, void *map,
> 			 int (*callback)(struct bpf_map *map, void *key,
> 					 void *value));
>
> @rh is a struct bpf_rcu_head embedded in a value of @map, so the
> callback runs as callback(map, key, value) for the element it lives in
> and needs no cookie.  A head can only be armed once, which bounds
> outstanding work by the number of elements.
>
> struct bpf_rcu_head holds the callback state inline rather than a
> pointer to it, as bpf_timer, bpf_wq and bpf_task_work do, because there
> is nothing to cancel and so nothing that has to outlive the map value.
> That avoids an allocation and a state machine on the arming path at the
> cost of 64 bytes per element, 48 of which are used today.  Embedding
> struct rcu_head ties part of a uapi struct to a definition outside of
> BPF, which is acceptable here only because it is two pointers, a
> callback and its argument, with no room to grow.
>
> An RCU callback cannot be cancelled, so everything it touches has to
> stay alive until it runs:
>
>   - The callback is the program's text, so arming takes a program
>     reference as bpf_timer, bpf_wq and bpf_task_work do, dropped once
>     the callback returns.  bpf_prog_inc_not_zero() also fails the arm
>     with -EBADF once the program is dying.
>
>   - The map is held by that reference through used_maps.  An inner map
>     is not, so bpf_rcu_head is rejected in one.
>
>   - The field is only accepted in BPF_MAP_TYPE_ARRAY, whose elements
>     are never freed individually.  A hash element can be deleted and
>     recycled while a callback is queued on it.
>
>   - The head is disarmed before the callback runs so it can be armed
>     again from there, which takes a new program reference before the
>     running callback drops its own.  Arming therefore fails with -EPERM
>     once the map is held by neither a process nor bpffs, which is what
>     bpf_timer and bpf_wq do at init time; bpf_task_work uses -EBUSY and
>     additionally cancels, which is not possible here.
>
> bpf_iter hands a program a writable pointer to the live element, which
> would let it overwrite a queued head, so bpf_iter_attach_map() rejects
> maps carrying one.
>
> The callback is verified non-sleepable even when the caller is
> sleepable, and RCU invokes it with BH disabled.
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  include/linux/bpf.h            | 10 +++++
>  include/uapi/linux/bpf.h       |  4 ++
>  kernel/bpf/btf.c               |  7 +++
>  kernel/bpf/helpers.c           | 75 +++++++++++++++++++++++++++++++
>  kernel/bpf/map_in_map.c        |  4 ++
>  kernel/bpf/map_iter.c          |  6 +++
>  kernel/bpf/syscall.c           | 11 ++++-
>  kernel/bpf/verifier.c          | 81 +++++++++++++++++++++++++++++++++-
>  tools/include/uapi/linux/bpf.h |  4 ++
>  9 files changed, 199 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index fd22db8bc6c50..e7c5e203edddb 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -215,6 +215,7 @@ enum btf_field_type {
>  	BPF_UPTR       = (1 << 11),
>  	BPF_RES_SPIN_LOCK = (1 << 12),
>  	BPF_TASK_WORK  = (1 << 13),
> +	BPF_RCU_HEAD   = (1 << 14),
>  };
>  
>  enum bpf_cgroup_storage_type {
> @@ -269,6 +270,7 @@ struct btf_record {
>  	int wq_off;
>  	int refcount_off;
>  	int task_work_off;
> +	int rcu_head_off;
>  	struct btf_field fields[];
>  };
>  
> @@ -374,6 +376,8 @@ static inline const char *btf_field_type_name(enum btf_field_type type)
>  		return "bpf_refcount";
>  	case BPF_TASK_WORK:
>  		return "bpf_task_work";
> +	case BPF_RCU_HEAD:
> +		return "bpf_rcu_head";
>  	default:
>  		WARN_ON_ONCE(1);
>  		return "unknown";
> @@ -414,6 +418,8 @@ static inline u32 btf_field_type_size(enum btf_field_type type)
>  		return sizeof(struct bpf_refcount);
>  	case BPF_TASK_WORK:
>  		return sizeof(struct bpf_task_work);
> +	case BPF_RCU_HEAD:
> +		return sizeof(struct bpf_rcu_head);
>  	default:
>  		WARN_ON_ONCE(1);
>  		return 0;
> @@ -448,6 +454,8 @@ static inline u32 btf_field_type_align(enum btf_field_type type)
>  		return __alignof__(struct bpf_refcount);
>  	case BPF_TASK_WORK:
>  		return __alignof__(struct bpf_task_work);
> +	case BPF_RCU_HEAD:
> +		return __alignof__(struct bpf_rcu_head);
>  	default:
>  		WARN_ON_ONCE(1);
>  		return 0;
> @@ -480,6 +488,7 @@ static inline void bpf_obj_init_field(const struct btf_field *field, void *addr)
>  	case BPF_KPTR_PERCPU:
>  	case BPF_UPTR:
>  	case BPF_TASK_WORK:
> +	case BPF_RCU_HEAD:
>  		break;
>  	default:
>  		WARN_ON_ONCE(1);
> @@ -925,6 +934,7 @@ enum bpf_arg_type {
>  	ARG_PTR_TO_RB_NODE,	/* pointer to bpf_rb_node */
>  	ARG_PTR_TO_WORKQUEUE,	/* pointer to bpf_wq */
>  	ARG_PTR_TO_TASK_WORK,	/* pointer to bpf_task_work */
> +	ARG_PTR_TO_RCU_HEAD,	/* pointer to bpf_rcu_head */
>  	ARG_PTR_TO_IRQ_FLAG,	/* pointer to saved IRQ flags on the stack */
>  	ARG_PTR_TO_RES_SPIN_LOCK,	/* pointer to bpf_res_spin_lock */
>  	ARG_PTR_TO_CTX_OUT,	/* hook output argument passed through from ctx */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 6330b7d745c57..eafeba23f5da7 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7611,6 +7611,10 @@ struct bpf_task_work {
>  	__u64 __opaque;
>  } __attribute__((aligned(8)));
>  
> +struct bpf_rcu_head {
> +	__u64 __opaque[8];
> +} __attribute__((aligned(8)));
> +
>  struct bpf_wq {
>  	__u64 __opaque[2];
>  } __attribute__((aligned(8)));
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 314ecb0e593b0..4a1fa4fbdf4e8 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3696,6 +3696,7 @@ static int btf_get_field_type(const struct btf *btf, const struct btf_type *var_
>  		{ BPF_TIMER, "bpf_timer", true },
>  		{ BPF_WORKQUEUE, "bpf_wq", true },
>  		{ BPF_TASK_WORK, "bpf_task_work", true },
> +		{ BPF_RCU_HEAD, "bpf_rcu_head", true },
>  		{ BPF_LIST_HEAD, "bpf_list_head", false },
>  		{ BPF_LIST_NODE, "bpf_list_node", false },
>  		{ BPF_RB_ROOT, "bpf_rb_root", false },
> @@ -3881,6 +3882,7 @@ static int btf_find_field_one(const struct btf *btf,
>  	case BPF_RB_NODE:
>  	case BPF_REFCOUNT:
>  	case BPF_TASK_WORK:
> +	case BPF_RCU_HEAD:
>  		ret = btf_find_struct(btf, var_type, off, sz, field_type,
>  				      info_cnt ? &info[0] : &tmp);
>  		if (ret < 0)
> @@ -4176,6 +4178,7 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
>  	rec->wq_off = -EINVAL;
>  	rec->refcount_off = -EINVAL;
>  	rec->task_work_off = -EINVAL;
> +	rec->rcu_head_off = -EINVAL;
>  	for (i = 0; i < cnt; i++) {
>  		field_type_size = btf_field_type_size(info_arr[i].type);
>  		if (info_arr[i].off + field_type_size > value_size) {
> @@ -4219,6 +4222,10 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
>  			WARN_ON_ONCE(rec->task_work_off >= 0);
>  			rec->task_work_off = rec->fields[i].offset;
>  			break;
> +		case BPF_RCU_HEAD:
> +			WARN_ON_ONCE(rec->rcu_head_off >= 0);
> +			rec->rcu_head_off = rec->fields[i].offset;
> +			break;
>  		case BPF_REFCOUNT:
>  			WARN_ON_ONCE(rec->refcount_off >= 0);
>  			/* Cache offset for faster lookup at runtime */
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 501c7ce35cba9..8a01dd4058a03 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -4805,6 +4805,80 @@ __bpf_kfunc int bpf_task_work_schedule_resume(struct task_struct *task, struct b
>  	return bpf_task_work_schedule(task, tw, map__const_map, callback, aux, TWA_RESUME);
>  }
>  
> +typedef int (*bpf_rcu_callback_t)(struct bpf_map *map, void *key, void *value);
> +
> +/* Actual type for struct bpf_rcu_head */
> +struct bpf_rcu_head_kern {
> +	struct rcu_head rcu;
> +	bpf_callback_t callback_fn;
> +	struct bpf_map *map;
> +	struct bpf_prog *prog;
> +	u32 armed;
> +} __aligned(8);
> +
> +static void bpf_rcu_run_callback(struct rcu_head *rcu)
> +{
> +	struct bpf_rcu_head_kern *rh = container_of(rcu, struct bpf_rcu_head_kern, rcu);
> +	bpf_callback_t callback_fn = rh->callback_fn;
> +	struct bpf_prog *prog = rh->prog;
> +	struct bpf_map *map = rh->map;
> +	void *value, *key;
> +	u32 idx;
> +
> +	value = (void *)rh - map->record->rcu_head_off;
> +	key = map_key_from_value(map, value, &idx);
> +
> +	/* Pairs with the arming cmpxchg(): rh may be re-armed as soon as this store lands. */
> +	smp_store_release(&rh->armed, 0);
> +
> +	rcu_read_lock_dont_migrate();
> +	callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
> +	rcu_read_unlock_migrate();
> +
> +	bpf_prog_put(prog);
> +}
> +
> +/**
> + * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
> + * @rh: struct bpf_rcu_head in a BPF map value
> + * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> + * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> + * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> + *
> + * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> + * nor bpffs, or -EBADF if the calling program is going away.
> + */
> +__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> +			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +{
> +	struct bpf_rcu_head_kern *rhk = (void *)rh;
> +	struct bpf_map *map = map__const_map;
> +	struct bpf_prog *prog;
> +
> +	BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));
> +	BUILD_BUG_ON(__alignof__(struct bpf_rcu_head_kern) != __alignof__(struct bpf_rcu_head));
> +	BTF_TYPE_EMIT(struct bpf_rcu_head);
> +
> +	/* A queued callback cannot be cancelled, so a self-rearming one would pin prog and map. */
> +	if (!atomic64_read(&map->usercnt))
> +		return -EPERM;
> +
> +	if (cmpxchg(&rhk->armed, 0, 1))
> +		return -EBUSY;
> +
> +	prog = bpf_prog_inc_not_zero(aux->prog);
> +	if (IS_ERR(prog)) {
> +		WRITE_ONCE(rhk->armed, 0);
> +		return -EBADF;
> +	}

can we drop prog_inc and remove prog pointer from rhk ?
with something like if (prog->has_call_rcu) rcu_barrier() during prog unload?

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Puranjay Mohan]

On Tue, Sep 22, 2026 at 2:53 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon Sep 21, 2026 at 7:14 PM UTC, Puranjay Mohan wrote:
> > BPF programs that manage their own objects have no way to run their own
> > logic once an RCU grace period has elapsed.  bpf_obj_drop() defers a
> > free, but returning an index to an allocator or unpinning a resource
> > once readers are done has no equivalent.  sched_ext's BPF library works
> > around this today by pushing freed nodes onto a list and having a
> > userspace thread call membarrier(MEMBARRIER_CMD_GLOBAL) and then run a
> > BPF program to reclaim them.
> >
> > Add:
> >
> >       int bpf_call_rcu(struct bpf_rcu_head *rh, void *map,
> >                        int (*callback)(struct bpf_map *map, void *key,
> >                                        void *value));
> >
> > @rh is a struct bpf_rcu_head embedded in a value of @map, so the
> > callback runs as callback(map, key, value) for the element it lives in
> > and needs no cookie.  A head can only be armed once, which bounds
> > outstanding work by the number of elements.
> >
> > struct bpf_rcu_head holds the callback state inline rather than a
> > pointer to it, as bpf_timer, bpf_wq and bpf_task_work do, because there
> > is nothing to cancel and so nothing that has to outlive the map value.
> > That avoids an allocation and a state machine on the arming path at the
> > cost of 64 bytes per element, 48 of which are used today.  Embedding
> > struct rcu_head ties part of a uapi struct to a definition outside of
> > BPF, which is acceptable here only because it is two pointers, a
> > callback and its argument, with no room to grow.
> >
> > An RCU callback cannot be cancelled, so everything it touches has to
> > stay alive until it runs:
> >
> >   - The callback is the program's text, so arming takes a program
> >     reference as bpf_timer, bpf_wq and bpf_task_work do, dropped once
> >     the callback returns.  bpf_prog_inc_not_zero() also fails the arm
> >     with -EBADF once the program is dying.
> >
> >   - The map is held by that reference through used_maps.  An inner map
> >     is not, so bpf_rcu_head is rejected in one.
> >
> >   - The field is only accepted in BPF_MAP_TYPE_ARRAY, whose elements
> >     are never freed individually.  A hash element can be deleted and
> >     recycled while a callback is queued on it.
> >
> >   - The head is disarmed before the callback runs so it can be armed
> >     again from there, which takes a new program reference before the
> >     running callback drops its own.  Arming therefore fails with -EPERM
> >     once the map is held by neither a process nor bpffs, which is what
> >     bpf_timer and bpf_wq do at init time; bpf_task_work uses -EBUSY and
> >     additionally cancels, which is not possible here.
> >
> > bpf_iter hands a program a writable pointer to the live element, which
> > would let it overwrite a queued head, so bpf_iter_attach_map() rejects
> > maps carrying one.
> >
> > The callback is verified non-sleepable even when the caller is
> > sleepable, and RCU invokes it with BH disabled.
> >
> > Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> > ---
> >  include/linux/bpf.h            | 10 +++++
> >  include/uapi/linux/bpf.h       |  4 ++
> >  kernel/bpf/btf.c               |  7 +++
> >  kernel/bpf/helpers.c           | 75 +++++++++++++++++++++++++++++++
> >  kernel/bpf/map_in_map.c        |  4 ++
> >  kernel/bpf/map_iter.c          |  6 +++
> >  kernel/bpf/syscall.c           | 11 ++++-
> >  kernel/bpf/verifier.c          | 81 +++++++++++++++++++++++++++++++++-
> >  tools/include/uapi/linux/bpf.h |  4 ++
> >  9 files changed, 199 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index fd22db8bc6c50..e7c5e203edddb 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -215,6 +215,7 @@ enum btf_field_type {
> >       BPF_UPTR       = (1 << 11),
> >       BPF_RES_SPIN_LOCK = (1 << 12),
> >       BPF_TASK_WORK  = (1 << 13),
> > +     BPF_RCU_HEAD   = (1 << 14),
> >  };
> >
> >  enum bpf_cgroup_storage_type {
> > @@ -269,6 +270,7 @@ struct btf_record {
> >       int wq_off;
> >       int refcount_off;
> >       int task_work_off;
> > +     int rcu_head_off;
> >       struct btf_field fields[];
> >  };
> >
> > @@ -374,6 +376,8 @@ static inline const char *btf_field_type_name(enum btf_field_type type)
> >               return "bpf_refcount";
> >       case BPF_TASK_WORK:
> >               return "bpf_task_work";
> > +     case BPF_RCU_HEAD:
> > +             return "bpf_rcu_head";
> >       default:
> >               WARN_ON_ONCE(1);
> >               return "unknown";
> > @@ -414,6 +418,8 @@ static inline u32 btf_field_type_size(enum btf_field_type type)
> >               return sizeof(struct bpf_refcount);
> >       case BPF_TASK_WORK:
> >               return sizeof(struct bpf_task_work);
> > +     case BPF_RCU_HEAD:
> > +             return sizeof(struct bpf_rcu_head);
> >       default:
> >               WARN_ON_ONCE(1);
> >               return 0;
> > @@ -448,6 +454,8 @@ static inline u32 btf_field_type_align(enum btf_field_type type)
> >               return __alignof__(struct bpf_refcount);
> >       case BPF_TASK_WORK:
> >               return __alignof__(struct bpf_task_work);
> > +     case BPF_RCU_HEAD:
> > +             return __alignof__(struct bpf_rcu_head);
> >       default:
> >               WARN_ON_ONCE(1);
> >               return 0;
> > @@ -480,6 +488,7 @@ static inline void bpf_obj_init_field(const struct btf_field *field, void *addr)
> >       case BPF_KPTR_PERCPU:
> >       case BPF_UPTR:
> >       case BPF_TASK_WORK:
> > +     case BPF_RCU_HEAD:
> >               break;
> >       default:
> >               WARN_ON_ONCE(1);
> > @@ -925,6 +934,7 @@ enum bpf_arg_type {
> >       ARG_PTR_TO_RB_NODE,     /* pointer to bpf_rb_node */
> >       ARG_PTR_TO_WORKQUEUE,   /* pointer to bpf_wq */
> >       ARG_PTR_TO_TASK_WORK,   /* pointer to bpf_task_work */
> > +     ARG_PTR_TO_RCU_HEAD,    /* pointer to bpf_rcu_head */
> >       ARG_PTR_TO_IRQ_FLAG,    /* pointer to saved IRQ flags on the stack */
> >       ARG_PTR_TO_RES_SPIN_LOCK,       /* pointer to bpf_res_spin_lock */
> >       ARG_PTR_TO_CTX_OUT,     /* hook output argument passed through from ctx */
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 6330b7d745c57..eafeba23f5da7 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -7611,6 +7611,10 @@ struct bpf_task_work {
> >       __u64 __opaque;
> >  } __attribute__((aligned(8)));
> >
> > +struct bpf_rcu_head {
> > +     __u64 __opaque[8];
> > +} __attribute__((aligned(8)));
> > +
> >  struct bpf_wq {
> >       __u64 __opaque[2];
> >  } __attribute__((aligned(8)));
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 314ecb0e593b0..4a1fa4fbdf4e8 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -3696,6 +3696,7 @@ static int btf_get_field_type(const struct btf *btf, const struct btf_type *var_
> >               { BPF_TIMER, "bpf_timer", true },
> >               { BPF_WORKQUEUE, "bpf_wq", true },
> >               { BPF_TASK_WORK, "bpf_task_work", true },
> > +             { BPF_RCU_HEAD, "bpf_rcu_head", true },
> >               { BPF_LIST_HEAD, "bpf_list_head", false },
> >               { BPF_LIST_NODE, "bpf_list_node", false },
> >               { BPF_RB_ROOT, "bpf_rb_root", false },
> > @@ -3881,6 +3882,7 @@ static int btf_find_field_one(const struct btf *btf,
> >       case BPF_RB_NODE:
> >       case BPF_REFCOUNT:
> >       case BPF_TASK_WORK:
> > +     case BPF_RCU_HEAD:
> >               ret = btf_find_struct(btf, var_type, off, sz, field_type,
> >                                     info_cnt ? &info[0] : &tmp);
> >               if (ret < 0)
> > @@ -4176,6 +4178,7 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
> >       rec->wq_off = -EINVAL;
> >       rec->refcount_off = -EINVAL;
> >       rec->task_work_off = -EINVAL;
> > +     rec->rcu_head_off = -EINVAL;
> >       for (i = 0; i < cnt; i++) {
> >               field_type_size = btf_field_type_size(info_arr[i].type);
> >               if (info_arr[i].off + field_type_size > value_size) {
> > @@ -4219,6 +4222,10 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
> >                       WARN_ON_ONCE(rec->task_work_off >= 0);
> >                       rec->task_work_off = rec->fields[i].offset;
> >                       break;
> > +             case BPF_RCU_HEAD:
> > +                     WARN_ON_ONCE(rec->rcu_head_off >= 0);
> > +                     rec->rcu_head_off = rec->fields[i].offset;
> > +                     break;
> >               case BPF_REFCOUNT:
> >                       WARN_ON_ONCE(rec->refcount_off >= 0);
> >                       /* Cache offset for faster lookup at runtime */
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 501c7ce35cba9..8a01dd4058a03 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -4805,6 +4805,80 @@ __bpf_kfunc int bpf_task_work_schedule_resume(struct task_struct *task, struct b
> >       return bpf_task_work_schedule(task, tw, map__const_map, callback, aux, TWA_RESUME);
> >  }
> >
> > +typedef int (*bpf_rcu_callback_t)(struct bpf_map *map, void *key, void *value);
> > +
> > +/* Actual type for struct bpf_rcu_head */
> > +struct bpf_rcu_head_kern {
> > +     struct rcu_head rcu;
> > +     bpf_callback_t callback_fn;
> > +     struct bpf_map *map;
> > +     struct bpf_prog *prog;
> > +     u32 armed;
> > +} __aligned(8);
> > +
> > +static void bpf_rcu_run_callback(struct rcu_head *rcu)
> > +{
> > +     struct bpf_rcu_head_kern *rh = container_of(rcu, struct bpf_rcu_head_kern, rcu);
> > +     bpf_callback_t callback_fn = rh->callback_fn;
> > +     struct bpf_prog *prog = rh->prog;
> > +     struct bpf_map *map = rh->map;
> > +     void *value, *key;
> > +     u32 idx;
> > +
> > +     value = (void *)rh - map->record->rcu_head_off;
> > +     key = map_key_from_value(map, value, &idx);
> > +
> > +     /* Pairs with the arming cmpxchg(): rh may be re-armed as soon as this store lands. */
> > +     smp_store_release(&rh->armed, 0);
> > +
> > +     rcu_read_lock_dont_migrate();
> > +     callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
> > +     rcu_read_unlock_migrate();
> > +
> > +     bpf_prog_put(prog);
> > +}
> > +
> > +/**
> > + * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
> > + * @rh: struct bpf_rcu_head in a BPF map value
> > + * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> > + * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> > + * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> > + *
> > + * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> > + * nor bpffs, or -EBADF if the calling program is going away.
> > + */
> > +__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> > +                          bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> > +{
> > +     struct bpf_rcu_head_kern *rhk = (void *)rh;
> > +     struct bpf_map *map = map__const_map;
> > +     struct bpf_prog *prog;
> > +
> > +     BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));
> > +     BUILD_BUG_ON(__alignof__(struct bpf_rcu_head_kern) != __alignof__(struct bpf_rcu_head));
> > +     BTF_TYPE_EMIT(struct bpf_rcu_head);
> > +
> > +     /* A queued callback cannot be cancelled, so a self-rearming one would pin prog and map. */
> > +     if (!atomic64_read(&map->usercnt))
> > +             return -EPERM;
> > +
> > +     if (cmpxchg(&rhk->armed, 0, 1))
> > +             return -EBUSY;
> > +
> > +     prog = bpf_prog_inc_not_zero(aux->prog);
> > +     if (IS_ERR(prog)) {
> > +             WRITE_ONCE(rhk->armed, 0);
> > +             return -EBADF;
> > +     }
>
> can we drop prog_inc and remove prog pointer from rhk ?
> with something like if (prog->has_call_rcu) rcu_barrier() during prog unload?

That could work but we would need more than just the calls to
rcu_barrier_tasks_trace() and/or  rcu_barrier(). As the rcu_barrier
only waits for callbacks enqueued before it was called, the registered
callback and re-queue itself and cause problems. So, we would need a
flag that is set before the call to rcu_barrier() which stops further
queueing of callbacks from that program. Also, assuming we add these
barrier calls in bpf_prog_free_deferred() which is runs from
system_percpu_wq, it could make this run for much longer than
expected. What is the motivation for using rcu_barrier() instead of
bpf_prog_inc_not_zero()? is it the 8B pointer in rhk or the atomic
operation in bpf_prog_inc_not_zero()?

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Alexei Starovoitov]

On Tue Sep 22, 2026 at 2:17 PM UTC, Puranjay Mohan wrote:
>> > +     prog = bpf_prog_inc_not_zero(aux->prog);
>> > +     if (IS_ERR(prog)) {
>> > +             WRITE_ONCE(rhk->armed, 0);
>> > +             return -EBADF;
>> > +     }
>>
>> can we drop prog_inc and remove prog pointer from rhk ?
>> with something like if (prog->has_call_rcu) rcu_barrier() during prog unload?
>
> That could work but we would need more than just the calls to
> rcu_barrier_tasks_trace() and/or  rcu_barrier(). As the rcu_barrier
> only waits for callbacks enqueued before it was called, the registered
> callback and re-queue itself and cause problems. So, we would need a
> flag that is set before the call to rcu_barrier() which stops further
> queueing of callbacks from that program. Also, assuming we add these
> barrier calls in bpf_prog_free_deferred() which is runs from
> system_percpu_wq, it could make this run for much longer than
> expected. What is the motivation for using rcu_barrier() instead of
> bpf_prog_inc_not_zero()? is it the 8B pointer in rhk or the atomic
> operation in bpf_prog_inc_not_zero()?

ok. fair enough. let's keep prog inc.

>> > That avoids an allocation and a state machine on the arming path at the
>> > cost of 64 bytes per element, 48 of which are used today.

What was the thinking here? Why ask all progs to waste 16 bytes
for future extensibility that may never happen?
If rcu_head growth we can switch to dynamic alloc while
keeping the struct at 48 bytes.

I feel it's a better trade off.

pw-bot: cr

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Puranjay Mohan]

On Tue, Sep 22, 2026 at 7:35 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue Sep 22, 2026 at 2:17 PM UTC, Puranjay Mohan wrote:
> >> > +     prog = bpf_prog_inc_not_zero(aux->prog);
> >> > +     if (IS_ERR(prog)) {
> >> > +             WRITE_ONCE(rhk->armed, 0);
> >> > +             return -EBADF;
> >> > +     }
> >>
> >> can we drop prog_inc and remove prog pointer from rhk ?
> >> with something like if (prog->has_call_rcu) rcu_barrier() during prog unload?
> >
> > That could work but we would need more than just the calls to
> > rcu_barrier_tasks_trace() and/or  rcu_barrier(). As the rcu_barrier
> > only waits for callbacks enqueued before it was called, the registered
> > callback and re-queue itself and cause problems. So, we would need a
> > flag that is set before the call to rcu_barrier() which stops further
> > queueing of callbacks from that program. Also, assuming we add these
> > barrier calls in bpf_prog_free_deferred() which is runs from
> > system_percpu_wq, it could make this run for much longer than
> > expected. What is the motivation for using rcu_barrier() instead of
> > bpf_prog_inc_not_zero()? is it the 8B pointer in rhk or the atomic
> > operation in bpf_prog_inc_not_zero()?
>
> ok. fair enough. let's keep prog inc.
>
> >> > That avoids an allocation and a state machine on the arming path at the
> >> > cost of 64 bytes per element, 48 of which are used today.
>
> What was the thinking here? Why ask all progs to waste 16 bytes
> for future extensibility that may never happen?

My broken thought process, rcu_head won't grow anytime soon, I will
fix it in the next version.

Re: [PATCH bpf-next v5 1/4] bpf: Add bpf_call_rcu() kfunc

[Paul E. McKenney]

On Tue, Sep 22, 2026 at 08:09:44PM +0100, Puranjay Mohan wrote:
> On Tue, Sep 22, 2026 at 7:35 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue Sep 22, 2026 at 2:17 PM UTC, Puranjay Mohan wrote:
> > >> > +     prog = bpf_prog_inc_not_zero(aux->prog);
> > >> > +     if (IS_ERR(prog)) {
> > >> > +             WRITE_ONCE(rhk->armed, 0);
> > >> > +             return -EBADF;
> > >> > +     }
> > >>
> > >> can we drop prog_inc and remove prog pointer from rhk ?
> > >> with something like if (prog->has_call_rcu) rcu_barrier() during prog unload?
> > >
> > > That could work but we would need more than just the calls to
> > > rcu_barrier_tasks_trace() and/or  rcu_barrier(). As the rcu_barrier
> > > only waits for callbacks enqueued before it was called, the registered
> > > callback and re-queue itself and cause problems. So, we would need a
> > > flag that is set before the call to rcu_barrier() which stops further
> > > queueing of callbacks from that program. Also, assuming we add these
> > > barrier calls in bpf_prog_free_deferred() which is runs from
> > > system_percpu_wq, it could make this run for much longer than
> > > expected. What is the motivation for using rcu_barrier() instead of
> > > bpf_prog_inc_not_zero()? is it the 8B pointer in rhk or the atomic
> > > operation in bpf_prog_inc_not_zero()?
> >
> > ok. fair enough. let's keep prog inc.
> >
> > >> > That avoids an allocation and a state machine on the arming path at the
> > >> > cost of 64 bytes per element, 48 of which are used today.
> >
> > What was the thinking here? Why ask all progs to waste 16 bytes
> > for future extensibility that may never happen?
> 
> My broken thought process, rcu_head won't grow anytime soon, I will
> fix it in the next version.

Just confirming that there are no plans to expand rcu_head.  Furthermore,
in the past, such suggestions have not been warmly welcomed, especially by
maintainers having small highly-replicated structures containing rcu_head.
Alternatively, you could say that they were welcomed with considerable
warmth, but of a different sort.  ;-)

							Thanx, Paul

[PATCH bpf-next v5 2/4] selftests/bpf: Add tests for bpf_call_rcu()

[Puranjay Mohan]

Cover the callback running after a grace period with the right map, key
and value, -EBUSY on a second arm, reuse of the head once disarmed, a
callback arming itself again, and teardown with a callback queued.

struct bpf_rcu_head is not the first member of the map value, so the
callback's recovery of the value from the head is exercised.  arm()
wraps both arms in an RCU read section, otherwise a grace period may
elapse between them and the second one legitimately succeeds.

Negative tests: a hash map created with the same BTF, the map used as an
inner map, and an iterator attach, all checked for -EOPNOTSUPP; plus
verifier rejection of a mismatched map, a map with no bpf_rcu_head, a
head at the wrong offset, a head on the stack, and a sleepable callback.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 .../selftests/bpf/prog_tests/call_rcu.c       | 271 ++++++++++++++++++
 tools/testing/selftests/bpf/progs/call_rcu.c  |  93 ++++++
 .../selftests/bpf/progs/call_rcu_fail.c       | 114 ++++++++
 3 files changed, 478 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/call_rcu.c
 create mode 100644 tools/testing/selftests/bpf/progs/call_rcu.c
 create mode 100644 tools/testing/selftests/bpf/progs/call_rcu_fail.c

diff --git a/tools/testing/selftests/bpf/prog_tests/call_rcu.c b/tools/testing/selftests/bpf/prog_tests/call_rcu.c
new file mode 100644
index 0000000000000..bfa6512b9052a
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/call_rcu.c
@@ -0,0 +1,271 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+#include "call_rcu.skel.h"
+#include "call_rcu_fail.skel.h"
+
+struct elem {
+	__u64 pad;
+	struct bpf_rcu_head rh;
+	__u64 val;
+};
+
+/*
+ * Force the grace period, then poll: the callback still has to be invoked
+ * afterwards, and call_rcu() is lazy on a CONFIG_RCU_LAZY kernel.
+ */
+static bool wait_for_callbacks(struct call_rcu *skel, int expected)
+{
+	int i, got = 0;
+
+	kern_sync_rcu();
+	for (i = 0; i < 3000; i++) {
+		got = __atomic_load_n(&skel->bss->callbacks, __ATOMIC_ACQUIRE);
+		if (got >= expected)
+			return true;
+		usleep(10000);
+	}
+	return ASSERT_GE(got, expected, "callbacks");
+}
+
+static void test_call_rcu_run(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct call_rcu *skel;
+	struct elem elem;
+	__u32 key = 1;
+	int err;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts);
+	if (!ASSERT_OK(err, "test_run") || !ASSERT_EQ(opts.retval, 0, "retval"))
+		goto out;
+
+	ASSERT_EQ(skel->bss->arm_err, 0, "arm_err");
+	ASSERT_EQ(skel->bss->busy_err, -EBUSY, "busy_err");
+
+	if (!wait_for_callbacks(skel, 1))
+		goto out;
+
+	ASSERT_EQ(skel->bss->cb_key, key, "cb_key");
+	ASSERT_EQ(skel->bss->cb_val, 0xdeadbeef, "cb_val");
+	ASSERT_EQ(skel->bss->cb_max_entries, bpf_map__max_entries(skel->maps.arr), "cb_map");
+
+	err = bpf_map__lookup_elem(skel->maps.arr, &key, sizeof(key), &elem, sizeof(elem), 0);
+	if (ASSERT_OK(err, "lookup"))
+		ASSERT_EQ(elem.val, 0, "value_cleared");
+
+	/* The head is disarmed before the callback runs, so it can be reused. */
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts);
+	if (!ASSERT_OK(err, "test_run_again"))
+		goto out;
+	ASSERT_EQ(skel->bss->arm_err, 0, "rearm_err");
+	wait_for_callbacks(skel, 2);
+out:
+	call_rcu__destroy(skel);
+}
+
+static void test_call_rcu_chain(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct call_rcu *skel;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	skel->bss->chain = 1;
+	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts), "test_run"))
+		goto out;
+
+	if (!wait_for_callbacks(skel, 1))
+		goto out;
+	ASSERT_EQ(skel->bss->chain_err, 0, "chain_err");
+	wait_for_callbacks(skel, 2);
+out:
+	call_rcu__destroy(skel);
+}
+
+/*
+ * A callback that keeps re-arming must stop once the map loses its last user
+ * reference, or it pins the program for good.  Hold an independent fd on .bss
+ * so the refusal is still readable after the skeleton is gone.
+ */
+static void test_call_rcu_teardown(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	int i, err, fd = 0, bss_fd = -1;
+	struct bpf_prog_info pinfo = {};
+	struct bpf_map_info minfo = {};
+	__u32 len, prog_id, zero = 0;
+	struct call_rcu *skel;
+	char *buf = NULL;
+	size_t off, vsz;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	len = sizeof(pinfo);
+	if (!ASSERT_OK(bpf_prog_get_info_by_fd(bpf_program__fd(skel->progs.arm), &pinfo, &len),
+		       "prog_info"))
+		goto out;
+	prog_id = pinfo.id;
+
+	len = sizeof(minfo);
+	if (!ASSERT_OK(bpf_map_get_info_by_fd(bpf_map__fd(skel->maps.bss), &minfo, &len),
+		       "bss_info"))
+		goto out;
+	bss_fd = bpf_map_get_fd_by_id(minfo.id);
+	if (!ASSERT_GE(bss_fd, 0, "bss_fd"))
+		goto out;
+
+	vsz = bpf_map__value_size(skel->maps.bss);
+	off = (char *)&skel->bss->chain_err - (char *)skel->bss;
+	buf = malloc(vsz);
+	if (!ASSERT_OK_PTR(buf, "buf"))
+		goto out;
+
+	skel->bss->chain = INT_MAX;
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts);
+	if (!ASSERT_OK(err, "test_run") || !ASSERT_EQ(opts.retval, 0, "retval"))
+		goto out;
+	if (!ASSERT_EQ(skel->bss->arm_err, 0, "arm_err"))
+		goto out;
+	/* The chain has to be running before the map reference goes away. */
+	if (!wait_for_callbacks(skel, 2))
+		goto out;
+
+	call_rcu__destroy(skel);
+	skel = NULL;
+
+	for (i = 0; i < 3000; i++) {
+		fd = bpf_prog_get_fd_by_id(prog_id);
+		if (fd < 0)
+			break;
+		close(fd);
+		usleep(10000);
+	}
+	if (!ASSERT_EQ(fd, -ENOENT, "prog_freed"))
+		goto out;
+
+	if (ASSERT_OK(bpf_map_lookup_elem(bss_fd, &zero, buf), "bss_lookup"))
+		ASSERT_EQ(*(int *)(buf + off), -EPERM, "chain_refused");
+out:
+	free(buf);
+	if (bss_fd >= 0)
+		close(bss_fd);
+	call_rcu__destroy(skel);
+}
+
+static void test_call_rcu_bad_map(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts);
+	struct call_rcu *skel;
+	int fd;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	opts.btf_fd = bpf_object__btf_fd(skel->obj);
+	opts.btf_key_type_id = bpf_map__btf_key_type_id(skel->maps.arr);
+	opts.btf_value_type_id = bpf_map__btf_value_type_id(skel->maps.arr);
+
+	fd = bpf_map_create(BPF_MAP_TYPE_HASH, "rcu_hash", sizeof(__u32),
+			    bpf_map__value_size(skel->maps.arr), 1, &opts);
+	ASSERT_EQ(fd, -EOPNOTSUPP, "hash_rejected");
+	if (fd >= 0)
+		close(fd);
+
+	call_rcu__destroy(skel);
+}
+
+/* Iterating would hand the program a writable pointer to the head. */
+static void test_call_rcu_iter(void)
+{
+	LIBBPF_OPTS(bpf_iter_attach_opts, opts);
+	union bpf_iter_link_info linfo = {};
+	struct bpf_link *link;
+	struct call_rcu *skel;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	linfo.map.map_fd = bpf_map__fd(skel->maps.arr);
+	opts.link_info = &linfo;
+	opts.link_info_len = sizeof(linfo);
+
+	link = bpf_program__attach_iter(skel->progs.dump, &opts);
+	if (!ASSERT_ERR_PTR(link, "iter_rejected"))
+		bpf_link__destroy(link);
+	else
+		ASSERT_EQ(libbpf_get_error(link), -EOPNOTSUPP, "iter_errno");
+
+	call_rcu__destroy(skel);
+}
+
+static void test_call_rcu_inner_map(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts);
+	struct call_rcu *skel;
+	int fd;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	opts.inner_map_fd = bpf_map__fd(skel->maps.arr);
+	fd = bpf_map_create(BPF_MAP_TYPE_ARRAY_OF_MAPS, "rcu_outer",
+			    sizeof(__u32), sizeof(__u32), 1, &opts);
+	ASSERT_EQ(fd, -EOPNOTSUPP, "inner_map_rejected");
+	if (fd >= 0)
+		close(fd);
+
+	call_rcu__destroy(skel);
+}
+
+/* Two heads in the same map must be independent. */
+static void test_call_rcu_two_heads(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct call_rcu *skel;
+
+	skel = call_rcu__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm_both), &opts),
+		       "test_run"))
+		goto out;
+	if (!ASSERT_EQ(skel->bss->arm_err, 0, "arm_err"))
+		goto out;
+	if (!wait_for_callbacks(skel, 2))
+		goto out;
+	ASSERT_EQ(skel->bss->cb_keys, 0x3, "both_keys");
+out:
+	call_rcu__destroy(skel);
+}
+
+void test_call_rcu(void)
+{
+	if (test__start_subtest("run"))
+		test_call_rcu_run();
+	if (test__start_subtest("chain"))
+		test_call_rcu_chain();
+	if (test__start_subtest("two_heads"))
+		test_call_rcu_two_heads();
+	if (test__start_subtest("teardown"))
+		test_call_rcu_teardown();
+	if (test__start_subtest("bad_map"))
+		test_call_rcu_bad_map();
+	if (test__start_subtest("iter"))
+		test_call_rcu_iter();
+	if (test__start_subtest("inner_map"))
+		test_call_rcu_inner_map();
+	RUN_TESTS(call_rcu_fail);
+}
diff --git a/tools/testing/selftests/bpf/progs/call_rcu.c b/tools/testing/selftests/bpf/progs/call_rcu.c
new file mode 100644
index 0000000000000..94d92cee88a70
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/call_rcu.c
@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+/* rh is not at offset 0, so the callback's value recovery is exercised. */
+struct elem {
+	__u64 pad;
+	struct bpf_rcu_head rh;
+	__u64 val;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 2);
+	__type(key, __u32);
+	__type(value, struct elem);
+} arr SEC(".maps");
+
+__u32 cb_key;
+__u64 cb_keys;
+__u64 cb_val;
+__u32 cb_max_entries;
+int callbacks;
+int arm_err;
+int busy_err;
+int chain;		/* set by userspace: number of times to re-arm from the callback */
+int chain_err;
+
+static int reclaim(struct bpf_map *map, void *key, void *value)
+{
+	struct elem *e = value;
+
+	cb_key = *(__u32 *)key;
+	__sync_fetch_and_or(&cb_keys, 1ULL << cb_key);
+	cb_val = e->val;
+	cb_max_entries = map->max_entries;
+	e->val = 0;
+
+	if (chain > 0) {
+		chain--;
+		chain_err = bpf_call_rcu(&e->rh, &arr, reclaim);
+	}
+
+	__sync_fetch_and_add(&callbacks, 1);
+	return 0;
+}
+
+SEC("syscall")
+int arm(void *ctx)
+{
+	__u32 key = 1;
+	struct elem *e;
+
+	e = bpf_map_lookup_elem(&arr, &key);
+	if (!e)
+		return 1;
+
+	e->val = 0xdeadbeef;
+	/* Keep a grace period from elapsing between the two arms. */
+	bpf_rcu_read_lock();
+	arm_err = bpf_call_rcu(&e->rh, &arr, reclaim);
+	busy_err = bpf_call_rcu(&e->rh, &arr, reclaim);
+	bpf_rcu_read_unlock();
+	return 0;
+}
+
+SEC("syscall")
+int arm_both(void *ctx)
+{
+	__u32 key0 = 0, key1 = 1;
+	struct elem *e0, *e1;
+
+	e0 = bpf_map_lookup_elem(&arr, &key0);
+	e1 = bpf_map_lookup_elem(&arr, &key1);
+	if (!e0 || !e1)
+		return 1;
+
+	e0->val = 0xdeadbeef;
+	e1->val = 0xdeadbeef;
+	arm_err = bpf_call_rcu(&e0->rh, &arr, reclaim);
+	arm_err |= bpf_call_rcu(&e1->rh, &arr, reclaim);
+	return 0;
+}
+
+SEC("iter/bpf_map_elem")
+int dump(struct bpf_iter__bpf_map_elem *ctx)
+{
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/progs/call_rcu_fail.c b/tools/testing/selftests/bpf/progs/call_rcu_fail.c
new file mode 100644
index 0000000000000..bf60a27731fb3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/call_rcu_fail.c
@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+const void *user_ptr = NULL;
+
+struct elem {
+	__u64 pad;
+	struct bpf_rcu_head rh;
+	__u64 val;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, struct elem);
+} arr SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, struct elem);
+} arr2 SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u64);
+} plain SEC(".maps");
+
+__u32 key = 0;
+
+static int reclaim(struct bpf_map *map, void *key, void *value)
+{
+	return 0;
+}
+
+static int sleepable_reclaim(struct bpf_map *map, void *key, void *value)
+{
+	struct elem *e = value;
+
+	bpf_copy_from_user(&e->val, sizeof(e->val), user_ptr);
+	return 0;
+}
+
+SEC("syscall")
+__failure __msg("bpf_rcu_head pointer in R1") __msg("doesn't match map pointer in R2")
+int mismatch_map(void *ctx)
+{
+	struct elem *e;
+
+	e = bpf_map_lookup_elem(&arr, &key);
+	if (!e)
+		return 0;
+	bpf_call_rcu(&e->rh, &arr2, reclaim);
+	return 0;
+}
+
+SEC("syscall")
+__failure __msg("map 'plain' has no valid bpf_rcu_head")
+int no_rcu_head(void *ctx)
+{
+	__u64 *val;
+
+	val = bpf_map_lookup_elem(&plain, &key);
+	if (!val)
+		return 0;
+	bpf_call_rcu((struct bpf_rcu_head *)val, &plain, reclaim);
+	return 0;
+}
+
+SEC("syscall")
+__failure __msg("doesn't point to 'struct bpf_rcu_head' that is at 8")
+int wrong_offset(void *ctx)
+{
+	struct elem *e;
+
+	e = bpf_map_lookup_elem(&arr, &key);
+	if (!e)
+		return 0;
+	bpf_call_rcu((struct bpf_rcu_head *)&e->pad, &arr, reclaim);
+	return 0;
+}
+
+SEC("syscall")
+__failure __msg("R1 type=fp expected=map_value")
+int rcu_head_on_stack(void *ctx)
+{
+	struct bpf_rcu_head rh;
+
+	bpf_call_rcu(&rh, &arr, reclaim);
+	return 0;
+}
+
+SEC("syscall")
+__failure __msg("sleepable helper bpf_copy_from_user") __msg("in non-sleepable prog")
+int sleepable_callback(void *ctx)
+{
+	struct elem *e;
+
+	e = bpf_map_lookup_elem(&arr, &key);
+	if (!e)
+		return 0;
+	bpf_call_rcu(&e->rh, &arr, sleepable_reclaim);
+	return 0;
+}
-- 
2.53.0-Meta

Re: [PATCH bpf-next v5 2/4] selftests/bpf: Add tests for bpf_call_rcu()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpf: selftest sleepable_callback fails to properly test callback restriction
--

commit 8dcd9155da335ebebbf25f7c26266814bb232e74
Author: Puranjay Mohan <puranjay@kernel.org>

selftests/bpf: Add tests for bpf_call_rcu()
    
This commit adds selftests to cover bpf_call_rcu() functionality, checking
grace periods, callback re-arming, teardown handling, and negative tests
including verifier rejection of a sleepable callback.

> diff --git a/tools/testing/selftests/bpf/progs/call_rcu_fail.c b/tools/testing/selftests/bpf/progs/call_rcu_fail.c
> new file mode 100644
> index 0000000000000..bf60a27731fb3
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/call_rcu_fail.c

[ ... ]

> +SEC("syscall")
> +__failure __msg("sleepable helper bpf_copy_from_user") __msg("in non-sleepable prog")
> +int sleepable_callback(void *ctx)
> +{

[Severity: Medium]
Does SEC("syscall") cause this test to pass for the wrong reason?

Since the main program is marked as non-sleepable, the verifier will reject
the sleepable helper regardless of the callback context. Should this be
changed to SEC("syscall.s") so the main program is sleepable, which would
properly force the verifier to reject the helper purely because it is inside
the non-sleepable callback context?

> +	struct elem *e;
> +
> +	e = bpf_map_lookup_elem(&arr, &key);
> +	if (!e)
> +		return 0;
> +	bpf_call_rcu(&e->rh, &arr, sleepable_reclaim);
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260921191407.1742386-1-puranjay@kernel.org?part=2

[PATCH bpf-next v5 3/4] bpf: Add bpf_call_rcu_tasks_trace() kfunc

[Puranjay Mohan]

Sleepable BPF programs hold rcu_read_lock_trace(), not rcu_read_lock(),
so a plain RCU grace period does not wait for them.  A program whose
readers are sleepable needs this flavour to defer reclaim safely.

call_rcu_tasks_trace() is call_srcu() on rcu_tasks_trace_srcu_struct and
SRCU invokes callbacks with BH disabled, so the callback is still not
sleepable.  It does run from a kworker rather than softirq or the
rcuc/rcuo kthread, so a callback must not assume anything about current.

Only the queueing call differs, so the two share struct bpf_rcu_head and
all of the verifier plumbing.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/helpers.c  | 57 +++++++++++++++++++++++++++++++------------
 kernel/bpf/verifier.c |  7 ++++--
 2 files changed, 47 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 8a01dd4058a03..301b35bd85c8a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4838,21 +4838,10 @@ static void bpf_rcu_run_callback(struct rcu_head *rcu)
 	bpf_prog_put(prog);
 }
 
-/**
- * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
- * @rh: struct bpf_rcu_head in a BPF map value
- * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
- * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
- * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
- *
- * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
- * nor bpffs, or -EBADF if the calling program is going away.
- */
-__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
-			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
+static int __bpf_call_rcu(struct bpf_rcu_head *rh, struct bpf_map *map, void *callback,
+			  struct bpf_prog_aux *aux, bool trace)
 {
 	struct bpf_rcu_head_kern *rhk = (void *)rh;
-	struct bpf_map *map = map__const_map;
 	struct bpf_prog *prog;
 
 	BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));
@@ -4872,13 +4861,50 @@ __bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
 		return -EBADF;
 	}
 
-	rhk->callback_fn = (bpf_callback_t)(void *)callback;
+	rhk->callback_fn = (bpf_callback_t)callback;
 	rhk->map = map;
 	rhk->prog = prog;
-	call_rcu(&rhk->rcu, bpf_rcu_run_callback);
+	if (trace)
+		call_rcu_tasks_trace(&rhk->rcu, bpf_rcu_run_callback);
+	else
+		call_rcu(&rhk->rcu, bpf_rcu_run_callback);
 	return 0;
 }
 
+/**
+ * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
+ * @rh: struct bpf_rcu_head in a BPF map value
+ * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
+ * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
+ * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
+ *
+ * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
+ * nor bpffs, or -EBADF if the calling program is going away.
+ */
+__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
+			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
+{
+	return __bpf_call_rcu(rh, map__const_map, callback, aux, false);
+}
+
+/**
+ * bpf_call_rcu_tasks_trace - Invoke a BPF callback after an RCU tasks trace grace period
+ * @rh: struct bpf_rcu_head in a BPF map value
+ * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
+ * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
+ * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
+ *
+ * Waits for sleepable BPF programs too.  The callback itself is not sleepable either way.
+ *
+ * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
+ * nor bpffs, or -EBADF if the calling program is going away.
+ */
+__bpf_kfunc int bpf_call_rcu_tasks_trace(struct bpf_rcu_head *rh, void *map__const_map,
+					 bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
+{
+	return __bpf_call_rcu(rh, map__const_map, callback, aux, true);
+}
+
 static int make_file_dynptr(struct file *file, u32 flags, bool may_sleep,
 			    struct bpf_dynptr_kern *ptr)
 {
@@ -5175,6 +5201,7 @@ BTF_ID_FLAGS(func, bpf_stream_print_stack, KF_IMPLICIT_ARGS | KF_SPINLOCK_SAFE)
 BTF_ID_FLAGS(func, bpf_task_work_schedule_signal, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_task_work_schedule_resume, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_call_rcu, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, bpf_call_rcu_tasks_trace, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_dynptr_from_file)
 BTF_ID_FLAGS(func, bpf_dynptr_file_discard, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_timer_cancel_async)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 64db47964ff9f..a7c9e2d8965d5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -578,7 +578,7 @@ static bool is_async_cb_sleepable(struct bpf_verifier_env *env, struct bpf_insn
 	if (bpf_helper_call(insn) && insn->imm == BPF_FUNC_timer_set_callback)
 		return false;
 
-	/* bpf_call_rcu callbacks are never sleepable. */
+	/* bpf_call_rcu and bpf_call_rcu_tasks_trace callbacks are never sleepable. */
 	if (bpf_pseudo_kfunc_call(insn) && insn->off == 0 && is_call_rcu_kfunc(insn->imm))
 		return false;
 
@@ -12629,6 +12629,7 @@ enum special_kfunc_type {
 	KF_bpf_task_work_schedule_signal,
 	KF_bpf_task_work_schedule_resume,
 	KF_bpf_call_rcu,
+	KF_bpf_call_rcu_tasks_trace,
 	KF_bpf_arena_alloc_pages,
 	KF_bpf_arena_free_pages,
 	KF_bpf_arena_reserve_pages,
@@ -12723,6 +12724,7 @@ BTF_ID(func, __bpf_trap)
 BTF_ID(func, bpf_task_work_schedule_signal)
 BTF_ID(func, bpf_task_work_schedule_resume)
 BTF_ID(func, bpf_call_rcu)
+BTF_ID(func, bpf_call_rcu_tasks_trace)
 BTF_ID(func, bpf_arena_alloc_pages)
 BTF_ID(func, bpf_arena_free_pages)
 BTF_ID(func, bpf_arena_reserve_pages)
@@ -12796,7 +12798,8 @@ static bool is_bpf_rbtree_add_kfunc(u32 func_id)
 
 static bool is_call_rcu_kfunc(u32 func_id)
 {
-	return func_id == special_kfunc_list[KF_bpf_call_rcu];
+	return func_id == special_kfunc_list[KF_bpf_call_rcu] ||
+	       func_id == special_kfunc_list[KF_bpf_call_rcu_tasks_trace];
 }
 
 static bool is_task_work_add_kfunc(u32 func_id)
-- 
2.53.0-Meta

Re: [PATCH bpf-next v5 3/4] bpf: Add bpf_call_rcu_tasks_trace() kfunc

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] bpf: bpf_call_rcu/tasks_trace() deadlock and corruption from NMI context
--

commit 7357eb51c5d591938af8ac0c27619c6dee0b07c7
Author: Puranjay Mohan <puranjay@kernel.org>

    bpf: Add bpf_call_rcu_tasks_trace() kfunc

    This commit adds a new bpf kfunc, bpf_call_rcu_tasks_trace(), to defer
    reclaim until after an RCU tasks trace grace period. It is intended for
    sleepable BPF programs and shares the backend __bpf_call_rcu() with
    bpf_call_rcu().

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 8a01dd4058a03..301b35bd85c8a 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
[ ... ]
> @@ -4872,13 +4861,50 @@ __bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
>  		return -EBADF;
>  	}
>  
> -	rhk->callback_fn = (bpf_callback_t)(void *)callback;
> +	rhk->callback_fn = (bpf_callback_t)callback;
>  	rhk->map = map;
>  	rhk->prog = prog;
> -	call_rcu(&rhk->rcu, bpf_rcu_run_callback);
> +	if (trace)
> +		call_rcu_tasks_trace(&rhk->rcu, bpf_rcu_run_callback);

[Severity: High]
Can this branch cause a deadlock if called from NMI context?

A BPF program of type BPF_PROG_TYPE_PERF_EVENT executing in NMI context
could call bpf_call_rcu_tasks_trace(), which delegates to
call_rcu_tasks_trace() and ultimately call_srcu().

Since call_srcu() relies on acquiring a raw spinlock, could an NMI
interrupting a CPU already holding that lock lead to a guaranteed deadlock?

> +	else
> +		call_rcu(&rhk->rcu, bpf_rcu_run_callback);

[Severity: High]
Can this branch corrupt RCU callback lists if called from NMI context?

Similarly, bpf_call_rcu() delegates to call_rcu(), which modifies CPU-local
lists under local_irq_save(). Since local_irq_save() does not block NMIs,
could an NMI interrupting an existing call_rcu() on the same CPU result
in list corruption?

>  	return 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260921191407.1742386-1-puranjay@kernel.org?part=3

Re: [PATCH bpf-next v5 3/4] bpf: Add bpf_call_rcu_tasks_trace() kfunc

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 4791 bytes --]

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 8a01dd4058a03..301b35bd85c8a 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -4838,21 +4838,10 @@ static void bpf_rcu_run_callback(struct rcu_head *rcu)
>  	bpf_prog_put(prog);
>  }
>
> -/**
> - * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
> - * @rh: struct bpf_rcu_head in a BPF map value
> - * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> - * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> - * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> - *
> - * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> - * nor bpffs, or -EBADF if the calling program is going away.
> - */
> -__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> -			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +static int __bpf_call_rcu(struct bpf_rcu_head *rh, struct bpf_map *map, void *callback,
> +			  struct bpf_prog_aux *aux, bool trace)
>  {
>  	struct bpf_rcu_head_kern *rhk = (void *)rh;
> -	struct bpf_map *map = map__const_map;
>  	struct bpf_prog *prog;
>
>  	BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));

This isn't a bug, but why does __bpf_call_rcu() take 'void *callback'
instead of keeping 'bpf_rcu_callback_t callback' like the kfunc
wrappers? The two call sites both receive a typed bpf_rcu_callback_t and
pass it into the void * parameter, which is an implicit
function-pointer-to-object-pointer conversion (accepted by gcc/clang only
because the kernel doesn't build with -Wpedantic). Keeping
'bpf_rcu_callback_t callback' would preserve type checking at the two
call sites and match the cast pattern used elsewhere in this file
(helpers.c:bpf_timer_set_callback, bpf_wq_set_callback_impl) without
costing anything.

> @@ -4872,13 +4861,50 @@ __bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
>  		return -EBADF;
>  	}
>
> -	rhk->callback_fn = (bpf_callback_t)(void *)callback;
> +	rhk->callback_fn = (bpf_callback_t)callback;
>  	rhk->map = map;
>  	rhk->prog = prog;
> -	call_rcu(&rhk->rcu, bpf_rcu_run_callback);
> +	if (trace)
> +		call_rcu_tasks_trace(&rhk->rcu, bpf_rcu_run_callback);
> +	else
> +		call_rcu(&rhk->rcu, bpf_rcu_run_callback);
>  	return 0;
>  }

This isn't a bug, but would passing the call_rcu variant (or naming the
flag 'tasks_trace') read better than a bare 'bool trace' parameter? The
only behavioural difference between the two kfuncs is which queueing
function is used, but it's carried through a 'bool trace' flag that the
reader has to trace to the bottom of the helper to interpret. The name
'trace' is also ambiguous in a BPF file where 'trace' usually means
tracing programs rather than RCU-tasks-trace.

> +/**
> + * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
> + * @rh: struct bpf_rcu_head in a BPF map value
> + * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> + * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> + * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> + *
> + * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> + * nor bpffs, or -EBADF if the calling program is going away.
> + */
> +__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> +			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +{
> +	return __bpf_call_rcu(rh, map__const_map, callback, aux, false);
> +}
> +
> +/**
> + * bpf_call_rcu_tasks_trace - Invoke a BPF callback after an RCU tasks trace grace period
> + * @rh: struct bpf_rcu_head in a BPF map value
> + * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> + * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> + * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> + *
> + * Waits for sleepable BPF programs too.  The callback itself is not sleepable either way.
> + *
> + * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> + * nor bpffs, or -EBADF if the calling program is going away.
> + */
> +__bpf_kfunc int bpf_call_rcu_tasks_trace(struct bpf_rcu_head *rh, void *map__const_map,
> +					 bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +{
> +	return __bpf_call_rcu(rh, map__const_map, callback, aux, true);
> +}

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/35644498619

Re: [PATCH bpf-next v5 3/4] bpf: Add bpf_call_rcu_tasks_trace() kfunc

[Puranjay Mohan]

On Mon, Sep 21, 2026 at 9:18 PM <bot+bpf-ci@kernel.org> wrote:
>
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 8a01dd4058a03..301b35bd85c8a 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -4838,21 +4838,10 @@ static void bpf_rcu_run_callback(struct rcu_head *rcu)
> >       bpf_prog_put(prog);
> >  }
> >
> > -/**
> > - * bpf_call_rcu - Invoke a BPF callback after an RCU grace period
> > - * @rh: struct bpf_rcu_head in a BPF map value
> > - * @map__const_map: bpf_map that embeds struct bpf_rcu_head in the values
> > - * @callback: BPF subprogram, invoked as callback(map, key, value) for the value holding @rh
> > - * @aux: bpf_prog_aux of the caller, implicitly set by the verifier
> > - *
> > - * Return: 0, -EBUSY if @rh is already queued, -EPERM if @map is held by neither a process
> > - * nor bpffs, or -EBADF if the calling program is going away.
> > - */
> > -__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> > -                          bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> > +static int __bpf_call_rcu(struct bpf_rcu_head *rh, struct bpf_map *map, void *callback,
> > +                       struct bpf_prog_aux *aux, bool trace)
> >  {
> >       struct bpf_rcu_head_kern *rhk = (void *)rh;
> > -     struct bpf_map *map = map__const_map;
> >       struct bpf_prog *prog;
> >
> >       BUILD_BUG_ON(sizeof(struct bpf_rcu_head_kern) > sizeof(struct bpf_rcu_head));
>
> This isn't a bug, but why does __bpf_call_rcu() take 'void *callback'
> instead of keeping 'bpf_rcu_callback_t callback' like the kfunc
> wrappers? The two call sites both receive a typed bpf_rcu_callback_t and
> pass it into the void * parameter, which is an implicit
> function-pointer-to-object-pointer conversion (accepted by gcc/clang only
> because the kernel doesn't build with -Wpedantic). Keeping
> 'bpf_rcu_callback_t callback' would preserve type checking at the two
> call sites and match the cast pattern used elsewhere in this file
> (helpers.c:bpf_timer_set_callback, bpf_wq_set_callback_impl) without
> costing anything.
>
> > @@ -4872,13 +4861,50 @@ __bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> >               return -EBADF;
> >       }
> >
> > -     rhk->callback_fn = (bpf_callback_t)(void *)callback;
> > +     rhk->callback_fn = (bpf_callback_t)callback;
> >       rhk->map = map;
> >       rhk->prog = prog;
> > -     call_rcu(&rhk->rcu, bpf_rcu_run_callback);
> > +     if (trace)
> > +             call_rcu_tasks_trace(&rhk->rcu, bpf_rcu_run_callback);
> > +     else
> > +             call_rcu(&rhk->rcu, bpf_rcu_run_callback);
> >       return 0;
> >  }
>
> This isn't a bug, but would passing the call_rcu variant (or naming the
> flag 'tasks_trace') read better than a bare 'bool trace' parameter? The
> only behavioural difference between the two kfuncs is which queueing
> function is used, but it's carried through a 'bool trace' flag that the
> reader has to trace to the bottom of the helper to interpret. The name
> 'trace' is also ambiguous in a BPF file where 'trace' usually means
> tracing programs rather than RCU-tasks-trace.
>

No, I think "trace" is good enough, the readers know what this means
so we don't need to over explain here. Of Course if others think
otherwise I will send another version for this.

[PATCH bpf-next v5 4/4] selftests/bpf: Add a test for bpf_call_rcu_tasks_trace()

[Puranjay Mohan]

Run the same functional test against the tasks trace flavour.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 .../testing/selftests/bpf/prog_tests/call_rcu.c | 12 ++++++++----
 tools/testing/selftests/bpf/progs/call_rcu.c    | 17 +++++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/call_rcu.c b/tools/testing/selftests/bpf/prog_tests/call_rcu.c
index bfa6512b9052a..e4d271d43d878 100644
--- a/tools/testing/selftests/bpf/prog_tests/call_rcu.c
+++ b/tools/testing/selftests/bpf/prog_tests/call_rcu.c
@@ -28,9 +28,10 @@ static bool wait_for_callbacks(struct call_rcu *skel, int expected)
 	return ASSERT_GE(got, expected, "callbacks");
 }
 
-static void test_call_rcu_run(void)
+static void test_call_rcu_run(bool trace)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct bpf_program *prog;
 	struct call_rcu *skel;
 	struct elem elem;
 	__u32 key = 1;
@@ -40,7 +41,8 @@ static void test_call_rcu_run(void)
 	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
 		return;
 
-	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts);
+	prog = trace ? skel->progs.arm_trace : skel->progs.arm;
+	err = bpf_prog_test_run_opts(bpf_program__fd(prog), &opts);
 	if (!ASSERT_OK(err, "test_run") || !ASSERT_EQ(opts.retval, 0, "retval"))
 		goto out;
 
@@ -59,7 +61,7 @@ static void test_call_rcu_run(void)
 		ASSERT_EQ(elem.val, 0, "value_cleared");
 
 	/* The head is disarmed before the callback runs, so it can be reused. */
-	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arm), &opts);
+	err = bpf_prog_test_run_opts(bpf_program__fd(prog), &opts);
 	if (!ASSERT_OK(err, "test_run_again"))
 		goto out;
 	ASSERT_EQ(skel->bss->arm_err, 0, "rearm_err");
@@ -254,7 +256,9 @@ static void test_call_rcu_two_heads(void)
 void test_call_rcu(void)
 {
 	if (test__start_subtest("run"))
-		test_call_rcu_run();
+		test_call_rcu_run(false);
+	if (test__start_subtest("run_tasks_trace"))
+		test_call_rcu_run(true);
 	if (test__start_subtest("chain"))
 		test_call_rcu_chain();
 	if (test__start_subtest("two_heads"))
diff --git a/tools/testing/selftests/bpf/progs/call_rcu.c b/tools/testing/selftests/bpf/progs/call_rcu.c
index 94d92cee88a70..505008647fd6f 100644
--- a/tools/testing/selftests/bpf/progs/call_rcu.c
+++ b/tools/testing/selftests/bpf/progs/call_rcu.c
@@ -86,6 +86,23 @@ int arm_both(void *ctx)
 	return 0;
 }
 
+SEC("syscall")
+int arm_trace(void *ctx)
+{
+	__u32 key = 1;
+	struct elem *e;
+
+	e = bpf_map_lookup_elem(&arr, &key);
+	if (!e)
+		return 1;
+
+	e->val = 0xdeadbeef;
+	/* No lock needed: the enclosing rcu_read_lock_trace() already blocks the grace period. */
+	arm_err = bpf_call_rcu_tasks_trace(&e->rh, &arr, reclaim);
+	busy_err = bpf_call_rcu_tasks_trace(&e->rh, &arr, reclaim);
+	return 0;
+}
+
 SEC("iter/bpf_map_elem")
 int dump(struct bpf_iter__bpf_map_elem *ctx)
 {
-- 
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