bpf: introduce bpf_spin_lock

Introduce 'struct bpf_spin_lock' and bpf_spin_lock/unlock() helpers to let
bpf program serialize access to other variables.

Example:
struct hash_elem {
    int cnt;
    struct bpf_spin_lock lock;
};
struct hash_elem * val = bpf_map_lookup_elem(&hash_map, &key);
if (val) {
    bpf_spin_lock(&val->lock);
    val->cnt++;
    bpf_spin_unlock(&val->lock);
}

Restrictions and safety checks:
- bpf_spin_lock is only allowed inside HASH and ARRAY maps.
- BTF description of the map is mandatory for safety analysis.
- bpf program can take one bpf_spin_lock at a time, since two or more can
  cause dead locks.
- only one 'struct bpf_spin_lock' is allowed per map element.
  It drastically simplifies implementation yet allows bpf program to use
  any number of bpf_spin_locks.
- when bpf_spin_lock is taken the calls (either bpf2bpf or helpers) are not allowed.
- bpf program must bpf_spin_unlock() before return.
- bpf program can access 'struct bpf_spin_lock' only via
  bpf_spin_lock()/bpf_spin_unlock() helpers.
- load/store into 'struct bpf_spin_lock lock;' field is not allowed.
- to use bpf_spin_lock() helper the BTF description of map value must be
  a struct and have 'struct bpf_spin_lock anyname;' field at the top level.
  Nested lock inside another struct is not allowed.
- syscall map_lookup doesn't copy bpf_spin_lock field to user space.
- syscall map_update and program map_update do not update bpf_spin_lock field.
- bpf_spin_lock cannot be on the stack or inside networking packet.
  bpf_spin_lock can only be inside HASH or ARRAY map value.
- bpf_spin_lock is available to root only and to all program types.
- bpf_spin_lock is not allowed in inner maps of map-in-map.
- ld_abs is not allowed inside spin_lock-ed region.
- tracing progs and socket filter progs cannot use bpf_spin_lock due to
  insufficient preemption checks

Implementation details:
- cgroup-bpf class of programs can nest with xdp/tc programs.
  Hence bpf_spin_lock is equivalent to spin_lock_irqsave.
  Other solutions to avoid nested bpf_spin_lock are possible.
  Like making sure that all networking progs run with softirq disabled.
  spin_lock_irqsave is the simplest and doesn't add overhead to the
  programs that don't use it.
- arch_spinlock_t is used when its implemented as queued_spin_lock
- archs can force their own arch_spinlock_t
- on architectures where queued_spin_lock is not available and
  sizeof(arch_spinlock_t) != sizeof(__u32) trivial lock is used.
- presence of bpf_spin_lock inside map value could have been indicated via
  extra flag during map_create, but specifying it via BTF is cleaner.
  It provides introspection for map key/value and reduces user mistakes.

Next steps:
- allow bpf_spin_lock in other map types (like cgroup local storage)
- introduce BPF_F_LOCK flag for bpf_map_update() syscall and helper
  to request kernel to grab bpf_spin_lock before rewriting the value.
  That will serialize access to map elements.

Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: Alexei Starovoitov

include/linux/bpf.h

@@ -72,14 +72,15 @@ struct bpf_map {
 	u32 value_size;
 	u32 max_entries;
 	u32 map_flags;
-	u32 pages;
+	int spin_lock_off; /* >=0 valid offset, <0 error */
 	u32 id;
 	int numa_node;
 	u32 btf_key_type_id;
 	u32 btf_value_type_id;
 	struct btf *btf;
+	u32 pages;
 	bool unpriv_array;
-	/* 55 bytes hole */
+	/* 51 bytes hole */
 
 	/* The 3rd and 4th cacheline with misc members to avoid false sharing
 	 * particularly with refcounting.
@@ -91,6 +92,34 @@ struct bpf_map {
 	char name[BPF_OBJ_NAME_LEN];
 };
 
+static inline bool map_value_has_spin_lock(const struct bpf_map *map)
+{
+	return map->spin_lock_off >= 0;
+}
+
+static inline void check_and_init_map_lock(struct bpf_map *map, void *dst)
+{
+	if (likely(!map_value_has_spin_lock(map)))
+		return;
+	*(struct bpf_spin_lock *)(dst + map->spin_lock_off) =
+		(struct bpf_spin_lock){};
+}
+
+/* copy everything but bpf_spin_lock */
+static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
+{
+	if (unlikely(map_value_has_spin_lock(map))) {
+		u32 off = map->spin_lock_off;
+
+		memcpy(dst, src, off);
+		memcpy(dst + off + sizeof(struct bpf_spin_lock),
+		       src + off + sizeof(struct bpf_spin_lock),
+		       map->value_size - off - sizeof(struct bpf_spin_lock));
+	} else {
+		memcpy(dst, src, map->value_size);
+	}
+}
+
 struct bpf_offload_dev;
 struct bpf_offloaded_map;
 
@@ -162,6 +191,7 @@ enum bpf_arg_type {
 	ARG_PTR_TO_CTX,		/* pointer to context */
 	ARG_ANYTHING,		/* any (initialized) argument is ok */
 	ARG_PTR_TO_SOCKET,	/* pointer to bpf_sock */
+	ARG_PTR_TO_SPIN_LOCK,	/* pointer to bpf_spin_lock */
 };
 
 /* type of values returned from helper functions */
@@ -879,7 +909,8 @@ extern const struct bpf_func_proto bpf_msg_redirect_hash_proto;
 extern const struct bpf_func_proto bpf_msg_redirect_map_proto;
 extern const struct bpf_func_proto bpf_sk_redirect_hash_proto;
 extern const struct bpf_func_proto bpf_sk_redirect_map_proto;
-
+extern const struct bpf_func_proto bpf_spin_lock_proto;
+extern const struct bpf_func_proto bpf_spin_unlock_proto;
 extern const struct bpf_func_proto bpf_get_local_storage_proto;
 
 /* Shared helpers among cBPF and eBPF. */

include/linux/bpf_verifier.h

@@ -148,6 +148,7 @@ struct bpf_verifier_state {
 	/* call stack tracking */
 	struct bpf_func_state *frame[MAX_CALL_FRAMES];
 	u32 curframe;
+	u32 active_spin_lock;
 	bool speculative;
 };
 

include/linux/btf.h

@@ -50,6 +50,7 @@ u32 btf_id(const struct btf *btf);
 bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
 			   const struct btf_member *m,
 			   u32 expected_offset, u32 expected_size);
+int btf_find_spin_lock(const struct btf *btf, const struct btf_type *t);
 
 #ifdef CONFIG_BPF_SYSCALL
 const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);

include/uapi/linux/bpf.h

@@ -2422,7 +2422,9 @@ union bpf_attr {
 	FN(map_peek_elem),		\
 	FN(msg_push_data),		\
 	FN(msg_pop_data),		\
-	FN(rc_pointer_rel),
+	FN(rc_pointer_rel),		\
+	FN(spin_lock),			\
+	FN(spin_unlock),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -3056,4 +3058,7 @@ struct bpf_line_info {
 	__u32	line_col;
 };
 
+struct bpf_spin_lock {
+	__u32	val;
+};
 #endif /* _UAPI__LINUX_BPF_H__ */

kernel/Kconfig.locks

@@ -242,6 +242,9 @@ config QUEUED_SPINLOCKS
 	def_bool y if ARCH_USE_QUEUED_SPINLOCKS
 	depends on SMP
 
+config BPF_ARCH_SPINLOCK
+	bool
+
 config ARCH_USE_QUEUED_RWLOCKS
 	bool
 

kernel/bpf/arraymap.c

@@ -270,9 +270,10 @@ static int array_map_update_elem(struct bpf_map *map, void *key, void *value,
 		memcpy(this_cpu_ptr(array->pptrs[index & array->index_mask]),
 		       value, map->value_size);
 	else
-		memcpy(array->value +
-		       array->elem_size * (index & array->index_mask),
-		       value, map->value_size);
+		copy_map_value(map,
+			       array->value +
+			       array->elem_size * (index & array->index_mask),
+			       value);
 	return 0;
 }
 

kernel/bpf/btf.c

@@ -355,6 +355,11 @@ static bool btf_type_is_struct(const struct btf_type *t)
 	return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
 }
 
+static bool __btf_type_is_struct(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_STRUCT;
+}
+
 static bool btf_type_is_array(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_ARRAY;
@@ -2045,6 +2050,43 @@ static void btf_struct_log(struct btf_verifier_env *env,
 	btf_verifier_log(env, "size=%u vlen=%u", t->size, btf_type_vlen(t));
 }
 
+/* find 'struct bpf_spin_lock' in map value.
+ * return >= 0 offset if found
+ * and < 0 in case of error
+ */
+int btf_find_spin_lock(const struct btf *btf, const struct btf_type *t)
+{
+	const struct btf_member *member;
+	u32 i, off = -ENOENT;
+
+	if (!__btf_type_is_struct(t))
+		return -EINVAL;
+
+	for_each_member(i, t, member) {
+		const struct btf_type *member_type = btf_type_by_id(btf,
+								    member->type);
+		if (!__btf_type_is_struct(member_type))
+			continue;
+		if (member_type->size != sizeof(struct bpf_spin_lock))
+			continue;
+		if (strcmp(__btf_name_by_offset(btf, member_type->name_off),
+			   "bpf_spin_lock"))
+			continue;
+		if (off != -ENOENT)
+			/* only one 'struct bpf_spin_lock' is allowed */
+			return -E2BIG;
+		off = btf_member_bit_offset(t, member);
+		if (off % 8)
+			/* valid C code cannot generate such BTF */
+			return -EINVAL;
+		off /= 8;
+		if (off % __alignof__(struct bpf_spin_lock))
+			/* valid struct bpf_spin_lock will be 4 byte aligned */
+			return -EINVAL;
+	}
+	return off;
+}
+
 static void btf_struct_seq_show(const struct btf *btf, const struct btf_type *t,
 				u32 type_id, void *data, u8 bits_offset,
 				struct seq_file *m)

kernel/bpf/core.c

@@ -2002,6 +2002,8 @@ const struct bpf_func_proto bpf_map_delete_elem_proto __weak;
 const struct bpf_func_proto bpf_map_push_elem_proto __weak;
 const struct bpf_func_proto bpf_map_pop_elem_proto __weak;
 const struct bpf_func_proto bpf_map_peek_elem_proto __weak;
+const struct bpf_func_proto bpf_spin_lock_proto __weak;
+const struct bpf_func_proto bpf_spin_unlock_proto __weak;
 
 const struct bpf_func_proto bpf_get_prandom_u32_proto __weak;
 const struct bpf_func_proto bpf_get_smp_processor_id_proto __weak;

kernel/bpf/hashtab.c

@@ -718,21 +718,12 @@ static bool fd_htab_map_needs_adjust(const struct bpf_htab *htab)
 	       BITS_PER_LONG == 64;
 }
 
-static u32 htab_size_value(const struct bpf_htab *htab, bool percpu)
-{
-	u32 size = htab->map.value_size;
-
-	if (percpu || fd_htab_map_needs_adjust(htab))
-		size = round_up(size, 8);
-	return size;
-}
-
 static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 					 void *value, u32 key_size, u32 hash,
 					 bool percpu, bool onallcpus,
 					 struct htab_elem *old_elem)
 {
-	u32 size = htab_size_value(htab, percpu);
+	u32 size = htab->map.value_size;
 	bool prealloc = htab_is_prealloc(htab);
 	struct htab_elem *l_new, **pl_new;
 	void __percpu *pptr;
@@ -770,10 +761,13 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 			l_new = ERR_PTR(-ENOMEM);
 			goto dec_count;
 		}
+		check_and_init_map_lock(&htab->map,
+					l_new->key + round_up(key_size, 8));
 	}
 
 	memcpy(l_new->key, key, key_size);
 	if (percpu) {
+		size = round_up(size, 8);
 		if (prealloc) {
 			pptr = htab_elem_get_ptr(l_new, key_size);
 		} else {
@@ -791,8 +785,13 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 
 		if (!prealloc)
 			htab_elem_set_ptr(l_new, key_size, pptr);
-	} else {
+	} else if (fd_htab_map_needs_adjust(htab)) {
+		size = round_up(size, 8);
 		memcpy(l_new->key + round_up(key_size, 8), value, size);
+	} else {
+		copy_map_value(&htab->map,
+			       l_new->key + round_up(key_size, 8),
+			       value);
 	}
 
 	l_new->hash = hash;

kernel/bpf/helpers.c

@@ -221,6 +221,86 @@ const struct bpf_func_proto bpf_get_current_comm_proto = {
 	.arg2_type	= ARG_CONST_SIZE,
 };
 
+#if defined(CONFIG_QUEUED_SPINLOCKS) || defined(CONFIG_BPF_ARCH_SPINLOCK)
+
+static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
+{
+	arch_spinlock_t *l = (void *)lock;
+	union {
+		__u32 val;
+		arch_spinlock_t lock;
+	} u = { .lock = __ARCH_SPIN_LOCK_UNLOCKED };
+
+	compiletime_assert(u.val == 0, "__ARCH_SPIN_LOCK_UNLOCKED not 0");
+	BUILD_BUG_ON(sizeof(*l) != sizeof(__u32));
+	BUILD_BUG_ON(sizeof(*lock) != sizeof(__u32));
+	arch_spin_lock(l);
+}
+
+static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
+{
+	arch_spinlock_t *l = (void *)lock;
+
+	arch_spin_unlock(l);
+}
+
+#else
+
+static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
+{
+	atomic_t *l = (void *)lock;
+
+	BUILD_BUG_ON(sizeof(*l) != sizeof(*lock));
+	do {
+		atomic_cond_read_relaxed(l, !VAL);
+	} while (atomic_xchg(l, 1));
+}
+
+static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
+{
+	atomic_t *l = (void *)lock;
+
+	atomic_set_release(l, 0);
+}
+
+#endif
+
+static DEFINE_PER_CPU(unsigned long, irqsave_flags);
+
+notrace BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__bpf_spin_lock(lock);
+	__this_cpu_write(irqsave_flags, flags);
+	return 0;
+}
+
+const struct bpf_func_proto bpf_spin_lock_proto = {
+	.func		= bpf_spin_lock,
+	.gpl_only	= false,
+	.ret_type	= RET_VOID,
+	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
+};
+
+notrace BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
+{
+	unsigned long flags;
+
+	flags = __this_cpu_read(irqsave_flags);
+	__bpf_spin_unlock(lock);
+	local_irq_restore(flags);
+	return 0;
+}
+
+const struct bpf_func_proto bpf_spin_unlock_proto = {
+	.func		= bpf_spin_unlock,
+	.gpl_only	= false,
+	.ret_type	= RET_VOID,
+	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
+};
+
 #ifdef CONFIG_CGROUPS
 BPF_CALL_0(bpf_get_current_cgroup_id)
 {

kernel/bpf/map_in_map.c

@@ -37,6 +37,11 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
 		return ERR_PTR(-EINVAL);
 	}
 
+	if (map_value_has_spin_lock(inner_map)) {
+		fdput(f);
+		return ERR_PTR(-ENOTSUPP);
+	}
+
 	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)

kernel/bpf/syscall.c

@@ -463,7 +463,7 @@ int map_check_no_btf(const struct bpf_map *map,
 	return -ENOTSUPP;
 }
 
-static int map_check_btf(const struct bpf_map *map, const struct btf *btf,
+static int map_check_btf(struct bpf_map *map, const struct btf *btf,
 			 u32 btf_key_id, u32 btf_value_id)
 {
 	const struct btf_type *key_type, *value_type;
@@ -478,6 +478,21 @@ static int map_check_btf(const struct bpf_map *map, const struct btf *btf,
 	if (!value_type || value_size != map->value_size)
 		return -EINVAL;
 
+	map->spin_lock_off = btf_find_spin_lock(btf, value_type);
+
+	if (map_value_has_spin_lock(map)) {
+		if (map->map_type != BPF_MAP_TYPE_HASH &&
+		    map->map_type != BPF_MAP_TYPE_ARRAY)
+			return -ENOTSUPP;
+		if (map->spin_lock_off + sizeof(struct bpf_spin_lock) >
+		    map->value_size) {
+			WARN_ONCE(1,
+				  "verifier bug spin_lock_off %d value_size %d\n",
+				  map->spin_lock_off, map->value_size);
+			return -EFAULT;
+		}
+	}
+
 	if (map->ops->map_check_btf)
 		ret = map->ops->map_check_btf(map, btf, key_type, value_type);
 
@@ -542,6 +557,8 @@ static int map_create(union bpf_attr *attr)
 		map->btf = btf;
 		map->btf_key_type_id = attr->btf_key_type_id;
 		map->btf_value_type_id = attr->btf_value_type_id;
+	} else {
+		map->spin_lock_off = -EINVAL;
 	}
 
 	err = security_bpf_map_alloc(map);
@@ -740,7 +757,7 @@ static int map_lookup_elem(union bpf_attr *attr)
 			err = -ENOENT;
 		} else {
 			err = 0;
-			memcpy(value, ptr, value_size);
+			copy_map_value(map, value, ptr);
 		}
 		rcu_read_unlock();
 	}