bpf: Enable task local storage for tracing programs

To access per-task data, BPF programs usually creates a hash table with
pid as the key. This is not ideal because:
 1. The user need to estimate the proper size of the hash table, which may
    be inaccurate;
 2. Big hash tables are slow;
 3. To clean up the data properly during task terminations, the user need
    to write extra logic.

Task local storage overcomes these issues and offers a better option for
these per-task data. Task local storage is only available to BPF_LSM. Now
enable it for tracing programs.

Unlike LSM programs, tracing programs can be called in IRQ contexts.
Helpers that access task local storage are updated to use
raw_spin_lock_irqsave() instead of raw_spin_lock_bh().

Tracing programs can attach to functions on the task free path, e.g.
exit_creds(). To avoid allocating task local storage after
bpf_task_storage_free(). bpf_task_storage_get() is updated to not allocate
new storage when the task is not refcounted (task->usage == 0).

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210225234319.336131-2-songliubraving@fb.com

include/linux/bpf.h

@@ -1499,6 +1499,7 @@ struct bpf_prog *bpf_prog_by_id(u32 id);
 struct bpf_link *bpf_link_by_id(u32 id);
 
 const struct bpf_func_proto *bpf_base_func_proto(enum bpf_func_id func_id);
+void bpf_task_storage_free(struct task_struct *task);
 #else /* !CONFIG_BPF_SYSCALL */
 static inline struct bpf_prog *bpf_prog_get(u32 ufd)
 {
@@ -1684,6 +1685,10 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 {
 	return NULL;
 }
+
+static inline void bpf_task_storage_free(struct task_struct *task)
+{
+}
 #endif /* CONFIG_BPF_SYSCALL */
 
 void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
@@ -1886,6 +1891,8 @@ extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
 extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;
 extern const struct bpf_func_proto bpf_sock_from_file_proto;
 extern const struct bpf_func_proto bpf_get_socket_ptr_cookie_proto;
+extern const struct bpf_func_proto bpf_task_storage_get_proto;
+extern const struct bpf_func_proto bpf_task_storage_delete_proto;
 
 const struct bpf_func_proto *bpf_tracing_func_proto(
 	enum bpf_func_id func_id, const struct bpf_prog *prog);

include/linux/bpf_lsm.h

@@ -38,21 +38,9 @@ static inline struct bpf_storage_blob *bpf_inode(
 	return inode->i_security + bpf_lsm_blob_sizes.lbs_inode;
 }
 
-static inline struct bpf_storage_blob *bpf_task(
-	const struct task_struct *task)
-{
-	if (unlikely(!task->security))
-		return NULL;
-
-	return task->security + bpf_lsm_blob_sizes.lbs_task;
-}
-
 extern const struct bpf_func_proto bpf_inode_storage_get_proto;
 extern const struct bpf_func_proto bpf_inode_storage_delete_proto;
-extern const struct bpf_func_proto bpf_task_storage_get_proto;
-extern const struct bpf_func_proto bpf_task_storage_delete_proto;
 void bpf_inode_storage_free(struct inode *inode);
-void bpf_task_storage_free(struct task_struct *task);
 
 #else /* !CONFIG_BPF_LSM */
 
@@ -73,20 +61,10 @@ static inline struct bpf_storage_blob *bpf_inode(
 	return NULL;
 }
 
-static inline struct bpf_storage_blob *bpf_task(
-	const struct task_struct *task)
-{
-	return NULL;
-}
-
 static inline void bpf_inode_storage_free(struct inode *inode)
 {
 }
 
-static inline void bpf_task_storage_free(struct task_struct *task)
-{
-}
-
 #endif /* CONFIG_BPF_LSM */
 
 #endif /* _LINUX_BPF_LSM_H */

include/linux/bpf_types.h

@@ -109,8 +109,8 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_SOCKHASH, sock_hash_ops)
 #endif
 #ifdef CONFIG_BPF_LSM
 BPF_MAP_TYPE(BPF_MAP_TYPE_INODE_STORAGE, inode_storage_map_ops)
-BPF_MAP_TYPE(BPF_MAP_TYPE_TASK_STORAGE, task_storage_map_ops)
 #endif
+BPF_MAP_TYPE(BPF_MAP_TYPE_TASK_STORAGE, task_storage_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_CPUMAP, cpu_map_ops)
 #if defined(CONFIG_XDP_SOCKETS)
 BPF_MAP_TYPE(BPF_MAP_TYPE_XSKMAP, xsk_map_ops)

include/linux/sched.h

@@ -42,6 +42,7 @@ struct audit_context;
 struct backing_dev_info;
 struct bio_list;
 struct blk_plug;
+struct bpf_local_storage;
 struct capture_control;
 struct cfs_rq;
 struct fs_struct;
@@ -1348,6 +1349,10 @@ struct task_struct {
 	/* Used by LSM modules for access restriction: */
 	void				*security;
 #endif
+#ifdef CONFIG_BPF_SYSCALL
+	/* Used by BPF task local storage */
+	struct bpf_local_storage __rcu	*bpf_storage;
+#endif
 
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	unsigned long			lowest_stack;

kernel/bpf/Makefile

@@ -9,16 +9,15 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
 obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
 obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
+obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
 obj-${CONFIG_BPF_LSM}	  += bpf_inode_storage.o
-obj-${CONFIG_BPF_LSM}	  += bpf_task_storage.o
 obj-$(CONFIG_BPF_SYSCALL) += disasm.o
 obj-$(CONFIG_BPF_JIT) += trampoline.o
 obj-$(CONFIG_BPF_SYSCALL) += btf.o
 obj-$(CONFIG_BPF_JIT) += dispatcher.o
 ifeq ($(CONFIG_NET),y)
 obj-$(CONFIG_BPF_SYSCALL) += devmap.o
 obj-$(CONFIG_BPF_SYSCALL) += cpumap.o
-obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o
 obj-$(CONFIG_BPF_SYSCALL) += offload.o
 obj-$(CONFIG_BPF_SYSCALL) += net_namespace.o
 endif

kernel/bpf/bpf_local_storage.c

@@ -140,17 +140,18 @@ static void __bpf_selem_unlink_storage(struct bpf_local_storage_elem *selem)
 {
 	struct bpf_local_storage *local_storage;
 	bool free_local_storage = false;
+	unsigned long flags;
 
 	if (unlikely(!selem_linked_to_storage(selem)))
 		/* selem has already been unlinked from sk */
 		return;
 
 	local_storage = rcu_dereference(selem->local_storage);
-	raw_spin_lock_bh(&local_storage->lock);
+	raw_spin_lock_irqsave(&local_storage->lock, flags);
 	if (likely(selem_linked_to_storage(selem)))
 		free_local_storage = bpf_selem_unlink_storage_nolock(
 			local_storage, selem, true);
-	raw_spin_unlock_bh(&local_storage->lock);
+	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 
 	if (free_local_storage)
 		kfree_rcu(local_storage, rcu);
@@ -167,28 +168,30 @@ void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)
 {
 	struct bpf_local_storage_map *smap;
 	struct bpf_local_storage_map_bucket *b;
+	unsigned long flags;
 
 	if (unlikely(!selem_linked_to_map(selem)))
 		/* selem has already be unlinked from smap */
 		return;
 
 	smap = rcu_dereference(SDATA(selem)->smap);
 	b = select_bucket(smap, selem);
-	raw_spin_lock_bh(&b->lock);
+	raw_spin_lock_irqsave(&b->lock, flags);
 	if (likely(selem_linked_to_map(selem)))
 		hlist_del_init_rcu(&selem->map_node);
-	raw_spin_unlock_bh(&b->lock);
+	raw_spin_unlock_irqrestore(&b->lock, flags);
 }
 
 void bpf_selem_link_map(struct bpf_local_storage_map *smap,
 			struct bpf_local_storage_elem *selem)
 {
 	struct bpf_local_storage_map_bucket *b = select_bucket(smap, selem);
+	unsigned long flags;
 
-	raw_spin_lock_bh(&b->lock);
+	raw_spin_lock_irqsave(&b->lock, flags);
 	RCU_INIT_POINTER(SDATA(selem)->smap, smap);
 	hlist_add_head_rcu(&selem->map_node, &b->list);
-	raw_spin_unlock_bh(&b->lock);
+	raw_spin_unlock_irqrestore(&b->lock, flags);
 }
 
 void bpf_selem_unlink(struct bpf_local_storage_elem *selem)
@@ -224,16 +227,18 @@ bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
 
 	sdata = SDATA(selem);
 	if (cacheit_lockit) {
+		unsigned long flags;
+
 		/* spinlock is needed to avoid racing with the
 		 * parallel delete.  Otherwise, publishing an already
 		 * deleted sdata to the cache will become a use-after-free
 		 * problem in the next bpf_local_storage_lookup().
 		 */
-		raw_spin_lock_bh(&local_storage->lock);
+		raw_spin_lock_irqsave(&local_storage->lock, flags);
 		if (selem_linked_to_storage(selem))
 			rcu_assign_pointer(local_storage->cache[smap->cache_idx],
 					   sdata);
-		raw_spin_unlock_bh(&local_storage->lock);
+		raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 	}
 
 	return sdata;
@@ -327,6 +332,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
 	struct bpf_local_storage_data *old_sdata = NULL;
 	struct bpf_local_storage_elem *selem;
 	struct bpf_local_storage *local_storage;
+	unsigned long flags;
 	int err;
 
 	/* BPF_EXIST and BPF_NOEXIST cannot be both set */
@@ -374,7 +380,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
 		}
 	}
 
-	raw_spin_lock_bh(&local_storage->lock);
+	raw_spin_lock_irqsave(&local_storage->lock, flags);
 
 	/* Recheck local_storage->list under local_storage->lock */
 	if (unlikely(hlist_empty(&local_storage->list))) {
@@ -428,11 +434,11 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
 	}
 
 unlock:
-	raw_spin_unlock_bh(&local_storage->lock);
+	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 	return SDATA(selem);
 
 unlock_err:
-	raw_spin_unlock_bh(&local_storage->lock);
+	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 	return ERR_PTR(err);
 }
 

kernel/bpf/bpf_lsm.c

@@ -115,10 +115,6 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_spin_lock_proto;
 	case BPF_FUNC_spin_unlock:
 		return &bpf_spin_unlock_proto;
-	case BPF_FUNC_task_storage_get:
-		return &bpf_task_storage_get_proto;
-	case BPF_FUNC_task_storage_delete:
-		return &bpf_task_storage_delete_proto;
 	case BPF_FUNC_bprm_opts_set:
 		return &bpf_bprm_opts_set_proto;
 	case BPF_FUNC_ima_inode_hash:

kernel/bpf/bpf_task_storage.c

@@ -15,7 +15,6 @@
 #include <linux/bpf_local_storage.h>
 #include <linux/filter.h>
 #include <uapi/linux/btf.h>
-#include <linux/bpf_lsm.h>
 #include <linux/btf_ids.h>
 #include <linux/fdtable.h>
 
@@ -24,12 +23,8 @@ DEFINE_BPF_STORAGE_CACHE(task_cache);
 static struct bpf_local_storage __rcu **task_storage_ptr(void *owner)
 {
 	struct task_struct *task = owner;
-	struct bpf_storage_blob *bsb;
 
-	bsb = bpf_task(task);
-	if (!bsb)
-		return NULL;
-	return &bsb->storage;
+	return &task->bpf_storage;
 }
 
 static struct bpf_local_storage_data *
@@ -38,13 +33,8 @@ task_storage_lookup(struct task_struct *task, struct bpf_map *map,
 {
 	struct bpf_local_storage *task_storage;
 	struct bpf_local_storage_map *smap;
-	struct bpf_storage_blob *bsb;
-
-	bsb = bpf_task(task);
-	if (!bsb)
-		return NULL;
 
-	task_storage = rcu_dereference(bsb->storage);
+	task_storage = rcu_dereference(task->bpf_storage);
 	if (!task_storage)
 		return NULL;
 
@@ -57,16 +47,12 @@ void bpf_task_storage_free(struct task_struct *task)
 	struct bpf_local_storage_elem *selem;
 	struct bpf_local_storage *local_storage;
 	bool free_task_storage = false;
-	struct bpf_storage_blob *bsb;
 	struct hlist_node *n;
-
-	bsb = bpf_task(task);
-	if (!bsb)
-		return;
+	unsigned long flags;
 
 	rcu_read_lock();
 
-	local_storage = rcu_dereference(bsb->storage);
+	local_storage = rcu_dereference(task->bpf_storage);
 	if (!local_storage) {
 		rcu_read_unlock();
 		return;
@@ -81,7 +67,7 @@ void bpf_task_storage_free(struct task_struct *task)
 	 * when unlinking elem from the local_storage->list and
 	 * the map's bucket->list.
 	 */
-	raw_spin_lock_bh(&local_storage->lock);
+	raw_spin_lock_irqsave(&local_storage->lock, flags);
 	hlist_for_each_entry_safe(selem, n, &local_storage->list, snode) {
 		/* Always unlink from map before unlinking from
 		 * local_storage.
@@ -90,7 +76,7 @@ void bpf_task_storage_free(struct task_struct *task)
 		free_task_storage = bpf_selem_unlink_storage_nolock(
 			local_storage, selem, false);
 	}
-	raw_spin_unlock_bh(&local_storage->lock);
+	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 	rcu_read_unlock();
 
 	/* free_task_storage should always be true as long as
@@ -150,7 +136,7 @@ static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
 	 */
 	WARN_ON_ONCE(!rcu_read_lock_held());
 	task = pid_task(pid, PIDTYPE_PID);
-	if (!task || !task_storage_ptr(task)) {
+	if (!task) {
 		err = -ENOENT;
 		goto out;
 	}
@@ -213,23 +199,16 @@ BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
 	if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE))
 		return (unsigned long)NULL;
 
-	/* explicitly check that the task_storage_ptr is not
-	 * NULL as task_storage_lookup returns NULL in this case and
-	 * bpf_local_storage_update expects the owner to have a
-	 * valid storage pointer.
-	 */
-	if (!task || !task_storage_ptr(task))
+	if (!task)
 		return (unsigned long)NULL;
 
 	sdata = task_storage_lookup(task, map, true);
 	if (sdata)
 		return (unsigned long)sdata->data;
 
-	/* This helper must only be called from places where the lifetime of the task
-	 * is guaranteed. Either by being refcounted or by being protected
-	 * by an RCU read-side critical section.
-	 */
-	if (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
+	/* only allocate new storage, when the task is refcounted */
+	if (refcount_read(&task->usage) &&
+	    (flags & BPF_LOCAL_STORAGE_GET_F_CREATE)) {
 		sdata = bpf_local_storage_update(
 			task, (struct bpf_local_storage_map *)map, value,
 			BPF_NOEXIST);

kernel/fork.c

@@ -96,6 +96,7 @@
 #include <linux/kasan.h>
 #include <linux/scs.h>
 #include <linux/io_uring.h>
+#include <linux/bpf.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>
@@ -734,6 +735,7 @@ void __put_task_struct(struct task_struct *tsk)
 	cgroup_free(tsk);
 	task_numa_free(tsk, true);
 	security_task_free(tsk);
+	bpf_task_storage_free(tsk);
 	exit_creds(tsk);
 	delayacct_tsk_free(tsk);
 	put_signal_struct(tsk->signal);
@@ -2062,6 +2064,9 @@ static __latent_entropy struct task_struct *copy_process(
 	p->sequential_io	= 0;
 	p->sequential_io_avg	= 0;
 #endif
+#ifdef CONFIG_BPF_SYSCALL
+	RCU_INIT_POINTER(p->bpf_storage, NULL);
+#endif
 
 	/* Perform scheduler related setup. Assign this task to a CPU. */
 	retval = sched_fork(clone_flags, p);

kernel/trace/bpf_trace.c

@@ -1367,6 +1367,10 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_per_cpu_ptr_proto;
 	case BPF_FUNC_this_cpu_ptr:
 		return &bpf_this_cpu_ptr_proto;
+	case BPF_FUNC_task_storage_get:
+		return &bpf_task_storage_get_proto;
+	case BPF_FUNC_task_storage_delete:
+		return &bpf_task_storage_delete_proto;
 	default:
 		return NULL;
 	}