Merge branch 'Support storing struct task_struct objects as kptrs'

David Vernet says:

====================

Now that BPF supports adding new kernel functions with kfuncs, and
storing kernel objects in maps with kptrs, we can add a set of kfuncs
which allow struct task_struct objects to be stored in maps as
referenced kptrs.

The possible use cases for doing this are plentiful.  During tracing,
for example, it would be useful to be able to collect some tasks that
performed a certain operation, and then periodically summarize who they
are, which cgroup they're in, how much CPU time they've utilized, etc.
Doing this now would require storing the tasks' pids along with some
relevant data to be exported to user space, and later associating the
pids to tasks in other event handlers where the data is recorded.
Another useful by-product of this is that it allows a program to pin a
task in a BPF program, and by proxy therefore also e.g. pin its task
local storage.

In order to support this, we'll need to expand KF_TRUSTED_ARGS to
support receiving trusted, non-refcounted pointers. It currently only
supports either PTR_TO_CTX pointers, or refcounted pointers. What this
means in terms of the implementation is that check_kfunc_args() would
have to also check for the PTR_TRUSTED or MEM_ALLOC type modifiers when
determining if a trusted KF_ARG_PTR_TO_ALLOC_BTF_ID or
KF_ARG_PTR_TO_BTF_ID pointer requires a refcount.

Note that PTR_UNTRUSTED is insufficient for this purpose, as it does not
cover all of the possible types of potentially unsafe pointers. For
example, a pointer obtained from walking a struct is not PTR_UNTRUSTED.
To account for this and enable us to expand KF_TRUSTED_ARGS to include
allow-listed arguments such as those passed by the kernel to tracepoints
and struct_ops callbacks, this patch set also introduces a new
PTR_TRUSTED type flag modifier which records if a pointer was obtained
passed from the kernel in a trusted context.

Currently, both PTR_TRUSTED and MEM_ALLOC are used to imply that a
pointer is trusted. Longer term, PTR_TRUSTED should be the sole source
of truth for whether a pointer is trusted. This requires us to set
PTR_TRUSTED when appropriate (e.g. when setting MEM_ALLOC), and unset it
when appropriate (e.g. when setting PTR_UNTRUSTED). We don't do that in
this patch, as we need to do more clean up before this can be done in a
clear and well-defined manner.

In closing, this patch set:

1. Adds the new PTR_TRUSTED register type modifier flag, and updates the
   verifier and existing selftests accordingly. Also expands
   KF_TRUSTED_ARGS to also include trusted pointers that were not obtained
   from walking structs.
2. Adds a new set of kfuncs that allows struct task_struct* objects to be
   used as kptrs.
3. Adds a new selftest suite to validate these new task kfuncs.
---
Changelog:
v8 -> v9:
- Moved check for release register back to where we check for
  !PTR_TO_BTF_ID || socket. Change the verifier log message to
  reflect really what's being tested (the presence of unsafe
  modifiers) (Alexei)
- Fix verifier_test error tests to reflect above changes
- Remove unneeded parens around bitwise operator checks (Alexei)
- Move updates to reg_type_str() which allow multiple type modifiers
  to be present in the prefix string, to a separate patch (Alexei)
- Increase TYPE_STR_BUF_LEN size to 128 to reflect larger prefix size
  in reg_type_str().

v7 -> v8:
- Rebased onto Kumar's latest patch set which, adds a new MEM_ALLOC reg
  type modifier for bpf_obj_new() calls.
- Added comments to bpf_task_kptr_get() describing some of the subtle
  races we're protecting against (Alexei and John)
- Slightly rework process_kf_arg_ptr_to_btf_id(), and add a new
  reg_has_unsafe_modifiers() function which validates that a register
  containing a kfunc release arg doesn't have unsafe modifiers. Note
  that this is slightly different than the check for KF_TRUSTED_ARGS.
  An alternative here would be to treat KF_RELEASE as implicitly
  requiring KF_TRUSTED_ARGS.
- Export inline bpf_type_has_unsafe_modifiers() function from
  bpf_verifier.h so that it can be used from bpf_tcp_ca.c. Eventually this
  function should likely be changed to bpf_type_is_trusted(), once
  PTR_TRUSTED is the real source of truth.

v6 -> v7:
- Removed the PTR_WALKED type modifier, and instead define a new
  PTR_TRUSTED type modifier which is set on registers containing
  pointers passed from trusted contexts (i.e. as tracepoint or
  struct_ops callback args) (Alexei)
- Remove the new KF_OWNED_ARGS kfunc flag. This can be accomplished
  by defining a new type that wraps an existing type, such as with
  struct nf_conn___init (Alexei)
- Add a test_task_current_acquire_release testcase which verifies we can
  acquire a task struct returned from bpf_get_current_task_btf().
- Make bpf_task_acquire() no longer return NULL, as it can no longer be
  called with a NULL task.
- Removed unnecessary is_test_kfunc_task() checks from failure
  testcases.

v5 -> v6:
- Add a new KF_OWNED_ARGS kfunc flag which may be used by kfuncs to
  express that they require trusted, refcounted args (Kumar)
- Rename PTR_NESTED -> PTR_WALKED in the verifier (Kumar)
- Convert reg_type_str() prefixes to use snprintf() instead of strncpy()
  (Kumar)
- Add PTR_TO_BTF_ID | PTR_WALKED to missing struct btf_reg_type
  instances -- specifically btf_id_sock_common_types, and
  percpu_btf_ptr_types.
- Add a missing PTR_TO_BTF_ID | PTR_WALKED switch case entry in
  check_func_arg_reg_off(), which is required when validating helper
  calls (Kumar)
- Update reg_type_mismatch_ok() to check base types for the registers
  (i.e. to accommodate type modifiers). Additionally, add a lengthy
  comment that explains why this is being done (Kumar)
- Update convert_ctx_accesses() to also issue probe reads for
  PTR_TO_BTF_ID | PTR_WALKED (Kumar)
- Update selftests to expect new prefix reg type strings.
- Rename task_kfunc_acquire_trusted_nested testcase to
  task_kfunc_acquire_trusted_walked, and fix a comment (Kumar)
- Remove KF_TRUSTED_ARGS from bpf_task_release(), which already includes
  KF_RELEASE (Kumar)
- Add bpf-next in patch subject lines (Kumar)

v4 -> v5:
- Fix an improperly formatted patch title.

v3 -> v4:
- Remove an unnecessary check from my repository that I forgot to remove
  after debugging something.

v2 -> v3:
- Make bpf_task_acquire() check for NULL, and include KF_RET_NULL
  (Martin)
- Include new PTR_NESTED register modifier type flag which specifies
  whether a pointer was obtained from walking a struct. Use this to
  expand the meaning of KF_TRUSTED_ARGS to include trusted pointers that
  were passed from the kernel (Kumar)
- Add more selftests to the task_kfunc selftest suite which verify that
  you cannot pass a walked pointer to bpf_task_acquire().
- Update bpf_task_acquire() to also specify KF_TRUSTED_ARGS.

v1 -> v2:
- Rename tracing_btf_ids to generic_kfunc_btf_ids, and add the new
  kfuncs to that list instead of making a separate btf id list (Alexei).
- Don't run the new selftest suite on s390x, which doesn't appear to
  support invoking kfuncs.
- Add a missing __diag_ignore block for -Wmissing-prototypes
  (lkp@intel.com).
- Fix formatting on some of the SPDX-License-Identifier tags.
- Clarified the function header comment a bit on bpf_task_kptr_get().
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/kfuncs.rst

@@ -161,22 +161,20 @@ KF_ACQUIRE and KF_RET_NULL flags.
 --------------------------
 
 The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It
-indicates that the all pointer arguments will always have a guaranteed lifetime,
-and pointers to kernel objects are always passed to helpers in their unmodified
-form (as obtained from acquire kfuncs).
-
-It can be used to enforce that a pointer to a refcounted object acquired from a
-kfunc or BPF helper is passed as an argument to this kfunc without any
-modifications (e.g. pointer arithmetic) such that it is trusted and points to
-the original object.
-
-Meanwhile, it is also allowed pass pointers to normal memory to such kfuncs,
-but those can have a non-zero offset.
-
-This flag is often used for kfuncs that operate (change some property, perform
-some operation) on an object that was obtained using an acquire kfunc. Such
-kfuncs need an unchanged pointer to ensure the integrity of the operation being
-performed on the expected object.
+indicates that the all pointer arguments are valid, and that all pointers to
+BTF objects have been passed in their unmodified form (that is, at a zero
+offset, and without having been obtained from walking another pointer).
+
+There are two types of pointers to kernel objects which are considered "valid":
+
+1. Pointers which are passed as tracepoint or struct_ops callback arguments.
+2. Pointers which were returned from a KF_ACQUIRE or KF_KPTR_GET kfunc.
+
+Pointers to non-BTF objects (e.g. scalar pointers) may also be passed to
+KF_TRUSTED_ARGS kfuncs, and may have a non-zero offset.
+
+The definition of "valid" pointers is subject to change at any time, and has
+absolutely no ABI stability guarantees.
 
 2.4.6 KF_SLEEPABLE flag
 -----------------------

include/linux/bpf.h

@@ -543,6 +543,35 @@ enum bpf_type_flag {
 	 */
 	MEM_ALLOC		= BIT(11 + BPF_BASE_TYPE_BITS),
 
+	/* PTR was passed from the kernel in a trusted context, and may be
+	 * passed to KF_TRUSTED_ARGS kfuncs or BPF helper functions.
+	 * Confusingly, this is _not_ the opposite of PTR_UNTRUSTED above.
+	 * PTR_UNTRUSTED refers to a kptr that was read directly from a map
+	 * without invoking bpf_kptr_xchg(). What we really need to know is
+	 * whether a pointer is safe to pass to a kfunc or BPF helper function.
+	 * While PTR_UNTRUSTED pointers are unsafe to pass to kfuncs and BPF
+	 * helpers, they do not cover all possible instances of unsafe
+	 * pointers. For example, a pointer that was obtained from walking a
+	 * struct will _not_ get the PTR_UNTRUSTED type modifier, despite the
+	 * fact that it may be NULL, invalid, etc. This is due to backwards
+	 * compatibility requirements, as this was the behavior that was first
+	 * introduced when kptrs were added. The behavior is now considered
+	 * deprecated, and PTR_UNTRUSTED will eventually be removed.
+	 *
+	 * PTR_TRUSTED, on the other hand, is a pointer that the kernel
+	 * guarantees to be valid and safe to pass to kfuncs and BPF helpers.
+	 * For example, pointers passed to tracepoint arguments are considered
+	 * PTR_TRUSTED, as are pointers that are passed to struct_ops
+	 * callbacks. As alluded to above, pointers that are obtained from
+	 * walking PTR_TRUSTED pointers are _not_ trusted. For example, if a
+	 * struct task_struct *task is PTR_TRUSTED, then accessing
+	 * task->last_wakee will lose the PTR_TRUSTED modifier when it's stored
+	 * in a BPF register. Similarly, pointers passed to certain programs
+	 * types such as kretprobes are not guaranteed to be valid, as they may
+	 * for example contain an object that was recently freed.
+	 */
+	PTR_TRUSTED		= BIT(12 + BPF_BASE_TYPE_BITS),
+
 	__BPF_TYPE_FLAG_MAX,
 	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,
 };
@@ -636,6 +665,7 @@ enum bpf_return_type {
 	RET_PTR_TO_RINGBUF_MEM_OR_NULL	= PTR_MAYBE_NULL | MEM_RINGBUF | RET_PTR_TO_MEM,
 	RET_PTR_TO_DYNPTR_MEM_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_MEM,
 	RET_PTR_TO_BTF_ID_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_BTF_ID,
+	RET_PTR_TO_BTF_ID_TRUSTED	= PTR_TRUSTED	 | RET_PTR_TO_BTF_ID,
 
 	/* This must be the last entry. Its purpose is to ensure the enum is
 	 * wide enough to hold the higher bits reserved for bpf_type_flag.

include/linux/bpf_verifier.h

@@ -19,7 +19,7 @@
  */
 #define BPF_MAX_VAR_SIZ	(1 << 29)
 /* size of type_str_buf in bpf_verifier. */
-#define TYPE_STR_BUF_LEN 64
+#define TYPE_STR_BUF_LEN 128
 
 /* Liveness marks, used for registers and spilled-regs (in stack slots).
  * Read marks propagate upwards until they find a write mark; they record that
@@ -680,4 +680,11 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
 	}
 }
 
+#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | PTR_TRUSTED)
+
+static inline bool bpf_type_has_unsafe_modifiers(u32 type)
+{
+	return type_flag(type) & ~BPF_REG_TRUSTED_MODIFIERS;
+}
+
 #endif /* _LINUX_BPF_VERIFIER_H */

include/linux/btf.h

@@ -19,36 +19,53 @@
 #define KF_RELEASE	(1 << 1) /* kfunc is a release function */
 #define KF_RET_NULL	(1 << 2) /* kfunc returns a pointer that may be NULL */
 #define KF_KPTR_GET	(1 << 3) /* kfunc returns reference to a kptr */
-/* Trusted arguments are those which are meant to be referenced arguments with
- * unchanged offset. It is used to enforce that pointers obtained from acquire
- * kfuncs remain unmodified when being passed to helpers taking trusted args.
+/* Trusted arguments are those which are guaranteed to be valid when passed to
+ * the kfunc. It is used to enforce that pointers obtained from either acquire
+ * kfuncs, or from the main kernel on a tracepoint or struct_ops callback
+ * invocation, remain unmodified when being passed to helpers taking trusted
+ * args.
  *
- * Consider
- *	struct foo {
- *		int data;
- *		struct foo *next;
- *	};
+ * Consider, for example, the following new task tracepoint:
  *
- *	struct bar {
- *		int data;
- *		struct foo f;
- *	};
+ *	SEC("tp_btf/task_newtask")
+ *	int BPF_PROG(new_task_tp, struct task_struct *task, u64 clone_flags)
+ *	{
+ *		...
+ *	}
  *
- *	struct foo *f = alloc_foo(); // Acquire kfunc
- *	struct bar *b = alloc_bar(); // Acquire kfunc
+ * And the following kfunc:
  *
- * If a kfunc set_foo_data() wants to operate only on the allocated object, it
- * will set the KF_TRUSTED_ARGS flag, which will prevent unsafe usage like:
+ *	BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
  *
- *	set_foo_data(f, 42);	   // Allowed
- *	set_foo_data(f->next, 42); // Rejected, non-referenced pointer
- *	set_foo_data(&f->next, 42);// Rejected, referenced, but wrong type
- *	set_foo_data(&b->f, 42);   // Rejected, referenced, but bad offset
+ * All invocations to the kfunc must pass the unmodified, unwalked task:
  *
- * In the final case, usually for the purposes of type matching, it is deduced
- * by looking at the type of the member at the offset, but due to the
- * requirement of trusted argument, this deduction will be strict and not done
- * for this case.
+ *	bpf_task_acquire(task);		    // Allowed
+ *	bpf_task_acquire(task->last_wakee); // Rejected, walked task
+ *
+ * Programs may also pass referenced tasks directly to the kfunc:
+ *
+ *	struct task_struct *acquired;
+ *
+ *	acquired = bpf_task_acquire(task);	// Allowed, same as above
+ *	bpf_task_acquire(acquired);		// Allowed
+ *	bpf_task_acquire(task);			// Allowed
+ *	bpf_task_acquire(acquired->last_wakee); // Rejected, walked task
+ *
+ * Programs may _not_, however, pass a task from an arbitrary fentry/fexit, or
+ * kprobe/kretprobe to the kfunc, as BPF cannot guarantee that all of these
+ * pointers are guaranteed to be safe. For example, the following BPF program
+ * would be rejected:
+ *
+ * SEC("kretprobe/free_task")
+ * int BPF_PROG(free_task_probe, struct task_struct *tsk)
+ * {
+ *	struct task_struct *acquired;
+ *
+ *	acquired = bpf_task_acquire(acquired); // Rejected, not a trusted pointer
+ *	bpf_task_release(acquired);
+ *
+ *	return 0;
+ * }
  */
 #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
 #define KF_SLEEPABLE    (1 << 5) /* kfunc may sleep */

kernel/bpf/btf.c

@@ -5799,6 +5799,11 @@ static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
 	return nr_args + 1;
 }
 
+static bool prog_type_args_trusted(enum bpf_prog_type prog_type)
+{
+	return prog_type == BPF_PROG_TYPE_TRACING || prog_type == BPF_PROG_TYPE_STRUCT_OPS;
+}
+
 bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		    const struct bpf_prog *prog,
 		    struct bpf_insn_access_aux *info)
@@ -5942,6 +5947,9 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	}
 
 	info->reg_type = PTR_TO_BTF_ID;
+	if (prog_type_args_trusted(prog->type))
+		info->reg_type |= PTR_TRUSTED;
+
 	if (tgt_prog) {
 		enum bpf_prog_type tgt_type;
 

kernel/bpf/helpers.c

@@ -1824,6 +1824,63 @@ struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head)
 	return __bpf_list_del(head, true);
 }
 
+/**
+ * bpf_task_acquire - Acquire a reference to a task. A task acquired by this
+ * kfunc which is not stored in a map as a kptr, must be released by calling
+ * bpf_task_release().
+ * @p: The task on which a reference is being acquired.
+ */
+struct task_struct *bpf_task_acquire(struct task_struct *p)
+{
+	refcount_inc(&p->rcu_users);
+	return p;
+}
+
+/**
+ * bpf_task_kptr_get - Acquire a reference on a struct task_struct kptr. A task
+ * kptr acquired by this kfunc which is not subsequently stored in a map, must
+ * be released by calling bpf_task_release().
+ * @pp: A pointer to a task kptr on which a reference is being acquired.
+ */
+struct task_struct *bpf_task_kptr_get(struct task_struct **pp)
+{
+	struct task_struct *p;
+
+	rcu_read_lock();
+	p = READ_ONCE(*pp);
+
+	/* Another context could remove the task from the map and release it at
+	 * any time, including after we've done the lookup above. This is safe
+	 * because we're in an RCU read region, so the task is guaranteed to
+	 * remain valid until at least the rcu_read_unlock() below.
+	 */
+	if (p && !refcount_inc_not_zero(&p->rcu_users))
+		/* If the task had been removed from the map and freed as
+		 * described above, refcount_inc_not_zero() will return false.
+		 * The task will be freed at some point after the current RCU
+		 * gp has ended, so just return NULL to the user.
+		 */
+		p = NULL;
+	rcu_read_unlock();
+
+	return p;
+}
+
+/**
+ * bpf_task_release - Release the reference acquired on a struct task_struct *.
+ * If this kfunc is invoked in an RCU read region, the task_struct is
+ * guaranteed to not be freed until the current grace period has ended, even if
+ * its refcount drops to 0.
+ * @p: The task on which a reference is being released.
+ */
+void bpf_task_release(struct task_struct *p)
+{
+	if (!p)
+		return;
+
+	put_task_struct_rcu_user(p);
+}
+
 __diag_pop();
 
 BTF_SET8_START(generic_btf_ids)
@@ -1836,6 +1893,9 @@ BTF_ID_FLAGS(func, bpf_list_push_front)
 BTF_ID_FLAGS(func, bpf_list_push_back)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 BTF_SET8_END(generic_btf_ids)
 
 static const struct btf_kfunc_id_set generic_kfunc_set = {
@@ -1843,14 +1903,26 @@ static const struct btf_kfunc_id_set generic_kfunc_set = {
 	.set   = &generic_btf_ids,
 };
 
+BTF_ID_LIST(generic_dtor_ids)
+BTF_ID(struct, task_struct)
+BTF_ID(func, bpf_task_release)
+
 static int __init kfunc_init(void)
 {
 	int ret;
+	const struct btf_id_dtor_kfunc generic_dtors[] = {
+		{
+			.btf_id       = generic_dtor_ids[0],
+			.kfunc_btf_id = generic_dtor_ids[1]
+		},
+	};
 
 	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &generic_kfunc_set);
-	if (ret)
-		return ret;
-	return register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &generic_kfunc_set);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &generic_kfunc_set);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &generic_kfunc_set);
+	return ret ?: register_btf_id_dtor_kfuncs(generic_dtors,
+						  ARRAY_SIZE(generic_dtors),
+						  THIS_MODULE);
 }
 
 late_initcall(kfunc_init);

kernel/bpf/verifier.c

@@ -557,7 +557,7 @@ static bool is_cmpxchg_insn(const struct bpf_insn *insn)
 static const char *reg_type_str(struct bpf_verifier_env *env,
 				enum bpf_reg_type type)
 {
-	char postfix[16] = {0}, prefix[32] = {0};
+	char postfix[16] = {0}, prefix[64] = {0};
 	static const char * const str[] = {
 		[NOT_INIT]		= "?",
 		[SCALAR_VALUE]		= "scalar",
@@ -589,16 +589,14 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
 			strncpy(postfix, "_or_null", 16);
 	}
 
-	if (type & MEM_RDONLY)
-		strncpy(prefix, "rdonly_", 32);
-	if (type & MEM_RINGBUF)
-		strncpy(prefix, "ringbuf_", 32);
-	if (type & MEM_USER)
-		strncpy(prefix, "user_", 32);
-	if (type & MEM_PERCPU)
-		strncpy(prefix, "percpu_", 32);
-	if (type & PTR_UNTRUSTED)
-		strncpy(prefix, "untrusted_", 32);
+	snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s",
+		 type & MEM_RDONLY ? "rdonly_" : "",
+		 type & MEM_RINGBUF ? "ringbuf_" : "",
+		 type & MEM_USER ? "user_" : "",
+		 type & MEM_PERCPU ? "percpu_" : "",
+		 type & PTR_UNTRUSTED ? "untrusted_" : "",
+		 type & PTR_TRUSTED ? "trusted_" : ""
+	);
 
 	snprintf(env->type_str_buf, TYPE_STR_BUF_LEN, "%s%s%s",
 		 prefix, str[base_type(type)], postfix);
@@ -3859,7 +3857,7 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
 			       struct bpf_reg_state *reg, u32 regno)
 {
 	const char *targ_name = kernel_type_name(kptr_field->kptr.btf, kptr_field->kptr.btf_id);
-	int perm_flags = PTR_MAYBE_NULL;
+	int perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED;
 	const char *reg_name = "";
 
 	/* Only unreferenced case accepts untrusted pointers */
@@ -4735,6 +4733,9 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 	if (type_flag(reg->type) & PTR_UNTRUSTED)
 		flag |= PTR_UNTRUSTED;
 
+	/* Any pointer obtained from walking a trusted pointer is no longer trusted. */
+	flag &= ~PTR_TRUSTED;
+
 	if (atype == BPF_READ && value_regno >= 0)
 		mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag);
 
@@ -5847,6 +5848,7 @@ static const struct bpf_reg_types btf_id_sock_common_types = {
 		PTR_TO_TCP_SOCK,
 		PTR_TO_XDP_SOCK,
 		PTR_TO_BTF_ID,
+		PTR_TO_BTF_ID | PTR_TRUSTED,
 	},
 	.btf_id = &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON],
 };
@@ -5887,8 +5889,18 @@ static const struct bpf_reg_types scalar_types = { .types = { SCALAR_VALUE } };
 static const struct bpf_reg_types context_types = { .types = { PTR_TO_CTX } };
 static const struct bpf_reg_types ringbuf_mem_types = { .types = { PTR_TO_MEM | MEM_RINGBUF } };
 static const struct bpf_reg_types const_map_ptr_types = { .types = { CONST_PTR_TO_MAP } };
-static const struct bpf_reg_types btf_ptr_types = { .types = { PTR_TO_BTF_ID } };
-static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_BTF_ID | MEM_PERCPU } };
+static const struct bpf_reg_types btf_ptr_types = {
+	.types = {
+		PTR_TO_BTF_ID,
+		PTR_TO_BTF_ID | PTR_TRUSTED,
+	},
+};
+static const struct bpf_reg_types percpu_btf_ptr_types = {
+	.types = {
+		PTR_TO_BTF_ID | MEM_PERCPU,
+		PTR_TO_BTF_ID | MEM_PERCPU | PTR_TRUSTED,
+	}
+};
 static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
 static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
 static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
@@ -5976,7 +5988,7 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 	return -EACCES;
 
 found:
-	if (reg->type == PTR_TO_BTF_ID) {
+	if (reg->type == PTR_TO_BTF_ID || reg->type & PTR_TRUSTED) {
 		/* For bpf_sk_release, it needs to match against first member
 		 * 'struct sock_common', hence make an exception for it. This
 		 * allows bpf_sk_release to work for multiple socket types.
@@ -6058,6 +6070,8 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	 */
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | MEM_ALLOC:
+	case PTR_TO_BTF_ID | PTR_TRUSTED:
+	case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
 		/* When referenced PTR_TO_BTF_ID is passed to release function,
 		 * it's fixed offset must be 0.	In the other cases, fixed offset
 		 * can be non-zero.
@@ -7942,6 +7956,25 @@ static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
 	return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
 }
 
+static bool is_trusted_reg(const struct bpf_reg_state *reg)
+{
+	/* A referenced register is always trusted. */
+	if (reg->ref_obj_id)
+		return true;
+
+	/* If a register is not referenced, it is trusted if it has either the
+	 * MEM_ALLOC or PTR_TRUSTED type modifiers, and no others. Some of the
+	 * other type modifiers may be safe, but we elect to take an opt-in
+	 * approach here as some (e.g. PTR_UNTRUSTED and PTR_MAYBE_NULL) are
+	 * not.
+	 *
+	 * Eventually, we should make PTR_TRUSTED the single source of truth
+	 * for whether a register is trusted.
+	 */
+	return type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS &&
+	       !bpf_type_has_unsafe_modifiers(reg->type);
+}
+
 static bool __kfunc_param_match_suffix(const struct btf *btf,
 				       const struct btf_param *arg,
 				       const char *suffix)
@@ -8223,7 +8256,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
 	const char *reg_ref_tname;
 	u32 reg_ref_id;
 
-	if (reg->type == PTR_TO_BTF_ID) {
+	if (base_type(reg->type) == PTR_TO_BTF_ID) {
 		reg_btf = reg->btf;
 		reg_ref_id = reg->btf_id;
 	} else {
@@ -8369,6 +8402,7 @@ static int check_reg_allocation_locked(struct bpf_verifier_env *env, struct bpf_
 		ptr = reg->map_ptr;
 		break;
 	case PTR_TO_BTF_ID | MEM_ALLOC:
+	case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
 		ptr = reg->btf;
 		break;
 	default:
@@ -8599,8 +8633,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_BTF_ID:
 			if (!is_kfunc_trusted_args(meta))
 				break;
-			if (!reg->ref_obj_id) {
-				verbose(env, "R%d must be referenced\n", regno);
+
+			if (!is_trusted_reg(reg)) {
+				verbose(env, "R%d must be referenced or trusted\n", regno);
 				return -EINVAL;
 			}
 			fallthrough;
@@ -8705,9 +8740,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			break;
 		case KF_ARG_PTR_TO_BTF_ID:
 			/* Only base_type is checked, further checks are done here */
-			if (reg->type != PTR_TO_BTF_ID &&
-			    (!reg2btf_ids[base_type(reg->type)] || type_flag(reg->type))) {
-				verbose(env, "arg#%d expected pointer to btf or socket\n", i);
+			if ((base_type(reg->type) != PTR_TO_BTF_ID ||
+			     bpf_type_has_unsafe_modifiers(reg->type)) &&
+			    !reg2btf_ids[base_type(reg->type)]) {
+				verbose(env, "arg#%d is %s ", i, reg_type_str(env, reg->type));
+				verbose(env, "expected %s or socket\n",
+					reg_type_str(env, base_type(reg->type) |
+							  (type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS)));
 				return -EINVAL;
 			}
 			ret = process_kf_arg_ptr_to_btf_id(env, reg, ref_t, ref_tname, ref_id, meta, i);
@@ -14716,6 +14755,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 			break;
 		case PTR_TO_BTF_ID:
 		case PTR_TO_BTF_ID | PTR_UNTRUSTED:
+		case PTR_TO_BTF_ID | PTR_TRUSTED:
 		/* PTR_TO_BTF_ID | MEM_ALLOC always has a valid lifetime, unlike
 		 * PTR_TO_BTF_ID, and an active ref_obj_id, but the same cannot
 		 * be said once it is marked PTR_UNTRUSTED, hence we must handle
@@ -14723,6 +14763,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		 * for this case.
 		 */
 		case PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED:
+		case PTR_TO_BTF_ID | PTR_UNTRUSTED | PTR_TRUSTED:
+		case PTR_TO_BTF_ID | PTR_UNTRUSTED | MEM_ALLOC | PTR_TRUSTED:
 			if (type == BPF_READ) {
 				insn->code = BPF_LDX | BPF_PROBE_MEM |
 					BPF_SIZE((insn)->code);

kernel/trace/bpf_trace.c

@@ -774,7 +774,7 @@ BPF_CALL_0(bpf_get_current_task_btf)
 const struct bpf_func_proto bpf_get_current_task_btf_proto = {
 	.func		= bpf_get_current_task_btf,
 	.gpl_only	= true,
-	.ret_type	= RET_PTR_TO_BTF_ID,
+	.ret_type	= RET_PTR_TO_BTF_ID_TRUSTED,
 	.ret_btf_id	= &btf_tracing_ids[BTF_TRACING_TYPE_TASK],
 };
 

net/ipv4/bpf_tcp_ca.c

@@ -61,7 +61,9 @@ static bool bpf_tcp_ca_is_valid_access(int off, int size,
 	if (!bpf_tracing_btf_ctx_access(off, size, type, prog, info))
 		return false;
 
-	if (info->reg_type == PTR_TO_BTF_ID && info->btf_id == sock_id)
+	if (base_type(info->reg_type) == PTR_TO_BTF_ID &&
+	    !bpf_type_has_unsafe_modifiers(info->reg_type) &&
+	    info->btf_id == sock_id)
 		/* promote it to tcp_sock */
 		info->btf_id = tcp_sock_id;
 

tools/testing/selftests/bpf/DENYLIST.s390x

@@ -54,6 +54,7 @@ skc_to_unix_sock                         # could not attach BPF object unexpecte
 socket_cookie                            # prog_attach unexpected error: -524                                          (trampoline)
 stacktrace_build_id                      # compare_map_keys stackid_hmap vs. stackmap err -2 errno 2                   (?)
 tailcalls                                # tail_calls are not allowed in non-JITed programs with bpf-to-bpf calls      (?)
+task_kfunc                               # JIT does not support calling kernel function
 task_local_storage                       # failed to auto-attach program 'trace_exit_creds': -524                      (trampoline)
 test_bpffs                               # bpffs test  failed 255                                                      (iterator)
 test_bprm_opts                           # failed to auto-attach program 'secure_exec': -524                           (trampoline)

tools/testing/selftests/bpf/prog_tests/task_kfunc.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#define _GNU_SOURCE
+#include <sys/wait.h>
+#include <test_progs.h>
+#include <unistd.h>
+
+#include "task_kfunc_failure.skel.h"
+#include "task_kfunc_success.skel.h"
+
+static size_t log_buf_sz = 1 << 20; /* 1 MB */
+static char obj_log_buf[1048576];
+
+static struct task_kfunc_success *open_load_task_kfunc_skel(void)
+{
+	struct task_kfunc_success *skel;
+	int err;
+
+	skel = task_kfunc_success__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return NULL;
+
+	skel->bss->pid = getpid();
+
+	err = task_kfunc_success__load(skel);
+	if (!ASSERT_OK(err, "skel_load"))
+		goto cleanup;
+
+	return skel;
+
+cleanup:
+	task_kfunc_success__destroy(skel);
+	return NULL;
+}
+
+static void run_success_test(const char *prog_name)
+{
+	struct task_kfunc_success *skel;
+	int status;
+	pid_t child_pid;
+	struct bpf_program *prog;
+	struct bpf_link *link = NULL;
+
+	skel = open_load_task_kfunc_skel();
+	if (!ASSERT_OK_PTR(skel, "open_load_skel"))
+		return;
+
+	if (!ASSERT_OK(skel->bss->err, "pre_spawn_err"))
+		goto cleanup;
+
+	prog = bpf_object__find_program_by_name(skel->obj, prog_name);
+	if (!ASSERT_OK_PTR(prog, "bpf_object__find_program_by_name"))
+		goto cleanup;
+
+	link = bpf_program__attach(prog);
+	if (!ASSERT_OK_PTR(link, "attached_link"))
+		goto cleanup;
+
+	child_pid = fork();
+	if (!ASSERT_GT(child_pid, -1, "child_pid"))
+		goto cleanup;
+	if (child_pid == 0)
+		_exit(0);
+	waitpid(child_pid, &status, 0);
+
+	ASSERT_OK(skel->bss->err, "post_wait_err");
+
+cleanup:
+	bpf_link__destroy(link);
+	task_kfunc_success__destroy(skel);
+}
+
+static const char * const success_tests[] = {
+	"test_task_acquire_release_argument",
+	"test_task_acquire_release_current",
+	"test_task_acquire_leave_in_map",
+	"test_task_xchg_release",
+	"test_task_get_release",
+	"test_task_current_acquire_release",
+};
+
+static struct {
+	const char *prog_name;
+	const char *expected_err_msg;
+} failure_tests[] = {
+	{"task_kfunc_acquire_untrusted", "R1 must be referenced or trusted"},
+	{"task_kfunc_acquire_fp", "arg#0 pointer type STRUCT task_struct must point"},
+	{"task_kfunc_acquire_unsafe_kretprobe", "reg type unsupported for arg#0 function"},
+	{"task_kfunc_acquire_trusted_walked", "R1 must be referenced or trusted"},
+	{"task_kfunc_acquire_null", "arg#0 pointer type STRUCT task_struct must point"},
+	{"task_kfunc_acquire_unreleased", "Unreleased reference"},
+	{"task_kfunc_get_non_kptr_param", "arg#0 expected pointer to map value"},
+	{"task_kfunc_get_non_kptr_acquired", "arg#0 expected pointer to map value"},
+	{"task_kfunc_get_null", "arg#0 expected pointer to map value"},
+	{"task_kfunc_xchg_unreleased", "Unreleased reference"},
+	{"task_kfunc_get_unreleased", "Unreleased reference"},
+	{"task_kfunc_release_untrusted", "arg#0 is untrusted_ptr_or_null_ expected ptr_ or socket"},
+	{"task_kfunc_release_fp", "arg#0 pointer type STRUCT task_struct must point"},
+	{"task_kfunc_release_null", "arg#0 is ptr_or_null_ expected ptr_ or socket"},
+	{"task_kfunc_release_unacquired", "release kernel function bpf_task_release expects"},
+};
+
+static void verify_fail(const char *prog_name, const char *expected_err_msg)
+{
+	LIBBPF_OPTS(bpf_object_open_opts, opts);
+	struct task_kfunc_failure *skel;
+	int err, i;
+
+	opts.kernel_log_buf = obj_log_buf;
+	opts.kernel_log_size = log_buf_sz;
+	opts.kernel_log_level = 1;
+
+	skel = task_kfunc_failure__open_opts(&opts);
+	if (!ASSERT_OK_PTR(skel, "task_kfunc_failure__open_opts"))
+		goto cleanup;
+
+	for (i = 0; i < ARRAY_SIZE(failure_tests); i++) {
+		struct bpf_program *prog;
+		const char *curr_name = failure_tests[i].prog_name;
+
+		prog = bpf_object__find_program_by_name(skel->obj, curr_name);
+		if (!ASSERT_OK_PTR(prog, "bpf_object__find_program_by_name"))
+			goto cleanup;
+
+		bpf_program__set_autoload(prog, !strcmp(curr_name, prog_name));
+	}
+
+	err = task_kfunc_failure__load(skel);
+	if (!ASSERT_ERR(err, "unexpected load success"))
+		goto cleanup;
+
+	if (!ASSERT_OK_PTR(strstr(obj_log_buf, expected_err_msg), "expected_err_msg")) {
+		fprintf(stderr, "Expected err_msg: %s\n", expected_err_msg);
+		fprintf(stderr, "Verifier output: %s\n", obj_log_buf);
+	}
+
+cleanup:
+	task_kfunc_failure__destroy(skel);
+}
+
+void test_task_kfunc(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(success_tests); i++) {
+		if (!test__start_subtest(success_tests[i]))
+			continue;
+
+		run_success_test(success_tests[i]);
+	}
+
+	for (i = 0; i < ARRAY_SIZE(failure_tests); i++) {
+		if (!test__start_subtest(failure_tests[i].prog_name))
+			continue;
+
+		verify_fail(failure_tests[i].prog_name, failure_tests[i].expected_err_msg);
+	}
+}

tools/testing/selftests/bpf/progs/task_kfunc_common.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#ifndef _TASK_KFUNC_COMMON_H
+#define _TASK_KFUNC_COMMON_H
+
+#include <errno.h>
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+struct __tasks_kfunc_map_value {
+	struct task_struct __kptr_ref * task;
+};
+
+struct hash_map {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__type(key, int);
+	__type(value, struct __tasks_kfunc_map_value);
+	__uint(max_entries, 1);
+} __tasks_kfunc_map SEC(".maps");
+
+struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
+struct task_struct *bpf_task_kptr_get(struct task_struct **pp) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+static inline struct __tasks_kfunc_map_value *tasks_kfunc_map_value_lookup(struct task_struct *p)
+{
+	s32 pid;
+	long status;
+
+	status = bpf_probe_read_kernel(&pid, sizeof(pid), &p->pid);
+	if (status)
+		return NULL;
+
+	return bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
+}
+
+static inline int tasks_kfunc_map_insert(struct task_struct *p)
+{
+	struct __tasks_kfunc_map_value local, *v;
+	long status;
+	struct task_struct *acquired, *old;
+	s32 pid;
+
+	status = bpf_probe_read_kernel(&pid, sizeof(pid), &p->pid);
+	if (status)
+		return status;
+
+	local.task = NULL;
+	status = bpf_map_update_elem(&__tasks_kfunc_map, &pid, &local, BPF_NOEXIST);
+	if (status)
+		return status;
+
+	v = bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
+	if (!v) {
+		bpf_map_delete_elem(&__tasks_kfunc_map, &pid);
+		return -ENOENT;
+	}
+
+	acquired = bpf_task_acquire(p);
+	old = bpf_kptr_xchg(&v->task, acquired);
+	if (old) {
+		bpf_task_release(old);
+		return -EEXIST;
+	}
+
+	return 0;
+}
+
+#endif /* _TASK_KFUNC_COMMON_H */

tools/testing/selftests/bpf/progs/task_kfunc_failure.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+#include "task_kfunc_common.h"
+
+char _license[] SEC("license") = "GPL";
+
+/* Prototype for all of the program trace events below:
+ *
+ * TRACE_EVENT(task_newtask,
+ *         TP_PROTO(struct task_struct *p, u64 clone_flags)
+ */
+
+static struct __tasks_kfunc_map_value *insert_lookup_task(struct task_struct *task)
+{
+	int status;
+
+	status = tasks_kfunc_map_insert(task);
+	if (status)
+		return NULL;
+
+	return tasks_kfunc_map_value_lookup(task);
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_acquire_untrusted, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired;
+	struct __tasks_kfunc_map_value *v;
+
+	v = insert_lookup_task(task);
+	if (!v)
+		return 0;
+
+	/* Can't invoke bpf_task_acquire() on an untrusted pointer. */
+	acquired = bpf_task_acquire(v->task);
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_acquire_fp, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired, *stack_task = (struct task_struct *)&clone_flags;
+
+	/* Can't invoke bpf_task_acquire() on a random frame pointer. */
+	acquired = bpf_task_acquire((struct task_struct *)&stack_task);
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("kretprobe/free_task")
+int BPF_PROG(task_kfunc_acquire_unsafe_kretprobe, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired;
+
+	acquired = bpf_task_acquire(task);
+	/* Can't release a bpf_task_acquire()'d task without a NULL check. */
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_acquire_trusted_walked, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired;
+
+	/* Can't invoke bpf_task_acquire() on a trusted pointer obtained from walking a struct. */
+	acquired = bpf_task_acquire(task->last_wakee);
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_acquire_null, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired;
+
+	/* Can't invoke bpf_task_acquire() on a NULL pointer. */
+	acquired = bpf_task_acquire(NULL);
+	if (!acquired)
+		return 0;
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_acquire_unreleased, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired;
+
+	acquired = bpf_task_acquire(task);
+
+	/* Acquired task is never released. */
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_get_non_kptr_param, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+
+	/* Cannot use bpf_task_kptr_get() on a non-kptr, even on a valid task. */
+	kptr = bpf_task_kptr_get(&task);
+	if (!kptr)
+		return 0;
+
+	bpf_task_release(kptr);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_get_non_kptr_acquired, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr, *acquired;
+
+	acquired = bpf_task_acquire(task);
+
+	/* Cannot use bpf_task_kptr_get() on a non-kptr, even if it was acquired. */
+	kptr = bpf_task_kptr_get(&acquired);
+	bpf_task_release(acquired);
+	if (!kptr)
+		return 0;
+
+	bpf_task_release(kptr);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_get_null, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+
+	/* Cannot use bpf_task_kptr_get() on a NULL pointer. */
+	kptr = bpf_task_kptr_get(NULL);
+	if (!kptr)
+		return 0;
+
+	bpf_task_release(kptr);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_xchg_unreleased, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+	struct __tasks_kfunc_map_value *v;
+
+	v = insert_lookup_task(task);
+	if (!v)
+		return 0;
+
+	kptr = bpf_kptr_xchg(&v->task, NULL);
+	if (!kptr)
+		return 0;
+
+	/* Kptr retrieved from map is never released. */
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_get_unreleased, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+	struct __tasks_kfunc_map_value *v;
+
+	v = insert_lookup_task(task);
+	if (!v)
+		return 0;
+
+	kptr = bpf_task_kptr_get(&v->task);
+	if (!kptr)
+		return 0;
+
+	/* Kptr acquired above is never released. */
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_release_untrusted, struct task_struct *task, u64 clone_flags)
+{
+	struct __tasks_kfunc_map_value *v;
+
+	v = insert_lookup_task(task);
+	if (!v)
+		return 0;
+
+	/* Can't invoke bpf_task_release() on an untrusted pointer. */
+	bpf_task_release(v->task);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_release_fp, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired = (struct task_struct *)&clone_flags;
+
+	/* Cannot release random frame pointer. */
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_release_null, struct task_struct *task, u64 clone_flags)
+{
+	struct __tasks_kfunc_map_value local, *v;
+	long status;
+	struct task_struct *acquired, *old;
+	s32 pid;
+
+	status = bpf_probe_read_kernel(&pid, sizeof(pid), &task->pid);
+	if (status)
+		return 0;
+
+	local.task = NULL;
+	status = bpf_map_update_elem(&__tasks_kfunc_map, &pid, &local, BPF_NOEXIST);
+	if (status)
+		return status;
+
+	v = bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
+	if (!v)
+		return -ENOENT;
+
+	acquired = bpf_task_acquire(task);
+
+	old = bpf_kptr_xchg(&v->task, acquired);
+
+	/* old cannot be passed to bpf_task_release() without a NULL check. */
+	bpf_task_release(old);
+	bpf_task_release(old);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(task_kfunc_release_unacquired, struct task_struct *task, u64 clone_flags)
+{
+	/* Cannot release trusted task pointer which was not acquired. */
+	bpf_task_release(task);
+
+	return 0;
+}

tools/testing/selftests/bpf/progs/task_kfunc_success.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+#include "task_kfunc_common.h"
+
+char _license[] SEC("license") = "GPL";
+
+int err, pid;
+
+/* Prototype for all of the program trace events below:
+ *
+ * TRACE_EVENT(task_newtask,
+ *         TP_PROTO(struct task_struct *p, u64 clone_flags)
+ */
+
+static bool is_test_kfunc_task(void)
+{
+	int cur_pid = bpf_get_current_pid_tgid() >> 32;
+
+	return pid == cur_pid;
+}
+
+static int test_acquire_release(struct task_struct *task)
+{
+	struct task_struct *acquired;
+
+	acquired = bpf_task_acquire(task);
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_acquire_release_argument, struct task_struct *task, u64 clone_flags)
+{
+	if (!is_test_kfunc_task())
+		return 0;
+
+	return test_acquire_release(task);
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_acquire_release_current, struct task_struct *task, u64 clone_flags)
+{
+	if (!is_test_kfunc_task())
+		return 0;
+
+	return test_acquire_release(bpf_get_current_task_btf());
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_acquire_leave_in_map, struct task_struct *task, u64 clone_flags)
+{
+	long status;
+
+	if (!is_test_kfunc_task())
+		return 0;
+
+	status = tasks_kfunc_map_insert(task);
+	if (status)
+		err = 1;
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_xchg_release, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+	struct __tasks_kfunc_map_value *v;
+	long status;
+
+	if (!is_test_kfunc_task())
+		return 0;
+
+	status = tasks_kfunc_map_insert(task);
+	if (status) {
+		err = 1;
+		return 0;
+	}
+
+	v = tasks_kfunc_map_value_lookup(task);
+	if (!v) {
+		err = 2;
+		return 0;
+	}
+
+	kptr = bpf_kptr_xchg(&v->task, NULL);
+	if (!kptr) {
+		err = 3;
+		return 0;
+	}
+
+	bpf_task_release(kptr);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_get_release, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *kptr;
+	struct __tasks_kfunc_map_value *v;
+	long status;
+
+	if (!is_test_kfunc_task())
+		return 0;
+
+	status = tasks_kfunc_map_insert(task);
+	if (status) {
+		err = 1;
+		return 0;
+	}
+
+	v = tasks_kfunc_map_value_lookup(task);
+	if (!v) {
+		err = 2;
+		return 0;
+	}
+
+	kptr = bpf_task_kptr_get(&v->task);
+	if (!kptr) {
+		err = 3;
+		return 0;
+	}
+
+	bpf_task_release(kptr);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_current_acquire_release, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *current, *acquired;
+
+	if (!is_test_kfunc_task())
+		return 0;
+
+	current = bpf_get_current_task_btf();
+	acquired = bpf_task_acquire(current);
+	bpf_task_release(acquired);
+
+	return 0;
+}

tools/testing/selftests/bpf/verifier/calls.c

@@ -109,7 +109,7 @@
 	},
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	.result = REJECT,
-	.errstr = "arg#0 expected pointer to btf or socket",
+	.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
 	.fixup_kfunc_btf_id = {
 		{ "bpf_kfunc_call_test_acquire", 3 },
 		{ "bpf_kfunc_call_test_release", 5 },

tools/testing/selftests/bpf/verifier/ref_tracking.c

@@ -142,7 +142,7 @@
 	.kfunc = "bpf",
 	.expected_attach_type = BPF_LSM_MAC,
 	.flags = BPF_F_SLEEPABLE,
-	.errstr = "arg#0 expected pointer to btf or socket",
+	.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
 	.fixup_kfunc_btf_id = {
 		{ "bpf_lookup_user_key", 2 },
 		{ "bpf_key_put", 4 },
@@ -163,7 +163,7 @@
 	.kfunc = "bpf",
 	.expected_attach_type = BPF_LSM_MAC,
 	.flags = BPF_F_SLEEPABLE,
-	.errstr = "arg#0 expected pointer to btf or socket",
+	.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
 	.fixup_kfunc_btf_id = {
 		{ "bpf_lookup_system_key", 1 },
 		{ "bpf_key_put", 3 },