bpf: Drop special callback reference handling

Logic to prevent callbacks from acquiring new references for the program
(i.e. leaving acquired references), and releasing caller references
(i.e. those acquired in parent frames) was introduced in commit
9d9d00a ("bpf: Fix reference state management for synchronous callbacks").

This was necessary because back then, the verifier simulated each
callback once (that could potentially be executed N times, where N can
be zero). This meant that callbacks that left lingering resources or
cleared caller resources could do it more than once, operating on
undefined state or leaking memory.

With the fixes to callback verification in commit
ab5cfac ("bpf: verify callbacks as if they are called unknown number of times"),
all of this extra logic is no longer necessary. Hence, drop it as part
of this commit.

Cc: Eduard Zingerman <[email protected]>
Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>

Author: kkdwivedi

include/linux/bpf_verifier.h

@@ -265,23 +265,10 @@ struct bpf_reference_state {
 	 * is used purely to inform the user of a reference leak.
 	 */
 	int insn_idx;
-	union {
-		/* There can be a case like:
-		 * main (frame 0)
-		 *  cb (frame 1)
-		 *   func (frame 3)
-		 *    cb (frame 4)
-		 * Hence for frame 4, if callback_ref just stored boolean, it would be
-		 * impossible to distinguish nested callback refs. Hence store the
-		 * frameno and compare that to callback_ref in check_reference_leak when
-		 * exiting a callback function.
-		 */
-		int callback_ref;
-		/* Use to keep track of the source object of a lock, to ensure
-		 * it matches on unlock.
-		 */
-		void *ptr;
-	};
+	/* Use to keep track of the source object of a lock, to ensure
+	 * it matches on unlock.
+	 */
+	void *ptr;
 };
 
 struct bpf_retval_range {

kernel/bpf/verifier.c

@@ -1358,7 +1358,6 @@ static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx)
 	state->refs[new_ofs].type = REF_TYPE_PTR;
 	state->refs[new_ofs].id = id;
 	state->refs[new_ofs].insn_idx = insn_idx;
-	state->refs[new_ofs].callback_ref = state->in_callback_fn ? state->frameno : 0;
 
 	return id;
 }
@@ -1392,9 +1391,6 @@ static int release_reference_state(struct bpf_func_state *state, int ptr_id)
 		if (state->refs[i].type != REF_TYPE_PTR)
 			continue;
 		if (state->refs[i].id == ptr_id) {
-			/* Cannot release caller references in callbacks */
-			if (state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
-				return -EINVAL;
 			if (last_idx && i != last_idx)
 				memcpy(&state->refs[i], &state->refs[last_idx],
 				       sizeof(*state->refs));
@@ -10267,17 +10263,10 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 		caller->regs[BPF_REG_0] = *r0;
 	}
 
-	/* callback_fn frame should have released its own additions to parent's
-	 * reference state at this point, or check_reference_leak would
-	 * complain, hence it must be the same as the caller. There is no need
-	 * to copy it back.
-	 */
-	if (!callee->in_callback_fn) {
-		/* Transfer references to the caller */
-		err = copy_reference_state(caller, callee);
-		if (err)
-			return err;
-	}
+	/* Transfer references to the caller */
+	err = copy_reference_state(caller, callee);
+	if (err)
+		return err;
 
 	/* for callbacks like bpf_loop or bpf_for_each_map_elem go back to callsite,
 	 * there function call logic would reschedule callback visit. If iteration
@@ -10447,14 +10436,12 @@ static int check_reference_leak(struct bpf_verifier_env *env, bool exception_exi
 	bool refs_lingering = false;
 	int i;
 
-	if (!exception_exit && state->frameno && !state->in_callback_fn)
+	if (!exception_exit && state->frameno)
 		return 0;
 
 	for (i = 0; i < state->acquired_refs; i++) {
 		if (state->refs[i].type != REF_TYPE_PTR)
 			continue;
-		if (!exception_exit && state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
-			continue;
 		verbose(env, "Unreleased reference id=%d alloc_insn=%d\n",
 			state->refs[i].id, state->refs[i].insn_idx);
 		refs_lingering = true;
@@ -17707,8 +17694,6 @@ static bool refsafe(struct bpf_func_state *old, struct bpf_func_state *cur,
 			return false;
 		switch (old->refs[i].type) {
 		case REF_TYPE_PTR:
-			if (old->refs[i].callback_ref != cur->refs[i].callback_ref)
-				return false;
 			break;
 		case REF_TYPE_LOCK:
 			if (old->refs[i].ptr != cur->refs[i].ptr)

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

@@ -11,8 +11,8 @@ struct {
 	const char *prog_name;
 	const char *err_msg;
 } cb_refs_tests[] = {
-	{ "underflow_prog", "reference has not been acquired before" },
-	{ "leak_prog", "Unreleased reference" },
+	{ "underflow_prog", "must point to scalar, or struct with scalar" },
+	{ "leak_prog", "Possibly NULL pointer passed to helper arg2" },
 	{ "nested_cb", "Unreleased reference id=4 alloc_insn=2" }, /* alloc_insn=2{4,5} */
 	{ "non_cb_transfer_ref", "Unreleased reference id=4 alloc_insn=1" }, /* alloc_insn=1{1,2} */
 };