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
9d9d00ac29d0 ("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
ab5cfac139ab ("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 <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241109231430.2475236-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

1 files changed, 5 insertions, 20 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d55ca27dc031..9f5de8d4fbd0 100644
--- a/kernel/bpf/verifier.c
+++ b/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)