[PATCH bpf-next v5 4/6] bpf: Inline bpf_iter_num_destroy() as a no-op

[Puranjay Mohan <puranjay@kernel.org>]

Once destroy() returns the stack slot is no longer tracked as iterator
state, so zeroing it is dead work. Make the kfunc a no-op and inline the
call to a single BPF_JA 0 (the fixup can't drop the instruction outright,
so emit a nop; the JITs elide it).

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/bpf_iter.c | 4 +---
 kernel/bpf/verifier.c | 4 ++++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index d19f1b2861d22..14a5fdfa04214 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -818,9 +818,7 @@ __bpf_kfunc int *bpf_iter_num_next(struct bpf_iter_num* it)
 
 __bpf_kfunc void bpf_iter_num_destroy(struct bpf_iter_num *it)
 {
-	struct bpf_iter_num_kern *s = (void *)it;
-
-	s->cur = s->end = 0;
+	/* no-op */
 }
 
 __bpf_kfunc_end_defs();
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6bfd715aaa379..64a142870e5b8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20047,6 +20047,10 @@ int bpf_fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		insn_buf[i++] = BPF_ST_MEM(BPF_DW, BPF_REG_1, 0, 0);
 		insn_buf[i++] = BPF_MOV64_IMM(BPF_REG_0, 0);
 		*cnt = i;
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_iter_num_destroy]) {
+		/* bpf_iter_num_destroy() is a no-op; emit a nop to drop the call */
+		insn_buf[0] = BPF_JMP_A(0);
+		*cnt = 1;
 	}
 
 	if (env->insn_aux_data[insn_idx].arg_prog) {
-- 
2.53.0-Meta