[PATCH bpf-next v3 3/9] bpf: Preserve reg->id of pointer objects after null-check

Amery Hung
2026-04-21T22:10:22Z

Preserve reg->id of pointer objects after null-checking the register so
that children objects derived from it can still refer to it in the new
object relationship tracking mechanism introduced in a later patch. This
change incurs a slight increase in the number of states in one selftest
bpf object, rbtree_search.bpf.o. For Meta bpf objects, the increase of
states is also negligible.

Selftest BPF objects with insns_diff > 0

Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF)
------------------------ --------- --------- -------------- ---------- ---------- -------------
rbtree_search 6820 7326 +506 (+7.42%) 379 398 +19 (+5.01%)

Meta BPF objects with insns_diff > 0

Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF)
------------------------ --------- --------- -------------- ---------- ---------- -------------
ned_imex_be_tclass 52 57 +5 (+9.62%) 5 6 +1 (+20.00%)
ned_imex_be_tclass 52 57 +5 (+9.62%) 5 6 +1 (+20.00%)
ned_skop_auto_flowlabel 523 526 +3 (+0.57%) 39 40 +1 (+2.56%)
ned_skop_mss 289 292 +3 (+1.04%) 20 20 +0 (+0.00%)
ned_skopt_bet_classifier 78 82 +4 (+5.13%) 8 8 +0 (+0.00%)
dctcp_update_alpha 252 320 +68 (+26.98%) 21 27 +6 (+28.57%)
dctcp_update_alpha 252 320 +68 (+26.98%) 21 27 +6 (+28.57%)
ned_ts_func 119 126 +7 (+5.88%) 6 7 +1 (+16.67%)
tw_egress 1119 1128 +9 (+0.80%) 95 96 +1 (+1.05%)
tw_ingress 1128 1137 +9 (+0.80%) 95 96 +1 (+1.05%)
tw_tproxy_router 4380 4465 +85 (+1.94%) 114 118 +4 (+3.51%)
tw_tproxy_router4 3093 3170 +77 (+2.49%) 83 88 +5 (+6.02%)
ttls_tc_ingress 34656 35717 +1061 (+3.06%) 936 970 +34 (+3.63%)
tw_twfw_egress 222327 222338 +11 (+0.00%) 10563 10564 +1 (+0.01%)
tw_twfw_ingress 78295 78299 +4 (+0.01%) 3825 3826 +1 (+0.03%)
tw_twfw_tc_eg 222839 222859 +20 (+0.01%) 10584 10585 +1 (+0.01%)
tw_twfw_tc_in 78295 78299 +4 (+0.01%) 3825 3826 +1 (+0.03%)
tw_twfw_egress 8080 8085 +5 (+0.06%) 456 456 +0 (+0.00%)
tw_twfw_ingress 8053 8056 +3 (+0.04%) 454 454 +0 (+0.00%)
tw_twfw_tc_eg 8154 8174 +20 (+0.25%) 456 457 +1 (+0.22%)
tw_twfw_tc_in 8060 8063 +3 (+0.04%) 455 455 +0 (+0.00%)
tw_twfw_egress 222327 222338 +11 (+0.00%) 10563 10564 +1 (+0.01%)
tw_twfw_ingress 78295 78299 +4 (+0.01%) 3825 3826 +1 (+0.03%)
tw_twfw_tc_eg 222839 222859 +20 (+0.01%) 10584 10585 +1 (+0.01%)
tw_twfw_tc_in 78295 78299 +4 (+0.01%) 3825 3826 +1 (+0.03%)
tw_twfw_egress 8080 8085 +5 (+0.06%) 456 456 +0 (+0.00%)
tw_twfw_ingress 8053 8056 +3 (+0.04%) 454 454 +0 (+0.00%)
tw_twfw_tc_eg 8154 8174 +20 (+0.25%) 456 457 +1 (+0.22%)
tw_twfw_tc_in 8060 8063 +3 (+0.04%) 455 455 +0 (+0.00%)

Looking into rbtree_search, the reason for such increase is that the
verifier has to explore the main loop shown below for one more iteration
until state pruning decides the current state is safe.

long rbtree_search(void *ctx)
{
...
bpf_spin_lock(&glock0);
rb_n = bpf_rbtree_root(&groot0);
while (can_loop) {
if (!rb_n) {
bpf_spin_unlock(&glock0);
return __LINE__;
}

n = rb_entry(rb_n, struct node_data, r0);
if (lookup_key == n->key0)
break;
if (nr_gc < NR_NODES)
gc_ns[nr_gc++] = rb_n;
if (lookup_key < n->key0)
rb_n = bpf_rbtree_left(&groot0, rb_n);
else
rb_n = bpf_rbtree_right(&groot0, rb_n);
}
...
}

Below is what the verifier sees at the start of each iteration
(65: may_goto) after preserving id of rb_n. Without id of rb_n, the
verifier stops exploring the loop at iter 16.

rb_n gc_ns[15]
iter 15 257 257

iter 16 290 257 rb_n: idmap add 257->290
gc_ns[15]: check 257 != 290 --> state not equal

iter 17 325 257 rb_n: idmap add 290->325
gc_ns[15]: idmap add 257->257 --> state safe

Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
kernel/bpf/verifier.c | 13 ++++---------
1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 93003a2a96b0..0313b7d5f6c9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15886,15 +15886,10 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,

mark_ptr_not_null_reg(reg);

- if (!reg_may_point_to_spin_lock(reg)) {
- /* For not-NULL ptr, reg->ref_obj_id will be reset
- * in release_reference().
- *
- * reg->id is still used by spin_lock ptr. Other
- * than spin_lock ptr type, reg->id can be reset.
- */
- reg->id = 0;
- }
+ /*
+ * reg->id is preserved for object relationship tracking
+ * and spin_lock lock state tracking
+ */
}
}

--
2.52.0