Re: [RFC PATCH bpf-next v2 05/11] bpf: Preserve reg->id of pointer objects after null-check
From: Alexei Starovoitov <hidden>
Date: 2026-03-11 22:26:56
Also in:
bpf
On Fri, Mar 6, 2026 at 10:44 PM Amery Hung [off-list ref] wrote:
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
Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF)
--------- --------- ------------- ---------- ---------- -------------
7309 7814 +505 (+6.91%) 394 413 +19 (+4.82%)
Meta BPF objects with insns_diff > 0
Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF)
--------- --------- -------------- ---------- ---------- -------------
52 57 +5 (+9.62%) 5 6 +1 (+20.00%)
52 57 +5 (+9.62%) 5 6 +1 (+20.00%)
676 679 +3 (+0.44%) 54 54 +0 (+0.00%)
289 292 +3 (+1.04%) 20 20 +0 (+0.00%)
78 82 +4 (+5.13%) 8 8 +0 (+0.00%)
252 320 +68 (+26.98%) 21 27 +6 (+28.57%)
252 320 +68 (+26.98%) 21 27 +6 (+28.57%)
119 126 +7 (+5.88%) 6 7 +1 (+16.67%)
1119 1128 +9 (+0.80%) 95 96 +1 (+1.05%)
1128 1137 +9 (+0.80%) 95 96 +1 (+1.05%)
4380 4465 +85 (+1.94%) 114 118 +4 (+3.51%)
3093 3170 +77 (+2.49%) 83 88 +5 (+6.02%)
30181 31224 +1043 (+3.46%) 832 863 +31 (+3.73%)
237608 237619 +11 (+0.00%) 11670 11671 +1 (+0.01%)
94832 94836 +4 (+0.00%) 4787 4788 +1 (+0.02%)
237387 237407 +20 (+0.01%) 11651 11652 +1 (+0.01%)
94832 94836 +4 (+0.00%) 4787 4788 +1 (+0.02%)
8103 8108 +5 (+0.06%) 459 459 +0 (+0.00%)
8076 8079 +3 (+0.04%) 457 457 +0 (+0.00%)
8177 8197 +20 (+0.24%) 459 460 +1 (+0.22%)
8083 8086 +3 (+0.04%) 458 458 +0 (+0.00%)
237608 237619 +11 (+0.00%) 11670 11671 +1 (+0.01%)
94832 94836 +4 (+0.00%) 4787 4788 +1 (+0.02%)
237387 237407 +20 (+0.01%) 11651 11652 +1 (+0.01%)
94832 94836 +4 (+0.00%) 4787 4788 +1 (+0.02%)
8103 8108 +5 (+0.06%) 459 459 +0 (+0.00%)
8076 8079 +3 (+0.04%) 457 457 +0 (+0.00%)
8177 8197 +20 (+0.24%) 459 460 +1 (+0.22%)
8083 8086 +3 (+0.04%) 458 458 +0 (+0.00%)The table is missing names. State columns can be dropped instead.
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 safeI'm not following. The verifier processes above as a bounded loop. All 16 (NR_NODES) iterations. Why presence of id on 'rb_n' makes a difference? It will still process 16 loops. Which insn is safe vs not in the above ? One after gc_ns[nr_gc++] = rb_n ?