From 6b31560c6bc1a8a7a70792c7b3ca4c1ea322063b Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Fri, 4 Sep 2026 01:33:25 -0700 Subject: [PATCH] selftests/bpf: No non-NULL inference from an imprecise zero register Check that a register-form NULL check does not lift PTR_MAYBE_NULL on a path where the compared register is non-zero. W/o the previous patch the program is accepted. Reported-by: Nicholas Carlini Suggested-by: Nicholas Carlini Signed-off-by: Eduard Zingerman Link: https://lore.kernel.org/bpf/20260904083325.2083493-8-eddyz87@gmail.com Signed-off-by: Kumar Kartikeya Dwivedi --- .../bpf/progs/verifier_jeq_infer_not_null.c | 34 +++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_jeq_infer_not_null.c b/tools/testing/selftests/bpf/progs/verifier_jeq_infer_not_null.c index 410acbf658c7..3c789c565b18 100644 --- a/tools/testing/selftests/bpf/progs/verifier_jeq_infer_not_null.c +++ b/tools/testing/selftests/bpf/progs/verifier_jeq_infer_not_null.c @@ -418,6 +418,40 @@ __naked void jmp32_ptr_vs_zero_jne(void) : __clobber_all); } +/* + * The below program is explored in two paths: r6 == 0 and r6 == 1. + * On the first path comparison "if r0 == r6 goto 2f" should mark r6 as precise, + * otherwise unsafe path with r6 == 1 would be incorrectly pruned. + */ +SEC("socket") +__failure +__flag(BPF_F_TEST_STATE_FREQ) +__msg("error: invalid dereference of R0 (a nullable map value pointer)") +__naked void imprecise_zero_does_not_infer_map_value_non_null(void) +{ + asm volatile (" \ + call %[bpf_get_prandom_u32]; \ + /* r6 is 0 on the path explored first, 1 on the other */\ + r6 = 1; \ + if r0 == 0 goto 1f; \ + r6 = 0; \ + /* r0 = bpf_map_lookup_elem(map_hash, &0); */ \ +1: *(u64 *)(r10 - 8) = 0; \ + r1 = %[map_hash] ll; \ + r2 = r10; \ + r2 += -8; \ + call %[bpf_map_lookup_elem]; \ + if r0 == r6 goto 2f; \ + r0 = *(u8 *)(r0 + 0); \ +2: r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32), + __imm(bpf_map_lookup_elem), + __imm_addr(map_hash) + : __clobber_all); +} + void kfunc_root(void) { bpf_rdonly_cast(0, 0);