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Fri, 04 Sep 2026 01:33:40 -0700 (PDT) Received: from ezingerman-fedora-PF4V722J ([38.34.87.7]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39b083e4fafsm9628057a91.1.2026.09.04.01.33.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Sep 2026 01:33:39 -0700 (PDT) From: Eduard Zingerman To: bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org Cc: daniel@iogearbox.net, martin.lau@linux.dev, kernel-team@fb.com, yonghong.song@linux.dev, eddyz87@gmail.com, memxor@gmail.com, npc@anthropic.com Subject: [PATCH bpf 2/8] selftests/bpf: no non-NULL inference from unbounded offset pointers Date: Fri, 4 Sep 2026 01:33:19 -0700 Message-ID: <20260904083325.2083493-2-eddyz87@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260904083325.2083493-1-eddyz87@gmail.com> References: <20260904083325.2083493-1-eddyz87@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Check that a comparison against a pointer whose offset is not bounded from above does not make the verifier infer that a nullable pointer is not NULL, and that a bounded offset still does. W/o the previous patch the first test is accepted. Reported-by: Nicholas Carlini Suggested-by: Nicholas Carlini Signed-off-by: Eduard Zingerman --- .../bpf/progs/verifier_jeq_infer_not_null.c | 80 +++++++++++++++++++ 1 file changed, 80 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 b412a542ef76..8657e4a0d601 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 @@ -311,6 +311,86 @@ __naked void untrusted_mem_does_not_infer_map_value_non_null(void) : __clobber_all); } +/* + * A pointer with an offset that is not bounded from above may be null at + * runtime, hence it is not a witness for the pointer it is compared with. + */ +SEC("socket") +__failure +__msg("error: invalid dereference of R7 (a nullable map value pointer)") +__naked void unbounded_offset_does_not_infer_map_value_non_null(void) +{ + asm volatile (" \ + /* r6 = bpf_map_lookup_elem(map_hash, &0); */ \ + *(u64 *)(r10 - 8) = 0; \ + r1 = %[map_hash] ll; \ + r2 = r10; \ + r2 += -8; \ + call %[bpf_map_lookup_elem]; \ + if r0 == 0 goto 1f; \ + r6 = r0; \ + /* r7 = bpf_map_lookup_elem(map_hash, &1); */ \ + *(u64 *)(r10 - 8) = 1; \ + r1 = %[map_hash] ll; \ + r2 = r10; \ + r2 += -8; \ + call %[bpf_map_lookup_elem]; \ + r7 = r0; \ + /* pointer - pointer is an unknown scalar */ \ + r8 = r7; \ + r8 -= r6; \ + /* r8 is in [0, S64_MAX] */ \ + r8 <<= 1; \ + r8 >>= 1; \ + /* r6 may wrap to zero at runtime */ \ + r6 += r8; \ + if r7 != r6 goto 1f; \ + r0 = *(u8 *)(r7 + 0); \ +1: r0 = 0; \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_hash) + : __clobber_all); +} + +/* Same, but the offset is bounded, so the inference is still done. */ +SEC("socket") +__success +__naked void bounded_offset_infers_map_value_non_null(void) +{ + asm volatile (" \ + /* r6 = bpf_map_lookup_elem(map_hash, &0); */ \ + *(u64 *)(r10 - 8) = 0; \ + r1 = %[map_hash] ll; \ + r2 = r10; \ + r2 += -8; \ + call %[bpf_map_lookup_elem]; \ + if r0 == 0 goto 1f; \ + r6 = r0; \ + /* r7 = bpf_map_lookup_elem(map_hash, &1); */ \ + *(u64 *)(r10 - 8) = 1; \ + r1 = %[map_hash] ll; \ + r2 = r10; \ + r2 += -8; \ + call %[bpf_map_lookup_elem]; \ + r7 = r0; \ + /* pointer - pointer is an unknown scalar */ \ + r8 = r7; \ + r8 -= r6; \ + /* r8 is in [0, 3] */ \ + r8 &= 3; \ + r6 += r8; \ + if r7 != r6 goto 1f; \ + r0 = *(u8 *)(r7 + 0); \ +1: r0 = 0; \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_hash) + : __clobber_all); +} + void kfunc_root(void) { bpf_rdonly_cast(0, 0); -- 2.55.0