From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4D2383242D2 for ; Tue, 3 Feb 2026 22:27:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770157654; cv=none; b=X6/0MCb2kiU9FFFy36V8w1fgYpIdMk3jxvAACg/KezC/6Kzb3m6ZBx7NhaaC+E6zdu3sAKpOkituicsHVBDaRJ9eKqmg2WcNY8vHAeyllQhVdxl0I2VUiA52f0aWl8yAFQUfltdPTp8cjUmxDnPSysgddmAp4N4fQ00w1UOwAXw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770157654; c=relaxed/simple; bh=xWcU6TYSCGkMYQjpSwHo71rjhAkKpoYVc77GCTEgllE=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=j5jzshxucUUzXRxFO8cffLvUSnvY7MhjCfis/dGkEf81f/UWRrtQcoTw7HVK8sPs2YFk68rdo15py9LurjM6GfmGp+0KNRH0VpB40xCWF27j0ON0/3m8c54U0gXylUWop2jqDqAGTLy9IMDAJkq53Qfap36NachIFY/w3Mt3oXw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ivR7g2Pm; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ivR7g2Pm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A9BF6C116D0; Tue, 3 Feb 2026 22:27:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770157653; bh=xWcU6TYSCGkMYQjpSwHo71rjhAkKpoYVc77GCTEgllE=; h=From:To:Cc:Subject:Date:From; b=ivR7g2PmNuwbPGbQkijKJLYNo296m9Hp5Tmo5uG6Ncz3JhJVbKYHA04uR1r/9RVpp SMiwKdv/FE/nXxfnFLFPuQQtPdFjri9m3piUcS2m701rYrP2Ao3jtPi+EtyO4bkEwG p2rjSXhLGTE5fSVBW7THRDcGF3PAhActqsoMwMmuWlKqmC6P8nfqp9F/c1N49mtSje tRvgFA9v/hdJNzykTvamrW0IjqrjtRcowqTYZL+yWJnGOGlay4LPkVk2navi6DWjR7 ngrsD4XOTEsK4RnKfRzEKEaDCHzCjuJSFknVrwTOfIl6CH4ah5Xz9HvZdk+nuXBwM5 Dj90GJzveK2RA== From: Puranjay Mohan To: bpf@vger.kernel.org Cc: Puranjay Mohan , Puranjay Mohan , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Eduard Zingerman , Kumar Kartikeya Dwivedi , Mykyta Yatsenko , kernel-team@meta.com Subject: [PATCH bpf-next v3 0/2] bpf: Improve linked register tracking Date: Tue, 3 Feb 2026 14:26:16 -0800 Message-ID: <20260203222643.994713-1-puranjay@kernel.org> X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit V2: https://lore.kernel.org/all/20260113152529.3217648-1-puranjay@kernel.org/ Changes in v2->v3: - Added another selftest showing a real usage pattern - Rebased on bpf-next/master v1: https://lore.kernel.org/bpf/20260107203941.1063754-1-puranjay@kernel.org/ Changes in v1->v2: - Add support for alu32 operations in linked register tracking (Alexei) - Squash the selftest fix with the first patch (Eduard) - Add more selftests to detect edge cases This series extends the BPF verifier's linked register tracking to handle negative offsets, BPF_SUB operations, and alu32 operations, enabling better bounds propagation for common arithmetic patterns. The verifier previously only tracked positive constant deltas between linked registers using BPF_ADD. This meant patterns using negative offsets or subtraction couldn't benefit from bounds propagation: void alu32_negative_offset(void) { volatile char path[5]; volatile int offset = bpf_get_prandom_u32(); int off = offset; if (off >= 5 && off < 10) path[off - 5] = '.'; } this gets compiled to: 0000000000000478 : 143: call 0x7 144: *(u32 *)(r10 - 0xc) = w0 145: w1 = *(u32 *)(r10 - 0xc) 146: w2 = w1 // w2 and w1 share the same id 147: w2 += -0x5 // verifier knows w1 = w2 + 5 148: if w2 > 0x4 goto +0x5 // in fall-through: verifier knows w2 ∈ [0,4] => w1 ∈ [5, 9] 149: r2 = r10 150: r2 += -0x5 // r2 = fp - 5 151: r2 += r1 // r2 = fp - 5 + r1 (∈ [5, 9]) => r2 ∈ [fp, fp + 4] 152: w1 = 0x2e 153: *(u8 *)(r2 - 0x5) = w1 // r2 ∈ [fp, fp + 4] => r2 - 5 ∈ [fp - 5, fp - 1] : 154: exit After the changes, the verifier could link 32-bit scalars and also supported -ve offsets for linking: 146: w2 = w1 147: w2 += -0x5 It allowed the verifier to correctly propagate bounds, without the changes in this patchset, verifier would reject this program with: invalid unbounded variable-offset write to stack R2 This program has been added as a selftest in the second patch. Veristat comparison on programs from sched_ext, selftests, and some meta internal programs: Scx Progs File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF) ----------------- ---------------- ----------- ----------- -------------- --------- --------- ------------- scx_layered.bpf.o layered_runnable success success MATCH 5674 6077 +403 (+7.10%) FB Progs File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF) ------------ ---------------- ----------- ----------- -------------- --------- --------- ----------------- bpf232.bpf.o layered_dump success success MATCH 1151 1218 +67 (+5.82%) bpf257.bpf.o layered_runnable success success MATCH 5743 6143 +400 (+6.97%) bpf252.bpf.o layered_runnable success success MATCH 5677 6075 +398 (+7.01%) bpf227.bpf.o layered_dump success success MATCH 915 982 +67 (+7.32%) bpf239.bpf.o layered_runnable success success MATCH 5459 5861 +402 (+7.36%) bpf246.bpf.o layered_runnable success success MATCH 5562 6008 +446 (+8.02%) bpf229.bpf.o layered_runnable success success MATCH 2559 3011 +452 (+17.66%) bpf231.bpf.o layered_runnable success success MATCH 2559 3011 +452 (+17.66%) bpf234.bpf.o layered_runnable success success MATCH 2549 3001 +452 (+17.73%) bpf019.bpf.o do_sendmsg success success MATCH 124823 153523 +28700 (+22.99%) bpf019.bpf.o do_parse success success MATCH 124809 153509 +28700 (+23.00%) bpf227.bpf.o layered_runnable success success MATCH 1915 2356 +441 (+23.03%) bpf228.bpf.o layered_runnable success success MATCH 1700 2152 +452 (+26.59%) bpf232.bpf.o layered_runnable success success MATCH 1499 1951 +452 (+30.15%) bpf312.bpf.o mount_exit success success MATCH 19253 62883 +43630 (+226.61%) bpf312.bpf.o umount_exit success success MATCH 19253 62883 +43630 (+226.61%) bpf311.bpf.o mount_exit success success MATCH 19226 62863 +43637 (+226.97%) bpf311.bpf.o umount_exit success success MATCH 19226 62863 +43637 (+226.97%) The above four programs have specific patters that make the verifier explore a lot more states: for (; depth < MAX_DIR_DEPTH; depth++) { const unsigned char* name = BPF_CORE_READ(dentry, d_name.name); if (offset >= MAX_PATH_LEN - MAX_DIR_LEN) { return depth; } int len = bpf_probe_read_kernel_str(&path[offset], MAX_DIR_LEN, name); offset += len; if (len == MAX_DIR_LEN) { if (offset - 2 < MAX_PATH_LEN) { // <---- (a) path[offset - 2] = '.'; } if (offset - 3 < MAX_PATH_LEN) { // <---- (b) path[offset - 3] = '.'; } if (offset - 4 < MAX_PATH_LEN) { // <---- (c) path[offset - 4] = '.'; } } } When at some depth == N false branches of conditions (a), (b) and (c) are scheduled for verification, constraints for offset at depth == N+1 are: 1. offset >= MAX_PATH_LEN + 2 2. offset >= MAX_PATH_LEN + 3 3. offset >= MAX_PATH_LEN + 4 (visited before others) And after offset += len it becomes: 1. offset >= MAX_PATH_LEN - 4093 2. offset >= MAX_PATH_LEN - 4092 3. offset >= MAX_PATH_LEN - 4091 (visited before others) Because of the DFS states exploration logic, the states above are visited in order 3, 2, 1; 3 is not a subset of 2 and 1 is not a subset of 2, so pruning logic does not kick in. Previously this was not a problem, because range for offset was not propagated through the statements (a), (b), (c). As the root cause of this regression is understood, this is not a blocker for this change. Selftest Progs File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF) ---------------------------------- ------------------------ ----------- ----------- -------------- --------- --------- -------------- linked_list_peek.bpf.o list_peek success success MATCH 152 88 -64 (-42.11%) verifier_iterating_callbacks.bpf.o cond_break2 success success MATCH 110 88 -22 (-20.00%) These are the added selftests that failed earlier but are passing now: verifier_linked_scalars.bpf.o alu32_negative_offset failure success MISMATCH 11 13 +2 (+18.18%) verifier_linked_scalars.bpf.o scalars_alu32_big_offset failure success MISMATCH 7 10 +3 (+42.86%) verifier_linked_scalars.bpf.o scalars_neg_alu32_add failure success MISMATCH 7 10 +3 (+42.86%) verifier_linked_scalars.bpf.o scalars_neg_alu32_sub failure success MISMATCH 7 10 +3 (+42.86%) verifier_linked_scalars.bpf.o scalars_neg failure success MISMATCH 7 10 +3 (+42.86%) verifier_linked_scalars.bpf.o scalars_neg_sub failure success MISMATCH 7 10 +3 (+42.86%) verifier_linked_scalars.bpf.o scalars_sub_neg_imm failure success MISMATCH 7 10 +3 (+42.86%) iters.bpf.o iter_obfuscate_counter success success MATCH 83 119 +36 (+43.37%) bpf_cubic.bpf.o bpf_cubic_acked success success MATCH 243 430 +187 (+76.95%) Puranjay Mohan (2): bpf: Support negative offsets, BPF_SUB, and alu32 for linked register tracking selftests/bpf: Add tests for improved linked register tracking include/linux/bpf_verifier.h | 6 +- kernel/bpf/verifier.c | 49 ++- .../selftests/bpf/progs/verifier_bounds.c | 2 +- .../bpf/progs/verifier_linked_scalars.c | 300 +++++++++++++++++- 4 files changed, 342 insertions(+), 15 deletions(-) base-commit: f11f7cf90ee09dbcf76413818063ffc38ed2d9fe -- 2.47.3