From: Eduard Zingerman <eddyz87@gmail.com>
To: Harishankar Vishwanathan <harishankar.vishwanathan@gmail.com>,
ast@kernel.org
Cc: m.shachnai@rutgers.edu, srinivas.narayana@rutgers.edu,
santosh.nagarakatte@rutgers.edu,
Daniel Borkmann <daniel@iogearbox.net>,
John Fastabend <john.fastabend@gmail.com>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@fomichev.me>,
Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/2] bpf, verifier: Improve precision for BPF_ADD and BPF_SUB
Date: Tue, 24 Jun 2025 14:59:40 -0700 [thread overview]
Message-ID: <11d71d3c2ac47438e4f366ec555e1695880f454e.camel@gmail.com> (raw)
In-Reply-To: <20250623040359.343235-2-harishankar.vishwanathan@gmail.com>
On Mon, 2025-06-23 at 00:03 -0400, Harishankar Vishwanathan wrote:
> This patch improves the precison of the scalar(32)_min_max_add and
> scalar(32)_min_max_sub functions, which update the u(32)min/u(32)_max
> ranges for the BPF_ADD and BPF_SUB instructions. We discovered this more
> precise operator using a technique we are developing for automatically
> synthesizing functions for updating tnums and ranges.
>
> According to the BPF ISA [1], "Underflow and overflow are allowed during
> arithmetic operations, meaning the 64-bit or 32-bit value will wrap".
> Our patch leverages the wrap-around semantics of unsigned overflow and
> underflow to improve precision.
>
> Below is an example of our patch for scalar_min_max_add; the idea is
> analogous for all four functions.
>
> There are three cases to consider when adding two u64 ranges [dst_umin,
> dst_umax] and [src_umin, src_umax]. Consider a value x in the range
> [dst_umin, dst_umax] and another value y in the range [src_umin,
> src_umax].
>
> (a) No overflow: No addition x + y overflows. This occurs when even the
> largest possible sum, i.e., dst_umax + src_umax <= U64_MAX.
>
> (b) Partial overflow: Some additions x + y overflow. This occurs when
> the largest possible sum overflows (dst_umax + src_umax > U64_MAX), but
> the smallest possible sum does not overflow (dst_umin + src_umin <=
> U64_MAX).
>
> (c) Full overflow: All additions x + y overflow. This occurs when both
> the smallest possible sum and the largest possible sum overflow, i.e.,
> both (dst_umin + src_umin) and (dst_umax + src_umax) are > U64_MAX.
>
[...]
> Co-developed-by: Matan Shachnai <m.shachnai@rutgers.edu>
> Signed-off-by: Matan Shachnai <m.shachnai@rutgers.edu>
> Co-developed-by: Srinivas Narayana <srinivas.narayana@rutgers.edu>
> Signed-off-by: Srinivas Narayana <srinivas.narayana@rutgers.edu>
> Co-developed-by: Santosh Nagarakatte <santosh.nagarakatte@rutgers.edu>
> Signed-off-by: Santosh Nagarakatte <santosh.nagarakatte@rutgers.edu>
> Signed-off-by: Harishankar Vishwanathan <harishankar.vishwanathan@gmail.com>
> ---
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
(Please don't drop acks).
next prev parent reply other threads:[~2025-06-24 21:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-23 4:03 [PATCH v3 0/2] bpf, verifier: Improve precision of BPF_ADD and BPF_SUB Harishankar Vishwanathan
2025-06-23 4:03 ` [PATCH v3 1/2] bpf, verifier: Improve precision for " Harishankar Vishwanathan
2025-06-24 21:59 ` Eduard Zingerman [this message]
2025-06-23 4:03 ` [PATCH v3 2/2] selftests/bpf: Add testcases " Harishankar Vishwanathan
2025-06-24 22:45 ` Eduard Zingerman
2025-06-25 1:50 ` Alexei Starovoitov
2025-06-25 2:00 ` [PATCH v3 0/2] bpf, verifier: Improve precision of " patchwork-bot+netdevbpf
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