linux-kernel - Re: [PATCH v2 2/2] selftests/bpf: Add testcases for BPF_ADD and BPF

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Message-ID: <CAM=Ch044aYYx_wJ+wPoDu6u0jqGk_18SyAkkTWf=ygofzDT2bw@mail.gmail.com>
Date: Thu, 19 Jun 2025 18:34:04 -0400
From: Harishankar Vishwanathan <harishankar.vishwanathan@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: ast@...nel.org, m.shachnai@...gers.edu, srinivas.narayana@...gers.edu, 
	santosh.nagarakatte@...gers.edu, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, Matan Shachnai <m.shachnai@...il.com>, 
	Luis Gerhorst <luis.gerhorst@....de>, Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 2/2] selftests/bpf: Add testcases for BPF_ADD and BPF_SUB

On Thu, Jun 19, 2025 at 5:55 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Thu, 2025-06-19 at 17:13 -0400, Harishankar Vishwanathan wrote:
> > On Wed, Jun 18, 2025 at 5:22 PM Eduard Zingerman <eddyz87@...il.com> wrote:
> > >
> > > On Tue, 2025-06-17 at 19:17 -0400, Harishankar Vishwanathan wrote:
[...]
> Hm, I see, that's an interesting angle.
> The problem is, if I do something silly changing the code and this
> test fails I'd have a hard time understanding the expected output.
> Therefore, I'd prefer something more obvious.
>
> Maybe let's go with this:
>
>   SEC("tc")
>   __success
>   __naked void test1(void)
>   {
>         asm volatile (
>         "r3 = 0xa000000000000000 ll;"
>         "r4 = 0x0;"
>         "r4 = -r4;"
>         "r3 |= r4;"
>         "r3 += r3;"
>         "r0 = 1;"
>         "exit;"
>         :
>         : __imm(bpf_get_prandom_u32)
>         : __clobber_all);
>   }
>
> Here is verifier log comparison:
>
>   master: 5: (0f) r3 += r3     ; R3_w=scalar()
>   branch: 5: (0f) r3 += r3     ; R3_w=scalar(umin=0x4000000000000000,umax=0xfffffffffffffffe)
>
> ?

Okay, this seems both readable and also demonstrates precision gains.
I'll follow up with a
v3 with similar updated test cases for full overflow and partial
overflow for all the four functions.

[...]