From: David Laight <david.laight.linux@gmail.com>
To: bot+bpf-ci@kernel.org
Cc: jolsa@kernel.org, andrii@kernel.org, bpf@vger.kernel.org,
linux-kernel@vger.kernel.org, songliubraving@fb.com, yhs@fb.com,
john.fastabend@gmail.com, ast@kernel.org, daniel@iogearbox.net,
martin.lau@kernel.org, eddyz87@gmail.com,
yonghong.song@linux.dev, clm@meta.com, ihor.solodrai@linux.dev
Subject: Re: [PATCHv4 bpf-next 2/5] libbpf: Add support to detect nop,nop5 instructions combo for usdt probe
Date: Sat, 21 Feb 2026 12:00:35 +0000 [thread overview]
Message-ID: <20260221120035.4562df85@pumpkin> (raw)
In-Reply-To: <e46ea28bf48fb9ff5ba205036a6baf0427f7646870cafc98b387940f99a848d8@mail.kernel.org>
On Fri, 20 Feb 2026 11:24:45 +0000 (UTC)
bot+bpf-ci@kernel.org wrote:
> > diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> > index d1524f6f54ae..c81e0a58caf5 100644
> > --- a/tools/lib/bpf/usdt.c
> > +++ b/tools/lib/bpf/usdt.c
>
> [ ... ]
>
> > +#if defined(__x86_64__)
> > +static bool has_nop_combo(int fd, long off)
> > +{
> > + static unsigned char nop_combo[6] = {
> ^^^^^^^^^^^^^^^^^^^^^^
>
> Should nop_combo be declared const? It is a read-only lookup table
> used only in the memcmp() below.
Actually you either want 'static const unsigned char' or just 'unsigned char'.
In either case gcc optimises away the array, the call to memcmp(), and
just compares with 32bit and 64bit constants.
>
> > + 0x90, 0x0f, 0x1f, 0x44, 0x00, 0x00 /* nop,nop5 */
> > + };
> > + unsigned char buf[6] = {};
No need to initialise buf[];
David
> > +
> > + if (pread(fd, buf, 6, off) != 6)
> > + return false;
> > + return memcmp(buf, nop_combo, 6) == 0;
> > +}
>
> [ ... ]
>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22221513473
>
> AI-authorship-score: low
> AI-authorship-explanation: Domain-specific optimization from established BPF/tracing maintainer with iterative lore review history and deep x86_64 uprobe internals knowledge.
> issues-found: 1
> issue-severity-score: low
> issue-severity-explanation: Missing const qualifier on a static read-only byte array is a minor C best practice deviation with no runtime impact.
next prev parent reply other threads:[~2026-02-21 12:00 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-20 10:42 [PATCHv4 bpf-next 0/5] libbpf: Make optimized uprobes backward compatible Jiri Olsa
2026-02-20 10:42 ` [PATCHv4 bpf-next 1/5] libbpf: Add uprobe syscall feature detection Jiri Olsa
2026-02-20 10:42 ` [PATCHv4 bpf-next 2/5] libbpf: Add support to detect nop,nop5 instructions combo for usdt probe Jiri Olsa
2026-02-20 11:24 ` bot+bpf-ci
2026-02-21 12:00 ` David Laight [this message]
2026-02-22 14:32 ` Jiri Olsa
2026-02-20 10:42 ` [PATCHv4 bpf-next 3/5] selftests/bpf: Emit nop,nop5 instructions combo for x86_64 arch Jiri Olsa
2026-02-20 10:42 ` [PATCHv4 bpf-next 4/5] selftests/bpf: Add test for checking correct nop of optimized usdt Jiri Olsa
2026-02-20 10:42 ` [PATCHv4 bpf-next 5/5] selftests/bpf: Add usdt trigger bench Jiri Olsa
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