Re: [PATCH 11/11] libperf: Document code simplification case for widening struct perf_cpu

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The documentation added to `tools/lib/perf/TODO` incorrectly states that narrowing values > 32767 to `int16_t` wraps them to 'small positive numbers'. In two's complement arithmetic, values from 32768 to 65535 wrap to negative numbers (-32768 to -1) when truncated to a signed 16-bit integer.
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commit e8f5b26d672396c3173a77e6aaeee613f6d309bd
Author: Arnaldo Carvalho de Melo <acme@redhat.com>

libperf: Document code simplification case for widening struct perf_cpu

Add a bullet point to the libperf ABI TODO explaining the code
simplification benefit of widening struct perf_cpu.cpu from int16_t
to int.

> diff --git a/tools/lib/perf/TODO b/tools/lib/perf/TODO
> index 486dd95dc5720..1a3644aa1f38d 100644
> --- a/tools/lib/perf/TODO
> +++ b/tools/lib/perf/TODO
> @@ -11,6 +11,13 @@ together.
>       (x86_64 max is 8192, arm64 is 4096), but NR_CPUS limits keep
>       growing.  perf clamps to INT16_MAX in set_max_cpu_num() as a
>       safety net.
> +   - Code simplification: the int16_t forces defensive truncation
> +     checks at every boundary where a wider CPU index (int from
> +     sample->cpu, al->cpu, etc.) is narrowed into struct perf_cpu.
> +     Without these checks, values > 32767 silently wrap to small
> +     positive numbers, bypassing bounds validation.  Widening to int
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Severity: Low]
This isn't a bug, but is the arithmetic description accurate here?

In two's complement arithmetic, truncating values between 32768 and 65535
into a signed 16-bit integer (int16_t) wraps them to negative numbers
(-32768 to -1), rather than small positive numbers.

The overall conclusion remains correct since negative values trivially
bypass bounds checks, but the description of the wrapping behavior appears
mathematically inaccurate.

> +     eliminates this entire class of silent truncation bugs and
> +     removes the need for the INT16_MAX clamp in set_max_cpu_num().
>     - Scope: struct perf_cpu is embedded everywhere — perf_cpu_map__cpu(),

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Sashiko AI review · https://sashiko.dev/#/patchset/20260608201753.1979464-1-acme@kernel.org?part=11