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libperf: Document code simplification case for widening struct perf_cpu

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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: the narrow type forces defensive truncation checks at every
boundary where wider CPU indices are narrowed, and values > 32767
silently wrap to negative numbers (two's complement), bypassing
bounds validation without them.

Acked-by: Ian Rogers <irogers@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Arnaldo Carvalho de Melo 2026-06-08 15:32:32 -03:00
parent e33711d5e7
commit 7435c4069a
1 changed files with 8 additions and 0 deletions
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@ -11,6 +11,14 @@ 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 negative
numbers (two's complement), bypassing bounds validation.
Widening to int 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(),
perf_cpu_map__min(), perf_cpu_map__max(), perf_cpu_map__has(), the
for_each_cpu macros, and all internal callers. The perf_cpu_map