perf session: Add byte-swap and bounds check for PERF_RECORD_BPF_METADATA events

perf session: Add byte-swap and bounds check for PERF_RECORD_BPF_METADATA events

PERF_RECORD_BPF_METADATA has no entry in perf_event__swap_ops[], so its
nr_entries field is never byte-swapped when reading a cross-endian
perf.data file.  Downstream processing in
perf_event__fprintf_bpf_metadata() loops over nr_entries, so a
foreign-endian value causes out-of-bounds reads.

Add a swap handler that byte-swaps nr_entries after validating that
header.size is large enough.  The entries[] array contains only char
arrays (key/value strings), so no per-entry swap is needed — but ensure
NUL-termination on the writable cross-endian path.

Validate header.size, nr_entries, and string NUL-termination in the
common event delivery path so that native-endian files with malicious
values are also rejected.  Snapshot nr_entries via READ_ONCE() before
validation — the event is on a MAP_SHARED mmap that could theoretically
change between the bounds check and the loop.

Changes in v2:
- Snapshot event->header.size via READ_ONCE() into a local variable
  to prevent a double-fetch underflow in the max_entries calculation
  (Reported-by: sashiko-bot@kernel.org)
- Write back clamped nr_entries to the event on the swap path,
  consistent with NAMESPACES and STAT_CONFIG handlers — without
  writeback the native path sees the inflated nr and skips the
  event entirely (Reported-by: sashiko-bot@kernel.org)

Fixes: ab38e84ba9a8 ("perf record: collect BPF metadata from existing BPF programs")
Reported-by: sashiko-bot@kernel.org # Running on a local machine
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Blake Jones <blakejones@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6-1m
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>