perf session: Add minimum event size and alignment validation

perf session: Add minimum event size and alignment validation

Add a per-type minimum size table (perf_event__min_size[]) and
enforce it before swap and processing, so that both cross-endian
and native-endian paths are protected from accessing fields past
the event boundary.

The table uses offsetof() for types with trailing variable-length
fields (filenames, strings, msg arrays) and sizeof() for
fixed-size types.  Zero entries mean no minimum beyond the 8-byte
header already enforced by the reader.

Undersized events are skipped with a warning in process_event
and rejected in peek_event — both checked before the swap
handler runs, preventing OOB access on crafted event fields.

Also reject events whose header.size is not 8-byte aligned.  The
kernel aligns all event sizes to sizeof(u64) — see
perf_event_comm_event() (ALIGN), perf_event_mmap_event(),
perf_event_cgroup(), perf_event_ksymbol() (IS_ALIGNED loops),
and perf_event_text_poke() (ALIGN) in kernel/events/core.c.
An unaligned size means the file is corrupted or crafted; reject
early so downstream code that divides by sizeof(u64) to compute
array element counts gets exact results.

Three legacy user events are exempted from the alignment check:
TRACING_DATA (66) had a 12-byte struct before commit b39c915a4f36
("libperf event: Ensure tracing data is multiple of 8 sized")
added padding, COMPRESSED (81) carries raw ZSTD output (already
superseded by COMPRESSED2 with PERF_ALIGN), and HEADER_FEATURE
(80) uses do_write_string() with a 4-byte length prefix.

Also guard event_swap() against crafted event types >=
PERF_RECORD_HEADER_MAX to prevent OOB reads on the
perf_event__swap_ops[] array.

Changes in v2:
- Fix double-skip for unsupported event types: return 0 instead
  of event->header.size in perf_session__process_event() for
  HEADER_MAX, since reader__read_event() already advances by
  event->header.size (Reported-by: sashiko-bot@kernel.org)
- Exempt TRACING_DATA, COMPRESSED, and HEADER_FEATURE from the
  alignment check — these legacy user events predate the 8-byte
  alignment rule (Reported-by: sashiko-bot@kernel.org)
- peek_event: return 0 (skip) for unknown event types instead of
  -1 (error), consistent with process_event which already skips
  unsupported types gracefully (Reported-by: sashiko-bot@kernel.org)

Reported-by: sashiko-bot@kernel.org # Running on a local machine
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.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>