perf header: Validate feature section size and add read path bounds checking

perf header: Validate feature section size and add read path bounds checking

Harden feature section parsing against crafted perf.data files:

1. perf_header__process_sections() reads the feature section table
   and passes each section's offset and size directly to the
   processing callbacks without validating them against the actual
   file size.  A crafted section size would make all downstream
   bounds checks against ff->size ineffective since they compare
   against the untrusted, inflated bound.  Add an fstat() check
   with S_ISREG() guard and verify that each section's offset +
   size does not extend past EOF.

2. __do_read_buf() validates reads against ff->size (section size),
   but __do_read_fd() had no such check, so a malformed perf.data
   with an understated section size could cause reads past the end
   of the current section into the next section's data.  Add the
   bounds check in __do_read(), the common caller of both helpers,
   so it is enforced uniformly for both the fd and buf paths.
   Track the section-relative offset in __do_read_fd() so the
   check works for the fd path.  Reject negative sizes which on
   32-bit can occur when a u32 >= 0x80000000 is passed as ssize_t.

3. do_read_string() relied on file data being null-padded.  Add
   explicit null-termination (buf[len-1] = '\0') after reading
   and validate length (>= 1, fits within section) before
   allocating, so callers like process_cpu_topology() never
   receive an unterminated string.

4. Initialize feat_fd.offset to 0 (section-relative) instead of
   section->offset (file-absolute) so the bounds tracking is
   consistent with __do_read()'s section-relative comparison.
   Adjust process_build_id() to use lseek() for its file-absolute
   offset needs since it cannot rely on ff->offset for that.

5. Propagate ff->size to perf_file_section__fprintf_info() so its
   reads are also bounded.

Reported-by: sashiko-bot@kernel.org # Running on a local machine
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: David Carrillo-Cisneros <davidcc@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>