perf zstd: Fix multi-iteration decompression and error handling

zstd_decompress_stream() has two bugs in its multi-iteration loop:

1. After each ZSTD_decompressStream() call, the code advances
   output.dst by output.pos but doesn't reset output.pos to 0.
   ZSTD interprets output.pos relative to output.dst, so the
   next iteration writes at (dst + pos) + pos = dst + 2*pos,
   skipping a gap and potentially writing out of bounds.

2. On ZSTD_decompressStream() error, the loop executes break
   and returns output.pos (which is > 0 if some bytes were
   decompressed before the error).  The caller checks
   !decomp_size and skips the error, silently accepting
   truncated or corrupted data.

Fix both by removing the output buffer adjustment — ZSTD
correctly accumulates output.pos across calls without it.
Return 0 on decompression error so the caller detects it.
Add a no-progress guard to prevent infinite loops if the
output buffer fills before all input is consumed.

Note: the compressed event data_size is validated against
header.size by a subsequent patch in this series
("perf tools: Harden compressed event processing").

Reported-by: sashiko-bot@kernel.org # Running on a local machine
Reviewed-by: Ian Rogers <irogers@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>

0 files changed, 0 insertions, 0 deletions