[PATCH v1] perf session: fix decompression of PERF_RECORD_COMPRESSED records

[Alexey Budankov]

Avoid termination of trace loading in case the last record in 
the decompressed buffer partly resides in the following 
mmaped PERF_RECORD_COMPRESSED record. In this case NULL value
returned by fetch_mmaped_event() means to proceed to the next 
mmaped record then decompress it and load compressed events. 

The issue can be reproduced like this:

  $ perf record -z -- some_long_running_workload
  $ perf report --stdio -vv
  decomp (B): 44519 to 163000
  decomp (B): 48119 to 174800
  decomp (B): 65527 to 131072
  fetch_mmaped_event: head=0x1ffe0 event->header_size=0x28, mmap_size=0x20000: fuzzed perf.data?
  Error:
  failed to process sample
  ...

Testing:
71: Zstd perf.data compression/decompression              : Ok

Fixes: 57fc032ad643 ("perf session: Avoid infinite loop when seeing invalid header.size")
Link: https://marc.info/?l=linux-kernel&m=156580812427554&w=2
Co-developed-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
---
 tools/perf/util/session.c | 47 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index f07b8ecb91bc..3f6f812ec4ed 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1957,9 +1957,31 @@ static int __perf_session__process_pipe_events(struct perf_session *session)
 	return err;
 }
 
+static union perf_event *
+prefetch_event(char *buf, u64 head, size_t mmap_size,
+	       bool needs_swap, union perf_event *ret);
+
 static union perf_event *
 fetch_mmaped_event(struct perf_session *session,
 		   u64 head, size_t mmap_size, char *buf)
+{
+	return prefetch_event(buf, head, mmap_size,
+			      session->header.needs_swap,
+			      ERR_PTR(-EINVAL));
+}
+
+static union perf_event *
+fetch_decomp_event(struct perf_session *session,
+		   u64 head, size_t mmap_size, char *buf)
+{
+	return prefetch_event(buf, head, mmap_size,
+			      session->header.needs_swap,
+			      NULL);
+}
+
+static union perf_event *
+prefetch_event(char *buf, u64 head, size_t mmap_size,
+	       bool needs_swap, union perf_event *ret)
 {
 	union perf_event *event;
 
@@ -1971,20 +1993,20 @@ fetch_mmaped_event(struct perf_session *session,
 		return NULL;
 
 	event = (union perf_event *)(buf + head);
+	if (needs_swap)
+		perf_event_header__bswap(&event->header);
 
-	if (session->header.needs_swap)
+	if (head + event->header.size <= mmap_size)
+		return event;
+
+	/* We're not fetching the event so swap back again */
+	if (needs_swap)
 		perf_event_header__bswap(&event->header);
 
-	if (head + event->header.size > mmap_size) {
-		/* We're not fetching the event so swap back again */
-		if (session->header.needs_swap)
-			perf_event_header__bswap(&event->header);
-		pr_debug("%s: head=%#" PRIx64 " event->header_size=%#x, mmap_size=%#zx: fuzzed perf.data?\n",
-			 __func__, head, event->header.size, mmap_size);
-		return ERR_PTR(-EINVAL);
-	}
+	pr_debug("%s: head=%#" PRIx64 " event->header_size=%#x, mmap_size=%#zx\n",
+		__func__, head, event->header.size, mmap_size);
 
-	return event;
+	return ret;
 }
 
 static int __perf_session__process_decomp_events(struct perf_session *session)
@@ -1997,10 +2019,7 @@ static int __perf_session__process_decomp_events(struct perf_session *session)
 		return 0;
 
 	while (decomp->head < decomp->size && !session_done()) {
-		union perf_event *event = fetch_mmaped_event(session, decomp->head, decomp->size, decomp->data);
-
-		if (IS_ERR(event))
-			return PTR_ERR(event);
+		union perf_event *event = fetch_decomp_event(session, decomp->head, decomp->size, decomp->data);
 
 		if (!event)
 			break;