[PATCH v1] perf synthetic-events: Fix stale build ID in module MMAP2 records

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

perf_event__synthesize_modules() allocates a single union
perf_event and reuses it across every kernel module callback.
After the first module is processed,
perf_record_mmap2__read_build_id() sets
PERF_RECORD_MISC_MMAP_BUILD_ID in header.misc and writes that
module's build ID into the event. On subsequent iterations the
callback overwrites start, len, pid, and filename for the next
module but never clears the stale build ID fields or the
MMAP_BUILD_ID flag. When perf_record_mmap2__read_build_id()
runs for the second module it sees the flag, reads the stale
build ID into a dso_id, and __dso__improve_id() permanently
poisons the DSO with the wrong build ID. Every module after
the first therefore receives the first module's build ID in
its MMAP2 record. On a system with the sunrpc and nfsd modules
loaded, this causes perf script and perf report to show
[unknown] for all module symbols.

The latent bug has existed since commit d9f2ecbc5e47 ("perf
dso: Move build_id to dso_id") introduced the
PERF_RECORD_MISC_MMAP_BUILD_ID check in
perf_record_mmap2__read_build_id(). Commit 53b00ff358dc ("perf
record: Make --buildid-mmap the default") then exposed it to
all users by making the MMAP2-with-build-ID path the default.
Both commits were merged in the same series.

Clear the MMAP_BUILD_ID flag and zero the build_id union
before each call to perf_record_mmap2__read_build_id() so
that every module starts with a clean slate.

Fixes: d9f2ecbc5e47 ("perf dso: Move build_id to dso_id")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 tools/perf/util/synthetic-events.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index ef79433ebc3a..ddf1cbda1902 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -703,6 +703,11 @@ static int perf_event__synthesize_modules_maps_cb(struct map *map, void *data)
 
 		memcpy(event->mmap2.filename, dso__long_name(dso), dso__long_name_len(dso) + 1);
 
+		/* Clear stale build ID from previous module iteration */
+		event->mmap2.header.misc &= ~PERF_RECORD_MISC_MMAP_BUILD_ID;
+		memset(event->mmap2.build_id, 0, sizeof(event->mmap2.build_id));
+		event->mmap2.build_id_size = 0;
+
 		perf_record_mmap2__read_build_id(&event->mmap2, args->machine, false);
 	} else {
 		size = PERF_ALIGN(dso__long_name_len(dso) + 1, sizeof(u64));
-- 
2.53.0