[PATCH 08/23] perf tools: Fix thread__set_comm_from_proc() on empty comm file

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

thread__set_comm_from_proc() calls procfs__read_str() then strips
the trailing newline via comm[sz - 1] = '\0'.  procfs__read_str()
allocates the buffer before reading, so on an empty /proc/pid/comm
(reachable during late exit teardown) it returns success with sz = 0
and an unterminated heap buffer.

The sz - 1 underflow was the original sashiko finding: it writes a
null byte before the allocation.  But even with a sz > 0 guard on
the newline strip, the unterminated buffer would still be passed to
thread__set_comm() which calls strlen() — an unbounded heap read.

Fix by treating sz == 0 as failure: free the buffer and return -1.
This is consistent with pmu.c's perf_pmu__parse_scale/unit which
already treat len == 0 from filename__read_str as an error.

Fixes: 2f3027ac28bf6bc3 ("perf thread: Introduce method to set comm from /proc/pid/self")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/thread.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/perf/util/thread.c b/tools/perf/util/thread.c
index ba33c0dfc18fe242..e483ffcb5d937fbc 100644
--- a/tools/perf/util/thread.c
+++ b/tools/perf/util/thread.c
@@ -295,6 +295,11 @@ int thread__set_comm_from_proc(struct thread *thread)
 	if (!(snprintf(path, sizeof(path), "%d/task/%d/comm",
 		       thread__pid(thread), thread__tid(thread)) >= (int)sizeof(path)) &&
 	    procfs__read_str(path, &comm, &sz) == 0) {
+		/* sz==0: read got nothing, e.g. race during exit teardown */
+		if (sz == 0) {
+			free(comm);
+			return -1;
+		}
 		comm[sz - 1] = '\0';
 		err = thread__set_comm(thread, comm, 0);
 	}
-- 
2.54.0