[PATCH perf-tools-next v10 01/10] perf ftrace: Handle signals, non-blocking reads and pager exits in __cmd_ftrace()

[Aaron Tomlin]

In __cmd_ftrace(), trace_fd is configured for non-blocking I/O via
fcntl(trace_fd, F_SETFL, O_NONBLOCK) and polled using poll(). However,
the polling loop previously treated any negative return code from poll()
or read() as an unrecoverable failure.

If poll() or read() is interrupted by a non-fatal signal (i.e. SIGWINCH
upon terminal resizing), errno is set to EINTR. Furthermore, read() on
trace_pipe can return EAGAIN/EWOULDBLOCK even after poll() indicates
POLLIN (for instance, under concurrent readers, spurious wakeups, or
block-mode trace iterator options). In all such cases, breaking from the
loop causes perf ftrace to terminate tracing prematurely.

Additionally, output errors on stdout (i.e. when a pager or consumer
process exits, closing the downstream pipe) were ignored by fflush(),
leaving __cmd_ftrace() polling indefinitely until the workload
completes.

Resolve these concerns by retrying poll() upon receiving EINTR, retrying
read() on EAGAIN, EWOULDBLOCK, and EINTR, and breaking on EOF (n == 0).
Furthermore, detect stdout errors and broken pipes via fflush() and
ferror() to exit immediately when the downstream output sink closes, and
handle EINTR and ferror(stdout) cleanly in the subsequent buffer
draining loop.

Fixes: d01f4e8db22c ("perf ftrace: Introduce new 'ftrace' tool")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
 tools/perf/builtin-ftrace.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 6017493ff179..7cdc9ae2fbbb 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -14,6 +14,7 @@
 #include <poll.h>
 #include <semaphore.h>
 #include <signal.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <linux/capability.h>
@@ -806,17 +807,27 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 	}
 
 	while (!done) {
-		if (poll(&pollfd, 1, -1) < 0)
+		if (poll(&pollfd, 1, -1) < 0) {
+			if (errno == EINTR)
+				continue;
 			break;
+		}
 
 		if (pollfd.revents & POLLIN) {
 			int n = read(trace_fd, buf, sizeof(buf));
-			if (n < 0)
+
+			if (n < 0) {
+				if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR)
+					continue;
+				break;
+			}
+			if (n == 0)
 				break;
 			if (fwrite(buf, n, 1, stdout) != 1)
 				break;
 			/* flush output since stdout is in full buffering mode due to pager */
-			fflush(stdout);
+			if (fflush(stdout) < 0 || ferror(stdout))
+				break;
 		}
 	}
 
@@ -831,9 +842,15 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 	}
 
 	/* read remaining buffer contents */
-	while (true) {
+	while (!ferror(stdout)) {
 		int n = read(trace_fd, buf, sizeof(buf));
-		if (n <= 0)
+
+		if (n < 0) {
+			if (errno == EINTR)
+				continue;
+			break;
+		}
+		if (n == 0)
 			break;
 		if (fwrite(buf, n, 1, stdout) != 1)
 			break;
-- 
2.55.0