executor/linux: dump mount information when failed to open kcov file

There are many "lost connection to test machine (5)" reports where the
testing terminated due to ENOENT upon open("/sys/kernel/debug/kcov").

Since some testcase might be unintendedly modifying mount information,
let's start from checking whether/how mount is broken.
This commit might be reverted after the cause is identified and fixed.

1 files changed, 23 insertions, 1 deletions

diff --git a/executor/executor_linux.h b/executor/executor_linux.h
index 98b89fc9a..e8105f65f 100644
--- a/executor/executor_linux.h
+++ b/executor/executor_linux.h
@@ -94,8 +94,30 @@ static intptr_t execute_syscall(const call_t* c, intptr_t a[kMaxArgs])
 static void cover_open(cover_t* cov, bool extra)
 {
 	int fd = open("/sys/kernel/debug/kcov", O_RDWR);
-	if (fd == -1)
+	if (fd == -1) {
+		const int err = errno;
+		if (chdir("/sys/"))
+			fprintf(stderr, "/sys/ does not exist.\n");
+		else if (chdir("/sys/kernel/"))
+			fprintf(stderr, "/sys/kernel/ does not exist.\n");
+		else if (chdir("/sys/kernel/debug/"))
+			fprintf(stderr, "/sys/kernel/debug/ does not exist.\n");
+		fd = open("/proc/mounts", O_RDONLY);
+		if (fd == -1) {
+			fprintf(stderr, "open of /proc/mounts failed.\n");
+			if (chdir("/proc/"))
+				fprintf(stderr, "/proc/ does not exist.\n");
+		} else {
+			static char buffer[4096];
+			int len;
+			fprintf(stderr, "Content of /proc/mounts\n");
+			while ((len = read(fd, buffer, sizeof(buffer))) > 0)
+				fwrite(buffer, 1, len, stderr);
+			close(fd);
+		}
+		errno = err;
 		fail("open of /sys/kernel/debug/kcov failed");
+	}
 	if (dup2(fd, cov->fd) < 0)
 		fail("filed to dup2(%d, %d) cover fd", fd, cov->fd);
 	close(fd);