[LTP] [PATCH 2/2] fill_fs: Ensure written data is not easily compressed

[Richard Palethorpe]

If the stack is auto initialized to zero, then we will write all
zeros. Some FS may treat this as a special case and just record the
number of zero bytes or sectors.

This could alter the test behaviour in unpredictable ways. For example
a large number of (slow) syscalls may be required to fill up the
drive, extending the required test time. Or we could overflow the file
size/offset causing EFBIG.

So this writes random data to the buffer if it is zero.

Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
Cc: Cyril Hrubis <chrubis@suse.cz>
---
 lib/tst_fill_fs.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/tst_fill_fs.c b/lib/tst_fill_fs.c
index 121dd2f20..ade0a52ba 100644
--- a/lib/tst_fill_fs.c
+++ b/lib/tst_fill_fs.c
@@ -16,13 +16,20 @@ void tst_fill_fs(const char *path, int verbose)
 {
 	int i = 0;
 	char file[PATH_MAX];
-	char buf[4096];
+	static char buf[4096];
 	size_t len;
 	ssize_t ret;
 	int fd;
 	struct statvfs fi;
+
 	statvfs(path, &fi);
 
+	if (!(buf[0] || buf[4095])) {
+		fd = SAFE_OPEN("/dev/urandom", O_RDONLY);
+		SAFE_READ(0, fd, buf, 4096);
+		SAFE_CLOSE(fd);
+	}
+
 	for (;;) {
 		len = random() % (1024 * 102400);
 
-- 
2.38.1