[LTP] [PATCH] syscalls/mount03: Don't read() with an invalid buffer argument

[Peter Maydell]

The syscall test mount03 includes a code path to test the MS_NOATIME
mount flag. This code checks that an attempted read of a file
does not update the atime, but the read() it uses is passed
a NULL buffer pointer, which isn't valid. The test passes on the
kernel because the kernel happens to check for "is this file at
EOF?" before "is the buffer argument valid?", and so it returns 0
rather than -1/EFAULT. However the test fails when run under QEMU,
because QEMU checks for a valid buffer before EOF.

POSIX and the Linux documentation make no guarantees about what order
error cases are checked in; pass in a valid buffer so that we aren't
relying on incidental behaviour of the implementation of read
in a test for a different syscall.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
This is my first LTP patch, so please let me know if I got
anything wrong stylistically or submission-wise...

This test also has a bug in its error handling code paths:
if for instance this read() fails then we return from
test_rwflag() without doing a close() on the filedescriptor.
This then causes the umount() performed by tst_release_device()
to fail with EBUSY, and then the loopback device is left
mounted. Later, other test cases that try to use the loopback
device then fail unnecessarily. I'm not sure what the best
way to fix this is -- just call close() in all the error
handling paths, or is there some kind of automatic cleanup
on failure arrangement that could be used? In any case,
that's a matter for a different patch I think.

 testcases/kernel/syscalls/mount/mount03.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/testcases/kernel/syscalls/mount/mount03.c b/testcases/kernel/syscalls/mount/mount03.c
index 1568c50..1873f0f 100644
--- a/testcases/kernel/syscalls/mount/mount03.c
+++ b/testcases/kernel/syscalls/mount/mount03.c
@@ -132,6 +132,7 @@ int test_rwflag(int i, int cnt)
 	time_t atime;
 	struct passwd *ltpuser;
 	struct stat file_stat;
+	char readbuf[20];
 
 	switch (i) {
 	case 0:
@@ -319,7 +320,7 @@ int test_rwflag(int i, int cnt)
 
 		sleep(1);
 
-		if (read(fd, NULL, 20) == -1) {
+		if (read(fd, readbuf, sizeof(readbuf)) == -1) {
 			tst_resm(TWARN | TERRNO, "read %s failed", file);
 			return 1;
 		}
-- 
1.9.1