[LTP] [RESEND][PATCH v3 1/2] libltp: add support to mount tmpfs for EROFS testing

[Sandeep Patil <sspatil@google.com>]

Some tests go through losetup, create, format and mount filesystems only
to test for 'EROFS' return value from system calls. The tests end up
being flaky depending on the tools available on the platform. e.g.
mkfs.<filesystem> tool is required for mounting a device with
filesystem.

If the test is only to check for EROFS, this can be achieved by simply
by the bind mounting (MS_BIND) for '/' on 'mntpoint' and then remount
the same as 'read-only'.

Example of tests that can benefit from this are:
  access04, mkdirat02, mknodat02, acct01, fchown04, mknod07 etc.

This also allows for these tests to successfully run on Android.

Signed-off-by: Sandeep Patil <sspatil@google.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
---
v2->v3
------
- Added Tested-by from Jan Stancek

v1->v2
------
- Changed the interface to be at a higher level, i.e. instead of
  making tests specify that they need a read-only tmpfs mount, we
  do that automatically in the library as a preference and fallback
  to the old way if that doesn't succeed.

- The read-only bind mount of "/" option was tried and it worked just
  fine with an Android system, but broke my ubuntu instance badly (I lost root).
  I could never find out why, since hte patch worked fine on Android
  and it correctly did the bind mount. However, I figured if there is any 
  chance of possible breakage, we should simply avoid that path for now.
 include/tst_test.h |  1 +
 lib/tst_test.c     | 36 ++++++++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/include/tst_test.h b/include/tst_test.h
index e90312ae3..ad468e8cf 100644
--- a/include/tst_test.h
+++ b/include/tst_test.h
@@ -124,6 +124,7 @@ struct tst_test {
 	int needs_checkpoints:1;
 	int format_device:1;
 	int mount_device:1;
+	int needs_rofs:1;
 
 	/* Minimal device size in megabytes */
 	unsigned int dev_min_size;
diff --git a/lib/tst_test.c b/lib/tst_test.c
index 4c30edab5..828eae49d 100644
--- a/lib/tst_test.c
+++ b/lib/tst_test.c
@@ -21,6 +21,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <sys/mount.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <sys/time.h>
@@ -41,7 +42,7 @@ struct tst_test *tst_test;
 static int iterations = 1;
 static float duration = -1;
 static pid_t main_pid, lib_pid;
-static int device_mounted;
+static int mntpoint_active;
 
 struct results {
 	int passed;
@@ -701,7 +702,32 @@ static void do_setup(int argc, char *argv[])
 	if (needs_tmpdir() && !tst_tmpdir_created())
 		tst_tmpdir();
 
-	if (tst_test->needs_device) {
+	if (tst_test->mntpoint)
+		SAFE_MKDIR(tst_test->mntpoint, 0777);
+
+	if (tst_test->needs_rofs) {
+		if (!tst_test->mntpoint) {
+			tst_brk(TBROK,
+				"tst_test->mntpoint must be set!");
+		}
+
+		/* If we failed to do read-only bind mount for '/'. Fallback to
+		 * using a device with empty read-only filesystem.
+		 */
+		if (mount(NULL, tst_test->mntpoint, "tmpfs", MS_RDONLY, NULL)) {
+			tst_res(TINFO, "Cannot mount tmpfs read-only at %s, "
+					"setting up a device instead\n",
+					tst_test->mntpoint);
+			tst_test->mount_device = 1;
+			tst_test->needs_device = 1;
+			tst_test->format_device = 1;
+			tst_test->mnt_flags = MS_RDONLY;
+		} else {
+			mntpoint_active = 1;
+		}
+	}
+
+	if (tst_test->needs_device && !mntpoint_active) {
 		tdev.dev = tst_acquire_device_(NULL, tst_test->dev_min_size);
 
 		if (!tdev.dev)
@@ -721,16 +747,14 @@ static void do_setup(int argc, char *argv[])
 		}
 
 		if (tst_test->mount_device) {
-
 			if (!tst_test->mntpoint) {
 				tst_brk(TBROK,
 					"tst_test->mntpoint must be set!");
 			}
 
-			SAFE_MKDIR(tst_test->mntpoint, 0777);
 			SAFE_MOUNT(tdev.dev, tst_test->mntpoint, tdev.fs_type,
 				   tst_test->mnt_flags, tst_test->mnt_data);
-			device_mounted = 1;
+			mntpoint_active = 1;
 		}
 	}
 
@@ -751,7 +775,7 @@ static void do_test_setup(void)
 
 static void do_cleanup(void)
 {
-	if (device_mounted)
+	if (mntpoint_active)
 		tst_umount(tst_test->mntpoint);
 
 	if (tst_test->needs_device && tdev.dev)
-- 
2.14.1.690.gbb1197296e-goog