[PATCH v2] lib/loopdev.c: Retry LOOP_SET_STATUS64 on EAGAIN

[Romain Izard <romain.izard.pro@xxxxxxxxx>]

A recent bugfix in the Linux kernel made it possible for the
LOOP_SET_STATUS64 ioctl to fail when called with a non-zero offset,
with an EAGAIN errno:

5db470e229e2 loop: drop caches if offset or block_size are changed

This fix changes a silent failure (where mount could sometimes access
the backing loop image through the cache without the specified offset)
to an explicit failure, and it has also been backported on stable
branches.

On a 5.0 kernel, other changes to the loop driver make it hard to get
generate the EAGAIN error, but this bugfix has also been backported to
stables branches, without these changes. At least with the 4.14 stable
branch, the EAGAIN error can be quickly generated with the following loop:

while mount -o loop,offset=239 disk point && umount point; do :; done

Retry the ioctl when it fails with EAGAIN, which means that mount or
losetup will eventually succeed when encountering this case.

Signed-off-by: Romain Izard <romain.izard.pro@xxxxxxxxx>
---
v2: Add a delay between retries

---
 lib/loopdev.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/lib/loopdev.c b/lib/loopdev.c
index 5d2e95b7e..22305ecd9 100644
--- a/lib/loopdev.c
+++ b/lib/loopdev.c
@@ -1275,7 +1275,7 @@ static int loopcxt_check_size(struct loopdev_cxt *lc, int file_fd)
  */
 int loopcxt_setup_device(struct loopdev_cxt *lc)
 {
-	int file_fd, dev_fd, mode = O_RDWR, rc = -1, cnt = 0;
+	int file_fd, dev_fd, mode = O_RDWR, rc = -1, cnt = 0, err, again;
 	int errsv = 0;
 
 	if (!lc || !*lc->device || !lc->filename)
@@ -1354,7 +1354,13 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 		goto err;
 	}
 
-	if (ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info)) {
+	do {
+		err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info);
+		again = err && errno == EAGAIN;
+		if (again)
+			usleep(250000);
+	} while (again);
+	if (err) {
 		rc = -errno;
 		errsv = errno;
 		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64 failed: %m"));
@@ -1399,7 +1405,7 @@ err:
  */
 int loopcxt_ioctl_status(struct loopdev_cxt *lc)
 {
-	int dev_fd, rc = -1;
+	int dev_fd, rc = -1, err, again;
 
 	errno = 0;
 	dev_fd = loopcxt_get_fd(lc);
@@ -1410,7 +1416,13 @@ int loopcxt_ioctl_status(struct loopdev_cxt *lc)
 	}
 	DBG(SETUP, ul_debugobj(lc, "device open: OK"));
 
-	if (ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info)) {
+	do {
+		err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info);
+		again = err && errno == EAGAIN;
+		if (again)
+			usleep(250000);
+	} while (again);
+	if (err) {
 		rc = -errno;
 		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64 failed: %m"));
 		return rc;
-- 
2.17.1