[git commit] loop: fix a race when a free loop device is snatched

[Denys Vlasenko]

commit: https://git.busybox.net/busybox/commit/?id=cb8d2ea8c91b5671b05e06ab2282496104453378
branch: https://git.busybox.net/busybox/commit/?id=refs/heads/master

When /dev/loop-control exists and *device is empty,
the mount may fail if a concurrent mount is running.

function                                             old     new   delta
set_loop                                             809     807      -2

Signed-off-by: Xiaoming Ni <nixiaoming at huawei.com>
Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
---
 libbb/loop.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/libbb/loop.c b/libbb/loop.c
index cb8fa2442..750642ade 100644
--- a/libbb/loop.c
+++ b/libbb/loop.c
@@ -218,8 +218,17 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
 			}
 			/* failure, undo LOOP_SET_FD */
 			ioctl(lfd, LOOP_CLR_FD, 0); // actually, 0 param is unnecessary
+		} else {
+			/* device is not free (rc == 0), or error other than ENXIO */
+			if (rc == 0	/* device is not free? */
+			 && !*device	/* racing with other mount? */
+			 && try != dev	/* tried a _kernel-offered_ loopN? */
+			) {
+				free(try);
+				close(lfd);
+				goto get_free_loopN;
+			}
 		}
-		/* else: device is not free (rc == 0) or error other than ENXIO */
  close_and_try_next_loopN:
 		close(lfd);
  try_next_loopN: