mount: create loop devices with LO_FLAGS_AUTOCLEAR flag

The "autolooped" mount (mount [-oloop] IMAGE /DIR/DIR)
always creates AUTOCLEARed loopdevs, so that umounting
drops them (and this does not require any code in the
umount userspace).
This happens since circa linux-2.6.25:
	commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
	Date:    Wed Feb 6 01:36:27 2008 -0800
	Subject: Allow auto-destruction of loop devices
IOW: in this case, umount does not have to use -d
to drop the loopdev.

The explicit loop mount (mount /dev/loopN /DIR/DIR)
does not do this. In this case, umount without -d
should not drop loopdev.
Unfortunately, bbox umount currently always implies -d,
this probably needs fixing.

function                                             old     new   delta
set_loop                                             537     597     +60
singlemount                                         1101    1138     +37
losetup_main                                         419     432     +13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 110/0)             Total: 110 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 33 insertions, 16 deletions

diff --git a/libbb/loop.c b/libbb/loop.c
index d30b378d7..f0d4296ae 100644
--- a/libbb/loop.c
+++ b/libbb/loop.c
@@ -78,22 +78,24 @@ int FAST_FUNC del_loop(const char *device)
         return rc;
 }
 
-/* Returns 0 if mounted RW, 1 if mounted read-only, <0 for error.
-   *device is loop device to use, or if *device==NULL finds a loop device to
-   mount it on and sets *device to a strdup of that loop device name.  This
-   search will re-use an existing loop device already bound to that
-   file/offset if it finds one.
+/* Returns opened fd to the loop device, <0 on error.
+ * *device is loop device to use, or if *device==NULL finds a loop device to
+ * mount it on and sets *device to a strdup of that loop device name.  This
+ * search will re-use an existing loop device already bound to that
+ * file/offset if it finds one.
  */
-int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, int ro)
+int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, unsigned flags)
 {
         char dev[LOOP_NAMESIZE];
         char *try;
         bb_loop_info loopinfo;
         struct stat statbuf;
-        int i, dfd, ffd, mode, rc = -1;
+        int i, dfd, ffd, mode, rc;
+
+        rc = dfd = -1;
 
         /* Open the file.  Barf if this doesn't work.  */
-        mode = ro ? O_RDONLY : O_RDWR;
+        mode = (flags & BB_LO_FLAGS_READ_ONLY) ? O_RDONLY : O_RDWR;
  open_ffd:
         ffd = open(file, mode);
         if (ffd < 0) {
@@ -144,20 +146,35 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
 
                 /* If device is free, claim it.  */
                 if (rc && errno == ENXIO) {
-                        memset(&loopinfo, 0, sizeof(loopinfo));
-                        safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
-                        loopinfo.lo_offset = offset;
                         /* Associate free loop device with file.  */
                         if (ioctl(dfd, LOOP_SET_FD, ffd) == 0) {
-                                if (ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo) == 0)
-                                        rc = 0;
-                                else
+                                memset(&loopinfo, 0, sizeof(loopinfo));
+                                safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
+                                loopinfo.lo_offset = offset;
+                                /*
+                                 * Used by mount to set LO_FLAGS_AUTOCLEAR.
+                                 * LO_FLAGS_READ_ONLY is not set because RO is controlled by open type of the file.
+                                 * Note that closing LO_FLAGS_AUTOCLEARed dfd before mount
+                                 * is wrong (would free the loop device!)
+                                 */
+                                loopinfo.lo_flags = (flags & ~BB_LO_FLAGS_READ_ONLY);
+                                rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
+                                if (rc != 0 && (loopinfo.lo_flags & BB_LO_FLAGS_AUTOCLEAR)) {
+                                        /* Old kernel, does not support LO_FLAGS_AUTOCLEAR? */
+                                        /* (this code path is not tested) */
+                                        loopinfo.lo_flags -= BB_LO_FLAGS_AUTOCLEAR;
+                                        rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
+                                }
+                                if (rc != 0) {
                                         ioctl(dfd, LOOP_CLR_FD, 0);
+                                }
                         }
                 } else {
                         rc = -1;
                 }
-                close(dfd);
+                if (rc != 0) {
+                        close(dfd);
+                }
  try_again:
                 if (*device) break;
         }
@@ -165,7 +182,7 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
         if (rc == 0) {
                 if (!*device)
                         *device = xstrdup(dev);
-                return (mode == O_RDONLY); /* 1:ro, 0:rw */
+                return dfd;
         }
         return rc;
 }