Avoid races between init mounting loopback device and udev probing it

LOOP_SET_FD, LOOP_SET_STATUS ioctls trigger a `change` event with
loopback device in question. udev handles those events with
(builtin) blkid command. Probing a device with blkid takes a while,
so init might try to mount the loopback device in question while
`blkid` is still running. As a result init and udev block each other.
Eventually (after 3 minutes or whatever udev event timeout is)
`blkid` gets killed and boot proceeds. However such long delays are
very annoying. Therefore run `udev_settle` after each loop related
ioctl to avoid the concurrent access to the same loopback device.

Closes: #40687

lomount.c

@@ -31,6 +31,7 @@
 #include "log.h"
 #include "mount.h"
 #include "modules.h"
+#include "udev.h"
 
 #include "lomount.h"
 
@@ -98,6 +99,13 @@ set_loop (const char *device, const char *file)
 		close(ffd);
 		return 1;
 	}
+	/* Note: LOOP_SET_FD triggers change event. While processing it
+	 * udev reads the loop device with builtin blkid. This can race
+	 * with subsequent access by kernel due to LOOP_SET_STATUS (or
+	 * mounting the loop device). Therefore give udev a chance to
+	 * process the event without concurrent access to the loop device
+	 */
+	udev_settle();
 
 	if (ioctl(fd, LOOP_SET_STATUS, &loopinfo) < 0) {
 		(void) ioctl (fd, LOOP_CLR_FD, 0);
@@ -106,8 +114,21 @@ set_loop (const char *device, const char *file)
 		return 1;
 	}
 
+	/* Same here: LOOP_SET_STATUS triggers change event, give udev
+	 * a chance to process it without concurrent access to the loop
+	 * device. In other words, prevent the caller of this function
+	 * from mounting the device while udev still running blkid on it
+	 */
+	udev_settle();
+
 	close(fd);
 	close(ffd);
+	/* udev might be monitoring loopback device (with fanotify) and
+	 * will synthesize a change event on close. Give udev some time
+	 * to process without racing with the caller of this function
+	 * (which is going to mount the newly configured loopback device)
+	 */
+	udev_settle();
 	return 0;
 }
 

tools.c

@@ -47,6 +47,7 @@
 #ifdef SPAWN_SPLASH
 #include "init.h"
 #endif
+#include "udev.h"
 
 /* If we have more than that amount of memory (in bytes), we assume we can load the second stage as a ramdisk */
 #define MEM_LIMIT_RAMDISK (256 * 1024 * 1024)
@@ -650,6 +651,13 @@ int do_losetup(char * device, char * target)
 		close( targfd );
 		return -1;
 	}
+	/* Note: LOOP_SET_FD triggers change event. While processing it
+	 * udev reads the loop device with builtin blkid. This can race
+	 * with subsequent access by kernel due to LOOP_SET_STATUS (or
+	 * mounting the loop device). Therefore give udev a chance to
+	 * process the event without concurrent access to the loop device
+	 */
+	udev_settle();
 
 	memset(&loopInfo, 0, sizeof(loopInfo));
 	strcpy(loopInfo.lo_name, target);
@@ -657,7 +665,20 @@ int do_losetup(char * device, char * target)
 	if ( ioctl(loopfd, LOOP_SET_STATUS, &loopInfo) < 0 )
 		log_message( "losetup: error setting up loopback device: %s", strerror(errno) );
 
+	/* Same here: LOOP_SET_STATUS triggers change event, give udev
+	 * a chance to process it without concurrent access to the loop
+	 * device. In other words, prevent the caller of this function
+	 * from mounting the device while udev still running blkid on it
+	 */
+	udev_settle();
+
 	close( loopfd );
+	/* udev might be monitoring loopback device (with fanotify) and
+	 * will synthesize a change event on close. Give udev some time
+	 * to process without racing with the caller of this function
+	 * (which is going to mount the newly configured loopback device)
+	 */
+	udev_settle();
 	close( targfd );
 	return 0;
 }