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Avoid races between init mounting loopback device and udev probing it

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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
This commit is contained in:
Alexey Sheplyakov 2021-08-11 21:15:40 +04:00
parent 095e17ab91
commit db44ebdc74
2 changed files with 42 additions and 0 deletions
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21
lomount.c
View File

@ -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;
}

21
tools.c
View File

@ -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;
}