From: Karel Zak <kzak@redhat.com>
Date: Fri, 17 Jun 2022 16:04:06 +0000 (+0200)
Subject: libmount: remove mtab locking
X-Git-Tag: v2.39-rc1~604
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=61394559c695c1ce685890c535533e0ffca45303;p=thirdparty%2Futil-linux.git

libmount: remove mtab locking

Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/libmount/src/lock.c b/libmount/src/lock.c
index f730af18c7..4835406472 100644
--- a/libmount/src/lock.c
+++ b/libmount/src/lock.c
@@ -13,11 +13,11 @@
 /**
  * SECTION: lock
  * @title: Locking
- * @short_description: locking methods for /etc/mtab or another libmount files
+ * @short_description: locking methods for utab or another libmount files
+ *
+ * Since v2.39 libmount does nto support classic mtab locking. Now all is based
+ * on flock only.
  *
- * The mtab lock is backwards compatible with the standard linux /etc/mtab
- * locking.  Note, it's necessary to use the same locking schema in all
- * applications that access the file.
  */
 #include <sys/time.h>
 #include <time.h>
@@ -37,12 +37,10 @@
  */
 struct libmnt_lock {
 	char	*lockfile;	/* path to lock file (e.g. /etc/mtab~) */
-	char	*linkfile;	/* path to link file (e.g. /etc/mtab~.<id>) */
 	int	lockfile_fd;	/* lock file descriptor */
 
 	unsigned int	locked :1,	/* do we own the lock? */
-			sigblock :1,	/* block signals when locked */
-			simplelock :1;	/* use flock rather than normal mtab lock */
+			sigblock :1;	/* block signals when locked */
 
 	sigset_t oldsigmask;
 };
@@ -51,46 +49,37 @@ struct libmnt_lock {
 /**
  * mnt_new_lock:
  * @datafile: the file that should be covered by the lock
- * @id: unique linkfile identifier or 0 (default is getpid())
+ * @id: ignored by library
  *
  * Returns: newly allocated lock handler or NULL on case of error.
  */
-struct libmnt_lock *mnt_new_lock(const char *datafile, pid_t id)
+struct libmnt_lock *mnt_new_lock(const char *datafile, pid_t id __attribute__((__unused__)))
 {
 	struct libmnt_lock *ml = NULL;
-	char *lo = NULL, *ln = NULL;
+	char *lo = NULL;
 	size_t losz;
 
 	if (!datafile)
 		return NULL;
 
-	/* for flock we use "foo.lock, for mtab "foo~"
-	 */
 	losz = strlen(datafile) + sizeof(".lock");
 	lo = malloc(losz);
 	if (!lo)
 		goto err;
 
-	/* default is mtab~ lock */
-	snprintf(lo, losz, "%s~", datafile);
+	snprintf(lo, losz, "%s.lock", datafile);
 
-	if (asprintf(&ln, "%s~.%d", datafile, id ? : getpid()) == -1) {
-		ln = NULL;
-		goto err;
-	}
 	ml = calloc(1, sizeof(*ml) );
 	if (!ml)
 		goto err;
 
 	ml->lockfile_fd = -1;
-	ml->linkfile = ln;
 	ml->lockfile = lo;
 
-	DBG(LOCKS, ul_debugobj(ml, "alloc: default linkfile=%s, lockfile=%s", ln, lo));
+	DBG(LOCKS, ul_debugobj(ml, "alloc: lockfile=%s", lo));
 	return ml;
 err:
 	free(lo);
-	free(ln);
 	free(ml);
 	return NULL;
 }
@@ -108,7 +97,6 @@ void mnt_free_lock(struct libmnt_lock *ml)
 		return;
 	DBG(LOCKS, ul_debugobj(ml, "free%s", ml->locked ? " !!! LOCKED !!!" : ""));
 	free(ml->lockfile);
-	free(ml->linkfile);
 	free(ml);
 }
 
@@ -131,38 +119,6 @@ int mnt_lock_block_signals(struct libmnt_lock *ml, int enable)
 	return 0;
 }
 
-/* don't export this to API
- */
-int mnt_lock_use_simplelock(struct libmnt_lock *ml, int enable)
-{
-	size_t sz;
-
-	if (!ml)
-		return -EINVAL;
-
-	assert(ml->lockfile);
-
-	DBG(LOCKS, ul_debugobj(ml, "flock: %s", enable ? "ENABLED" : "DISABLED"));
-	ml->simplelock = enable ? 1 : 0;
-
-	sz = strlen(ml->lockfile);
-	assert(sz);
-
-	/* Change lock name:
-	 *
-	 *	flock:     "<name>.lock"
-	 *	mtab lock: "<name>~"
-	 */
-	if (ml->simplelock && endswith(ml->lockfile, "~"))
-		memcpy(ml->lockfile + sz - 1, ".lock", 6);
-
-	else if (!ml->simplelock && endswith(ml->lockfile, ".lock"))
-		 memcpy(ml->lockfile + sz - 5, "~", 2);
-
-	DBG(LOCKS, ul_debugobj(ml, "new lock filename: '%s'", ml->lockfile));
-	return 0;
-}
-
 /*
  * Returns path to lockfile.
  */
@@ -171,24 +127,12 @@ static const char *mnt_lock_get_lockfile(struct libmnt_lock *ml)
 	return ml ? ml->lockfile : NULL;
 }
 
-/*
- * Note that the filename is generated by mnt_new_lock() and depends on
- * getpid() or 'id' argument of the mnt_new_lock() function.
- *
- * Returns: unique (per process/thread) path to linkfile.
- */
-static const char *mnt_lock_get_linkfile(struct libmnt_lock *ml)
-{
-	return ml ? ml->linkfile : NULL;
-}
-
 /*
  * Simple flocking
  */
 static void unlock_simplelock(struct libmnt_lock *ml)
 {
 	assert(ml);
-	assert(ml->simplelock);
 
 	if (ml->lockfile_fd >= 0) {
 		DBG(LOCKS, ul_debugobj(ml, "%s: unflocking",
@@ -205,7 +149,6 @@ static int lock_simplelock(struct libmnt_lock *ml)
 	const mode_t lock_mask = S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH;
 
 	assert(ml);
-	assert(ml->simplelock);
 
 	lfile = mnt_lock_get_lockfile(ml);
 
@@ -257,281 +200,14 @@ err:
 	return rc;
 }
 
-/*
- * traditional mtab locking
- */
-
-static void mnt_lockalrm_handler(int sig __attribute__((__unused__)))
-{
-	/* do nothing, say nothing, be nothing */
-}
-
-/*
- * Waits for F_SETLKW, unfortunately we have to use SIGALRM here to interrupt
- * fcntl() to avoid neverending waiting.
- *
- * Returns: 0 on success, 1 on timeout, -errno on error.
- */
-static int mnt_wait_mtab_lock(struct libmnt_lock *ml, struct flock *fl, time_t maxtime)
-{
-	struct timeval now = { 0 };
-	struct sigaction sa, osa;
-	int ret = 0;
-
-	gettime_monotonic(&now);
-	DBG(LOCKS, ul_debugobj(ml, "(%d) waiting for F_SETLKW (now=%lu, maxtime=%lu, diff=%lu)",
-				getpid(),
-				(unsigned long) now.tv_sec,
-				(unsigned long) maxtime,
-				(unsigned long) (maxtime - now.tv_sec)));
-
-	if (now.tv_sec >= maxtime)
-		return 1;		/* timeout */
-
-	/* setup ALARM handler -- we don't want to wait forever */
-	sa.sa_flags = 0;
-	sa.sa_handler = mnt_lockalrm_handler;
-	sigfillset (&sa.sa_mask);
-
-	sigaction(SIGALRM, &sa, &osa);
-
-
-	alarm(maxtime - now.tv_sec);
-	if (fcntl(ml->lockfile_fd, F_SETLKW, fl) == -1)
-		ret = errno == EINTR ? 1 : -errno;
-	alarm(0);
-
-	/* restore old sigaction */
-	sigaction(SIGALRM, &osa, NULL);
-
-	DBG(LOCKS, ul_debugobj(ml, "(%d) leaving mnt_wait_setlkw(), rc=%d",
-				getpid(), ret));
-	return ret;
-}
-
-/*
- * Create the mtab lock file.
- *
- * The old code here used flock on a lock file /etc/mtab~ and deleted
- * this lock file afterwards. However, as rgooch remarks, that has a
- * race: a second mount may be waiting on the lock and proceed as
- * soon as the lock file is deleted by the first mount, and immediately
- * afterwards a third mount comes, creates a new /etc/mtab~, applies
- * flock to that, and also proceeds, so that the second and third mount
- * are now both scribbling in /etc/mtab.
- *
- * The new code uses a link() instead of a creat(), where we proceed
- * only if it was us that created the lock, and hence we always have
- * to delete the lock afterwards. Now the use of flock() is in principle
- * superfluous, but avoids an arbitrary sleep().
- *
- * Where does the link point to? Obvious choices are mtab and mtab~~.
- * HJLu points out that the latter leads to races. Right now we use
- * mtab~.pid instead.
- *
- *
- * The original mount locking code has used sleep(1) between attempts and
- * maximal number of attempts has been 5.
- *
- * There was a very small number of attempts and extremely long waiting (1s)
- * that is useless on machines with large number of mount processes.
- *
- * Now we wait for a few thousand microseconds between attempts and we have a global
- * time limit (30s) rather than a limit for the number of attempts. The advantage
- * is that this method also counts time which we spend in fcntl(F_SETLKW) and
- * the number of attempts is not restricted.
- * -- kzak@redhat.com [Mar-2007]
- *
- *
- * This mtab locking code has been refactored and moved to libmount. The mtab
- * locking is really not perfect (e.g. SIGALRM), but it's stable, reliable and
- * backwardly compatible code.
- *
- * Don't forget that this code has to be compatible with 3rd party mounts
- * (/sbin/mount.foo) and has to work with NFS.
- * -- kzak@redhat.com [May-2009]
- */
-
-/* maximum seconds between the first and the last attempt */
-#define MOUNTLOCK_MAXTIME		30
-
-/* sleep time (in microseconds, max=999999) between attempts */
-#define MOUNTLOCK_WAITTIME		5000
-
-static void unlock_mtab(struct libmnt_lock *ml)
-{
-	if (!ml)
-		return;
-
-	if (!ml->locked && ml->lockfile && ml->linkfile)
-	{
-		/* We (probably) have all the files, but we don't own the lock,
-		 * Really? Check it! Maybe ml->locked wasn't set properly
-		 * because the code was interrupted by a signal. Paranoia? Yes.
-		 *
-		 * We own the lock when linkfile == lockfile.
-		 */
-		struct stat lo, li;
-
-		if (!stat(ml->lockfile, &lo) && !stat(ml->linkfile, &li) &&
-		    lo.st_dev == li.st_dev && lo.st_ino == li.st_ino)
-			ml->locked = 1;
-	}
-
-	if (ml->linkfile)
-		unlink(ml->linkfile);
-	if (ml->lockfile_fd >= 0)
-		close(ml->lockfile_fd);
-	if (ml->locked && ml->lockfile) {
-		unlink(ml->lockfile);
-		DBG(LOCKS, ul_debugobj(ml, "unlink %s", ml->lockfile));
-	}
-}
-
-static int lock_mtab(struct libmnt_lock *ml)
-{
-	int i, rc = -1;
-	struct timespec waittime = { 0 };;
-	struct timeval maxtime = { 0 };
-	const char *lockfile, *linkfile;
-
-	if (!ml)
-		return -EINVAL;
-	if (ml->locked)
-		return 0;
-
-	lockfile = mnt_lock_get_lockfile(ml);
-	if (!lockfile)
-		return -EINVAL;
-	linkfile = mnt_lock_get_linkfile(ml);
-	if (!linkfile)
-		return -EINVAL;
-
-	if (ml->sigblock) {
-		/*
-		 * Block all signals when locked, mnt_unlock_file() will
-		 * restore the old mask.
-		 */
-		sigset_t sigs;
-
-		sigemptyset(&ml->oldsigmask);
-		sigfillset(&sigs);
-		sigdelset(&sigs, SIGTRAP);
-		sigdelset(&sigs, SIGALRM);
-		sigprocmask(SIG_BLOCK, &sigs, &ml->oldsigmask);
-	}
-
-	i = open(linkfile, O_WRONLY|O_CREAT|O_CLOEXEC, S_IRUSR|S_IWUSR);
-	if (i < 0) {
-		/* linkfile does not exist (as a file) and we cannot create it.
-		 * Read-only or full filesystem? Too many files open in the system?
-		 */
-		if (errno > 0)
-			rc = -errno;
-		goto failed;
-	}
-	close(i);
-
-	gettime_monotonic(&maxtime);
-	maxtime.tv_sec += MOUNTLOCK_MAXTIME;
-
-	waittime.tv_sec = 0;
-	waittime.tv_nsec = (1000 * MOUNTLOCK_WAITTIME);
-
-	/* Repeat until it was us who made the link */
-	while (!ml->locked) {
-		struct timeval now = { 0 };
-		struct flock flock;
-		int j;
-
-		j = link(linkfile, lockfile);
-		if (j == 0)
-			ml->locked = 1;
-
-		if (j < 0 && errno != EEXIST) {
-			if (errno > 0)
-				rc = -errno;
-			goto failed;
-		}
-		ml->lockfile_fd = open(lockfile, O_WRONLY|O_CLOEXEC);
-
-		if (ml->lockfile_fd < 0) {
-			/* Strange... Maybe the file was just deleted? */
-			int errsv = errno;
-			gettime_monotonic(&now);
-			if (errsv == ENOENT && now.tv_sec < maxtime.tv_sec) {
-				ml->locked = 0;
-				continue;
-			}
-			if (errsv > 0)
-				rc = -errsv;
-			goto failed;
-		}
-
-		flock.l_type = F_WRLCK;
-		flock.l_whence = SEEK_SET;
-		flock.l_start = 0;
-		flock.l_len = 0;
-
-		if (ml->locked) {
-			/* We made the link. Now claim the lock. */
-			if (fcntl (ml->lockfile_fd, F_SETLK, &flock) == -1) {
-				DBG(LOCKS, ul_debugobj(ml,
-					"%s: can't F_SETLK lockfile, errno=%d\n",
-					lockfile, errno));
-				/* proceed, since it was us who created the lockfile anyway */
-			}
-			break;
-		}
-
-		/* Someone else made the link. Wait. */
-		int err = mnt_wait_mtab_lock(ml, &flock, maxtime.tv_sec);
-
-		if (err == 1) {
-			DBG(LOCKS, ul_debugobj(ml,
-				"%s: can't create link: time out (perhaps "
-				"there is a stale lock file?)", lockfile));
-			rc = -ETIMEDOUT;
-			goto failed;
-
-		} else if (err < 0) {
-			rc = err;
-			goto failed;
-		}
-		nanosleep(&waittime, NULL);
-		close(ml->lockfile_fd);
-		ml->lockfile_fd = -1;
-	}
-	DBG(LOCKS, ul_debugobj(ml, "%s: (%d) successfully locked",
-					lockfile, getpid()));
-	unlink(linkfile);
-	return 0;
-
-failed:
-	mnt_unlock_file(ml);
-	return rc;
-}
-
-
 /**
  * mnt_lock_file
  * @ml: pointer to struct libmnt_lock instance
  *
- * Creates a lock file (e.g. /etc/mtab~). Note that this function may
- * use alarm().
- *
- * Your application always has to call mnt_unlock_file() before exit.
- *
- * Traditional mtab locking scheme:
- *
- *   1. create linkfile (e.g. /etc/mtab~.$PID)
- *   2. link linkfile --> lockfile (e.g. /etc/mtab~.$PID --> /etc/mtab~)
- *   3. a) link() success: setups F_SETLK lock (see fcntl(2))
- *      b) link() failed:  wait (max 30s) on F_SETLKW lock, goto 2.
- *
+ * Creates a lock file.
+*
  * Note that when the lock is used by mnt_update_table() interface then libmount
- * uses flock() for private library file /run/mount/utab. The fcntl(2) is used only
- * for backwardly compatible stuff like /etc/mtab.
+ * uses flock() for private library file /run/mount/utab.
  *
  * Returns: 0 on success or negative number in case of error (-ETIMEOUT is case
  * of stale lock file).
@@ -541,10 +217,7 @@ int mnt_lock_file(struct libmnt_lock *ml)
 	if (!ml)
 		return -EINVAL;
 
-	if (ml->simplelock)
-		return lock_simplelock(ml);
-
-	return lock_mtab(ml);
+	return lock_simplelock(ml);
 }
 
 /**
@@ -562,10 +235,7 @@ void mnt_unlock_file(struct libmnt_lock *ml)
 	DBG(LOCKS, ul_debugobj(ml, "(%d) %s", getpid(),
 			ml->locked ? "unlocking" : "cleaning"));
 
-	if (ml->simplelock)
-		unlock_simplelock(ml);
-	else
-		unlock_mtab(ml);
+	unlock_simplelock(ml);
 
 	ml->locked = 0;
 	ml->lockfile_fd = -1;
diff --git a/libmount/src/mountP.h b/libmount/src/mountP.h
index 7a8ed0b863..ffd9ba4fe3 100644
--- a/libmount/src/mountP.h
+++ b/libmount/src/mountP.h
@@ -394,9 +394,6 @@ struct libmnt_context
 /* Flags usable with MS_BIND|MS_REMOUNT */
 #define MNT_BIND_SETTABLE	(MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_NOATIME|MS_NODIRATIME|MS_RELATIME|MS_RDONLY|MS_NOSYMFOLLOW)
 
-/* lock.c */
-extern int mnt_lock_use_simplelock(struct libmnt_lock *ml, int enable);
-
 /* optmap.c */
 extern const struct libmnt_optmap *mnt_optmap_get_entry(
 			     struct libmnt_optmap const **maps,
diff --git a/libmount/src/tab_update.c b/libmount/src/tab_update.c
index b045b1dcb5..f914df4122 100644
--- a/libmount/src/tab_update.c
+++ b/libmount/src/tab_update.c
@@ -854,8 +854,6 @@ int mnt_update_table(struct libmnt_update *upd, struct libmnt_lock *lc)
 		if (lc)
 			mnt_lock_block_signals(lc, TRUE);
 	}
-	if (lc)
-		mnt_lock_use_simplelock(lc, TRUE);	/* use flock */
 
 	if (!upd->fs && upd->target)
 		rc = update_remove_entry(upd, lc);	/* umount */
@@ -890,8 +888,6 @@ int mnt_update_already_done(struct libmnt_update *upd, struct libmnt_lock *lc)
 		if (lc)
 			mnt_lock_block_signals(lc, TRUE);
 	}
-	if (lc)
-		mnt_lock_use_simplelock(lc, TRUE);	/* use flock */
 	if (lc) {
 		rc = mnt_lock_file(lc);
 		if (rc) {