fstab.c

Util-linux 软件包包含许多工具。其中比较重要的是加载、卸载、格式化、分区和管理硬盘驱动器

		    (streq(mc->m.mnt_dir, file) ||
		     streq(canonicalize(mc->m.mnt_dir), file))) {
			if (has_uuid(spec, mc->m.mnt_fsname+5))
				return mc;
		}
	}
	return NULL;
}

/* Find the dir FILE in fstab.  */
struct mntentchn *
getfsfile (const char *file) {
	struct mntentchn *mc, *mc0;

	mc0 = fstab_head();
	for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt)
		if (streq(mc->m.mnt_dir, file))
			return mc;
	return NULL;
}

/* Find the device SPEC in fstab.  */
struct mntentchn *
getfsspec (const char *spec) {
	struct mntentchn *mc, *mc0;

	mc0 = fstab_head();
	for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt)
		if (streq(mc->m.mnt_fsname, spec))
			return mc;
	return NULL;
}

/* Find the uuid UUID in fstab. */
struct mntentchn *
getfsuuidspec (const char *uuid) {
	struct mntentchn *mc, *mc0;

	mc0 = fstab_head();
	for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt)
		if (strncmp (mc->m.mnt_fsname, "UUID=", 5) == 0
		    && streq(mc->m.mnt_fsname + 5, uuid))
			return mc;
	return NULL;
}

/* Find the label LABEL in fstab. */
struct mntentchn *
getfsvolspec (const char *label) {
	struct mntentchn *mc, *mc0;

	mc0 = fstab_head();
	for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt)
		if (strncmp (mc->m.mnt_fsname, "LABEL=", 6) == 0
		    && streq(mc->m.mnt_fsname + 6, label))
			return mc;
	return NULL;
}

/* Updating mtab ----------------------------------------------*/

/* Flag for already existing lock file. */
static int we_created_lockfile = 0;

/* Flag to indicate that signals have been set up. */
static int signals_have_been_setup = 0;

/* Ensure that the lock is released if we are interrupted.  */
extern char *strsignal(int sig);	/* not always in <string.h> */

static void
handler (int sig) {
     die(EX_USER, "%s", strsignal(sig));
}

static void
setlkw_timeout (int sig) {
     /* nothing, fcntl will fail anyway */
}

/* Create the lock file.
   The lock file will be removed if we catch a signal or when we exit. */
/* 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
   now both are 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. Use 20 as upper bound for the length of %d. */
#define MOUNTLOCK_LINKTARGET		MOUNTED_LOCK "%d"
#define MOUNTLOCK_LINKTARGET_LTH	(sizeof(MOUNTED_LOCK)+20)

void
lock_mtab (void) {
	int tries = 3;
	char linktargetfile[MOUNTLOCK_LINKTARGET_LTH];

	if (!signals_have_been_setup) {
		int sig = 0;
		struct sigaction sa;

		sa.sa_handler = handler;
		sa.sa_flags = 0;
		sigfillset (&sa.sa_mask);
  
		while (sigismember (&sa.sa_mask, ++sig) != -1
		       && sig != SIGCHLD) {
			if (sig == SIGALRM)
				sa.sa_handler = setlkw_timeout;
			else
				sa.sa_handler = handler;
			sigaction (sig, &sa, (struct sigaction *) 0);
		}
		signals_have_been_setup = 1;
	}

	sprintf(linktargetfile, MOUNTLOCK_LINKTARGET, getpid ());

	/* Repeat until it was us who made the link */
	while (!we_created_lockfile) {
		struct flock flock;
		int errsv, fd, i, j;

		i = open (linktargetfile, O_WRONLY|O_CREAT, 0);
		if (i < 0) {
			int errsv = errno;
			/* linktargetfile does not exist (as a file)
			   and we cannot create it. Read-only filesystem?
			   Too many files open in the system?
			   Filesystem full? */
			die (EX_FILEIO, _("can't create lock file %s: %s "
			     "(use -n flag to override)"),
			     linktargetfile, strerror (errsv));
		}
		close(i);

		j = link(linktargetfile, MOUNTED_LOCK);
		errsv = errno;

		(void) unlink(linktargetfile);

		if (j == 0)
			we_created_lockfile = 1;

		if (j < 0 && errsv != EEXIST) {
			die (EX_FILEIO, _("can't link lock file %s: %s "
			     "(use -n flag to override)"),
			     MOUNTED_LOCK, strerror (errsv));
		}

		fd = open (MOUNTED_LOCK, O_WRONLY);

		if (fd < 0) {
			int errsv = errno;
			/* Strange... Maybe the file was just deleted? */
			if (errno == ENOENT && tries-- > 0)
				continue;
			die (EX_FILEIO, _("can't open lock file %s: %s "
			     "(use -n flag to override)"),
			     MOUNTED_LOCK, strerror (errsv));
		}

		flock.l_type = F_WRLCK;
		flock.l_whence = SEEK_SET;
		flock.l_start = 0;
		flock.l_len = 0;

		if (j == 0) {
			/* We made the link. Now claim the lock. */
			if (fcntl (fd, F_SETLK, &flock) == -1) {
				if (verbose) {
				    int errsv = errno;
				    printf(_("Can't lock lock file %s: %s\n"),
					   MOUNTED_LOCK, strerror (errsv));
				}
				/* proceed anyway */
			}
		} else {
			static int tries = 0;

			/* Someone else made the link. Wait. */
			alarm(LOCK_TIMEOUT);
			if (fcntl (fd, F_SETLKW, &flock) == -1) {
				int errsv = errno;
				die (EX_FILEIO, _("can't lock lock file %s: %s"),
				     MOUNTED_LOCK, (errno == EINTR) ?
				     _("timed out") : strerror (errsv));
			}
			alarm(0);
			/* Limit the number of iterations - maybe there
			   still is some old /etc/mtab~ */
			if (tries++ > 3) {
				if (tries > 5)
					die (EX_FILEIO, _("Cannot create link %s\n"
					    "Perhaps there is a stale lock file?\n"),
					     MOUNTED_LOCK);
				sleep(1);
			}
		}

		close(fd);
	}
}

/* Remove lock file.  */
void
unlock_mtab (void) {
	if (we_created_lockfile) {
		unlink (MOUNTED_LOCK);
		we_created_lockfile = 0;
	}
}

/*
 * Update the mtab.
 *  Used by umount with null INSTEAD: remove the last DIR entry.
 *  Used by mount upon a remount: update option part,
 *   and complain if a wrong device or type was given.
 *   [Note that often a remount will be a rw remount of /
 *    where there was no entry before, and we'll have to believe
 *    the values given in INSTEAD.]
 */

void
update_mtab (const char *dir, struct my_mntent *instead) {
	mntFILE *mfp, *mftmp;
	const char *fnam = MOUNTED;
	struct mntentchn mtabhead;	/* dummy */
	struct mntentchn *mc, *mc0, *absent = NULL;

	if (mtab_does_not_exist() || mtab_is_a_symlink())
		return;

	lock_mtab();

	/* having locked mtab, read it again */
	mc0 = mc = &mtabhead;
	mc->nxt = mc->prev = NULL;

	mfp = my_setmntent(fnam, "r");
	if (mfp == NULL || mfp->mntent_fp == NULL) {
		int errsv = errno;
		error (_("cannot open %s (%s) - mtab not updated"),
		       fnam, strerror (errsv));
		goto leave;
	}

	read_mntentchn(mfp, fnam, mc);

	/* find last occurrence of dir */
	for (mc = mc0->prev; mc && mc != mc0; mc = mc->prev)
		if (streq(mc->m.mnt_dir, dir))
			break;
	if (mc && mc != mc0) {
		if (instead == NULL) {
			/* An umount - remove entry */
			if (mc && mc != mc0) {
				mc->prev->nxt = mc->nxt;
				mc->nxt->prev = mc->prev;
				free(mc);
			}
		} else {
			/* A remount */
			mc->m.mnt_opts = instead->mnt_opts;
		}
	} else if (instead) {
		/* not found, add a new entry */
		absent = xmalloc(sizeof(*absent));
		absent->m = *instead;
		absent->nxt = mc0;
		absent->prev = mc0->prev;
		mc0->prev = absent;
		if (mc0->nxt == NULL)
			mc0->nxt = absent;
	}

	/* write chain to mtemp */
	mftmp = my_setmntent (MOUNTED_TEMP, "w");
	if (mftmp == NULL || mftmp->mntent_fp == NULL) {
		int errsv = errno;
		error (_("cannot open %s (%s) - mtab not updated"),
		       MOUNTED_TEMP, strerror (errsv));
		goto leave;
	}

	for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt) {
		if (my_addmntent(mftmp, &(mc->m)) == 1) {
			int errsv = errno;
			die (EX_FILEIO, _("error writing %s: %s"),
			     MOUNTED_TEMP, strerror (errsv));
		}
	}

	discard_mntentchn(mc0);

	if (fchmod (fileno (mftmp->mntent_fp),
		    S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH) < 0) {
		int errsv = errno;
		fprintf(stderr, _("error changing mode of %s: %s\n"),
			MOUNTED_TEMP, strerror (errsv));
	}
	my_endmntent (mftmp);

	{ /*
	   * If mount is setuid and some non-root user mounts sth,
	   * then mtab.tmp might get the group of this user. Copy uid/gid
	   * from the present mtab before renaming.
	   */
	    struct stat sbuf;
	    if (stat (MOUNTED, &sbuf) == 0)
		chown (MOUNTED_TEMP, sbuf.st_uid, sbuf.st_gid);
	}

	/* rename mtemp to mtab */
	if (rename (MOUNTED_TEMP, MOUNTED) < 0) {
		int errsv = errno;
		fprintf(stderr, _("can't rename %s to %s: %s\n"),
			MOUNTED_TEMP, MOUNTED, strerror(errsv));
	}

 leave:
	unlock_mtab();
}