[PATCH] mkfs.xfs fix detection of empty devices

[Christoph Hellwig <hch@infradead.org>]

We currently fail to detect that a device does indeed not contain any
signature and we are indeed fine to proceed with it due to mishandling
the return value of blkid_do_fullprobe.  Fix that up and add some
better diagnostics of the blkid detection.  Also remove the size == 0
check in check_overwrite as blkid handles that just fine.

from RH bugzilla https://bugzilla.redhat.com/show_bug.cgi?id=561870

# dd if=/dev/zero of=k bs=1MB count=2 seek=20; mkfs.xfs k 
# mkfs.xfs: probe of k failed, cannot detect existing filesystem.
# mkfs.xfs: Use the -f option to force overwrite

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: xfsprogs-dev/mkfs/xfs_mkfs.c
===================================================================
--- xfsprogs-dev.orig/mkfs/xfs_mkfs.c	2010-02-04 19:19:36.000000000 +0000
+++ xfsprogs-dev/mkfs/xfs_mkfs.c	2010-02-04 19:33:36.000000000 +0000
@@ -297,48 +297,47 @@ check_overwrite(
 	const char	*type;
 	blkid_probe	pr = NULL;
 	int		ret;
-	int		fd;
-	long long	size;
-	int		bsz;
 
 	if (!device || !*device)
 		return 0;
 
 	ret = -1; /* will reset on success of all setup calls */
 
-	fd = open(device, O_RDONLY);
-	if (fd < 0)
-		goto out;
-	platform_findsizes(device, fd, &size, &bsz);
-	close(fd);
-
-	/* nothing to overwrite on a 0-length device */
-	if (size == 0) {
-		ret = 0;
-		goto out;
-	}
-
 	pr = blkid_new_probe_from_filename(device);
 	if (!pr)
 		goto out;
 
-	if (blkid_probe_enable_partitions(pr, 1))
+	ret = blkid_probe_enable_partitions(pr, 1);
+	if (ret < 0)
+		goto out;
+
+	ret = blkid_do_fullprobe(pr);
+	if (ret < 0)
 		goto out;
 
-	if (blkid_do_fullprobe(pr))
+	/*
+	 * Blkid returns 1 for nothing found and 0 when it finds a signature,
+	 * but we want the exact opposite, so reverse the return value here.
+	 *
+	 * In addition print some useful diagnostics about what actually is
+	 * on the device.
+	 */
+	ret = !ret;
+	if (!ret)
 		goto out;
 
-	ret = 0;
 	if (!blkid_probe_lookup_value(pr, "TYPE", &type, NULL)) {
 		fprintf(stderr,
 			_("%s: %s appears to contain an existing "
 			"filesystem (%s).\n"), progname, device, type);
-		ret = 1;
 	} else if (!blkid_probe_lookup_value(pr, "PTTYPE", &type, NULL)) {
 		fprintf(stderr,
 			_("%s: %s appears to contain a partition "
 			"table (%s).\n"), progname, device, type);
-		ret = 1;
+	} else {
+		fprintf(stderr,
+			_("%s: %s appears to contain something weird "
+			"according to blkid\n"), progname, device);
 	}
 
 out:

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