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[PATCH] mkfs.xfs fix detection of empty devices

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: [PATCH] mkfs.xfs fix detection of empty devices
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 4 Feb 2010 14:40:00 -0500
Cc: xfs mailing list <xfs@xxxxxxxxxxx>, Jim Meyering <meyering@xxxxxxxxxx>
In-reply-to: <4B6AF6FC.6030101@xxxxxxxxxx>
References: <4B6AF6FC.6030101@xxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
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@xxxxxx>

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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