Fixed handling of short reads in readdisklabel() and writedisklabel().

These functions use DEV_STRATEGY() which can easily return a short
count (with no error) for reads near EOF.  EOF happens for "disks" too
small to contain a label sector (mainly for empty slices).  The functions
didn't understand this at all, and looked for labels in the garbage
in the buffer beyond what DEV_STRATEGY() returned.  The recent UMA
changes combined with my local changes and configuration resulted in
the garbage often containing a valid but garbage label left over from
a previous call.

Bugs in EOF handling in -current limited the problem to "disks" with
size precisely LABELSECTOR sectors.  LABELSECTOR happens to be a very
unusual "disk" size since it is only 0 for non-i386 arches that don't
usually have disks with DOS MBRs.

1 files changed, 6 insertions, 0 deletions

diff --git a/sys/kern/subr_disklabel.c b/sys/kern/subr_disklabel.c
index 05339cb..d69e906 100644
--- a/sys/kern/subr_disklabel.c
+++ b/sys/kern/subr_disklabel.c
@@ -219,6 +219,8 @@ readdisklabel(dev, lp)
 	DEV_STRATEGY(bp, 1);
 	if (bufwait(bp))
 		msg = "I/O error";
+	else if (bp->b_resid != 0)
+		msg = "disk too small for a label";
 	else for (dlp = (struct disklabel *)bp->b_data;
 	    dlp <= (struct disklabel *)((char *)bp->b_data +
 	    lp->d_secsize - sizeof(*dlp));
@@ -323,6 +325,10 @@ writedisklabel(dev, lp)
 	error = bufwait(bp);
 	if (error)
 		goto done;
+	if (bp->b_resid != 0) {
+		error = ENOSPC;
+		goto done;
+	}
 	for (dlp = (struct disklabel *)bp->b_data;
 	    dlp <= (struct disklabel *)
 	      ((char *)bp->b_data + lp->d_secsize - sizeof(*dlp));