Try to preserve the fdisk table in the primary bootstrap, should it

already exist.

Mention the cases where this doesn't work in the BUGS section of the
man page.

2 files changed, 47 insertions, 0 deletions

diff --git a/sbin/disklabel/disklabel.8 b/sbin/disklabel/disklabel.8
index 1cc33abe7aff..ce1b1689fb37 100644
--- a/sbin/disklabel/disklabel.8
+++ b/sbin/disklabel/disklabel.8
@@ -362,3 +362,23 @@ will not install bootstrap code that overlaps it.
 When a disk name is given without a full pathname,
 the constructed device name uses the ``a'' partition on the tahoe,
 the ``c'' partition on all others.
+.Pp
+For the i386 architecture, the primary bootstrap sector contains
+an embedded
+.Em fdisk
+table.
+.Nm Disklabel
+takes care to not clobber it when installing a bootstrap only
+.Pq Fl B ,
+or when editing an existing label
+.Pq Fl e ,
+but it unconditionally writes the primary bootstrap program onto
+the disk for
+.Fl w
+or
+.Fl R ,
+thus replacing the
+.Em fdisk
+table by the dummy one in the bootstrap program.  This is only of
+concern if the disk is fully dedicated, so that the BSD disklabel
+starts at absolute block 0 on the disk.
diff --git a/sbin/disklabel/disklabel.c b/sbin/disklabel/disklabel.c
index cf93fc4373ff..3de7004e0f41 100644
--- a/sbin/disklabel/disklabel.c
+++ b/sbin/disklabel/disklabel.c
@@ -547,6 +547,10 @@ makebootarea(boot, dp, f)
 #if NUMBOOT == 1
 	struct stat sb;
 #endif
+#ifdef __i386__
+	char *tmpbuf;
+	int i, found;
+#endif /* i386 */
 #endif
 
 	/* XXX */
@@ -628,9 +632,32 @@ makebootarea(boot, dp, f)
 	if (b < 0)
 		Perror(xxboot);
 #if NUMBOOT > 1
+#ifdef __i386__
+	/*
+	 * XXX Botch alert.
+	 * The i386 has the so-called fdisk table embedded into the
+	 * primary bootstrap.  We take care to not clobber it, but
+	 * only if it does already contain some data.  (Otherwise,
+	 * the xxboot provides a template.)
+	 */
+	if ((tmpbuf = (char *)malloc((int)dp->d_secsize)) == 0)
+		Perror(xxboot);
+	memcpy((void *)tmpbuf, (void *)boot, (int)dp->d_secsize);
+#endif /* i386 */
 	if (read(b, boot, (int)dp->d_secsize) < 0)
 		Perror(xxboot);
 	(void)close(b);
+#ifdef __i386__
+	for (i = DOSPARTOFF, found = 0;
+	     !found && i < DOSPARTOFF + NDOSPART*sizeof(struct dos_partition);
+	     i++)
+		found = tmpbuf[i] != 0;
+	if (found)
+		memcpy((void *)&boot[DOSPARTOFF],
+		       (void *)&tmpbuf[DOSPARTOFF],
+		       NDOSPART * sizeof(struct dos_partition));
+	free(tmpbuf);
+#endif /* i386 */
 	b = open(bootxx, O_RDONLY);
 	if (b < 0)
 		Perror(bootxx);