Have the GPT probe return a lower priority when the MBR is not a PMBR

The purpose of the PMBR is to have the disk appear in use to GPT
unaware utilities (like fdisk).  However, if the PMBR has been changed
by a GPT unaware utlity then we must assume that this was deliberate
(as it involved removal of the special slice) and we should not treat
the unmodified GPT-specific sectors as being valid.  By lowering the
probe priority in that case, the MBR scheme will take precedence and
the kernel will end up using the MBR and not the GPT. We will still
use the GPT if the kernel does not support the MBR scheme.

1 files changed, 15 insertions, 5 deletions

diff --git a/sys/geom/part/g_part_gpt.c b/sys/geom/part/g_part_gpt.c
index 13adf470d8bb..d10404756da4 100644
--- a/sys/geom/part/g_part_gpt.c
+++ b/sys/geom/part/g_part_gpt.c
@@ -759,8 +759,8 @@ static int
 g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 {
 	struct g_provider *pp;
-	char *buf;
-	int error, res;
+	u_char *buf;
+	int error, index, pri, res;
 
 	/* We don't nest, which means that our depth should be 0. */
 	if (table->gpt_depth != 0)
@@ -785,11 +785,21 @@ g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 	if (pp->sectorsize < MBRSIZE || pp->mediasize < 6 * pp->sectorsize)
 		return (ENOSPC);
 
-	/* Check that there's a MBR. */
+	/*
+	 * Check that there's a MBR or a PMBR. If it's a PMBR, we return
+	 * as the highest priority on a match, otherwise we assume some
+	 * GPT-unaware tool has destroyed the GPT by recreating a MBR and
+	 * we really want the MBR scheme to take precedence.
+	 */
 	buf = g_read_data(cp, 0L, pp->sectorsize, &error);
 	if (buf == NULL)
 		return (error);
 	res = le16dec(buf + DOSMAGICOFFSET);
+	pri = G_PART_PROBE_PRI_LOW;
+	for (index = 0; index < NDOSPART; index++) {
+		if (buf[DOSPARTOFF + DOSPARTSIZE * index + 4] == 0xee)
+			pri = G_PART_PROBE_PRI_HIGH;
+	}
 	g_free(buf);
 	if (res != DOSMAGIC) 
 		return (ENXIO);
@@ -801,7 +811,7 @@ g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 	res = memcmp(buf, GPT_HDR_SIG, 8);
 	g_free(buf);
 	if (res == 0)
-		return (G_PART_PROBE_PRI_HIGH);
+		return (pri);
 
 	/* No primary? Check that there's a secondary. */
 	buf = g_read_data(cp, pp->mediasize - pp->sectorsize, pp->sectorsize,
@@ -810,7 +820,7 @@ g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 		return (error);
 	res = memcmp(buf, GPT_HDR_SIG, 8); 
 	g_free(buf);
-	return ((res == 0) ? G_PART_PROBE_PRI_HIGH : ENXIO);
+	return ((res == 0) ? pri : ENXIO);
 }
 
 static int