Make detection of GPT a bit more reliable.

When we are detecting a partition table and didn't find PMBR, try to
read backup GPT header from the last sector and if it is correct,
assume that we have GPT.

Reviewed by:	rpokala
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D4282

1 files changed, 16 insertions, 15 deletions

diff --git a/sys/geom/part/g_part_gpt.c b/sys/geom/part/g_part_gpt.c
index 922e814..fa9973c 100644
--- a/sys/geom/part/g_part_gpt.c
+++ b/sys/geom/part/g_part_gpt.c
@@ -823,22 +823,23 @@ g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 		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);
+	if (res == DOSMAGIC) {
+		for (index = 0; index < NDOSPART; index++) {
+			if (buf[DOSPARTOFF + DOSPARTSIZE * index + 4] == 0xee)
+				pri = G_PART_PROBE_PRI_HIGH;
+		}
+		g_free(buf);
 
-	/* Check that there's a primary header. */
-	buf = g_read_data(cp, pp->sectorsize, pp->sectorsize, &error);
-	if (buf == NULL)
-		return (error);
-	res = memcmp(buf, GPT_HDR_SIG, 8);
-	g_free(buf);
-	if (res == 0)
-		return (pri);
+		/* Check that there's a primary header. */
+		buf = g_read_data(cp, pp->sectorsize, pp->sectorsize, &error);
+		if (buf == NULL)
+			return (error);
+		res = memcmp(buf, GPT_HDR_SIG, 8);
+		g_free(buf);
+		if (res == 0)
+			return (pri);
+	} else
+		g_free(buf);
 
 	/* No primary? Check that there's a secondary. */
 	buf = g_read_data(cp, pp->mediasize - pp->sectorsize, pp->sectorsize,