MFS r365667,r365920: extend kern.geom.part.check_integrity to work on GPT

There are multiple USB/SATA bridges on the market that unconditionally
cut some LBAs off connected media. This could be a problem
for pre-partitioned drives so GEOM complains and does not create
devices in /dev for slices/partitions preventing access to existing data.

We have a knob kern.geom.part.check_integrity that allows us to correct
partitioning if changed from default 1 to 0 but it works for MBR only.
If backup copy of GPT is unavailable due to decreased number of LBAs,
the kernel does not give access to partitions still and prints to dmesg:

GEOM: md0: corrupt or invalid GPT detected.
GEOM: md0: GPT rejected -- may not be recoverable.

This change makes it work for GPT too, so it created partitions in /dev
and prints to dmesg this instead:

GEOM: md0: the secondary GPT table is corrupt or invalid.
GEOM: md0: using the primary only -- recovery suggested.

Then "gpart recover" re-creates backup copy of GPT
and allows further manipulations with partitions.

This change is no-op for default configuration having
kern.geom.part.check_integrity=1

Reported by:	Alex Korchmar
Approved by:	re (gjb)

2 files changed, 10 insertions, 6 deletions

diff --git a/sys/geom/part/g_part.c b/sys/geom/part/g_part.c
index c969b9e7cc64..8b79b6404a6d 100644
--- a/sys/geom/part/g_part.c
+++ b/sys/geom/part/g_part.c
@@ -135,9 +135,9 @@ struct g_part_alias_list {
 SYSCTL_DECL(_kern_geom);
 SYSCTL_NODE(_kern_geom, OID_AUTO, part, CTLFLAG_RW, 0,
     "GEOM_PART stuff");
-static u_int check_integrity = 1;
+u_int geom_part_check_integrity = 1;
 SYSCTL_UINT(_kern_geom_part, OID_AUTO, check_integrity,
-    CTLFLAG_RWTUN, &check_integrity, 1,
+    CTLFLAG_RWTUN, &geom_part_check_integrity, 1,
     "Enable integrity checking");
 static u_int auto_resize = 1;
 SYSCTL_UINT(_kern_geom_part, OID_AUTO, auto_resize,
@@ -420,7 +420,7 @@ g_part_check_integrity(struct g_part_table *table, struct g_consumer *cp)
 	if (failed != 0) {
 		printf("GEOM_PART: integrity check failed (%s, %s)\n",
 		    pp->name, table->gpt_scheme->name);
-		if (check_integrity != 0)
+		if (geom_part_check_integrity != 0)
 			return (EINVAL);
 		table->gpt_corrupt = 1;
 	}
@@ -1846,7 +1846,8 @@ g_part_ctlreq(struct gctl_req *req, struct g_class *mp, const char *verb)
 		table = gpp.gpp_geom->softc;
 		if (table != NULL && table->gpt_corrupt &&
 		    ctlreq != G_PART_CTL_DESTROY &&
-		    ctlreq != G_PART_CTL_RECOVER) {
+		    ctlreq != G_PART_CTL_RECOVER &&
+		    geom_part_check_integrity) {
 			gctl_error(req, "%d table '%s' is corrupt",
 			    EPERM, gpp.gpp_geom->name);
 			return;
diff --git a/sys/geom/part/g_part_gpt.c b/sys/geom/part/g_part_gpt.c
index a17cb30ea806..8fd3b490c4fc 100644
--- a/sys/geom/part/g_part_gpt.c
+++ b/sys/geom/part/g_part_gpt.c
@@ -64,6 +64,8 @@ SYSCTL_UINT(_kern_geom_part_gpt, OID_AUTO, allow_nesting,
 CTASSERT(offsetof(struct gpt_hdr, padding) == 92);
 CTASSERT(sizeof(struct gpt_ent) == 128);
 
+extern u_int geom_part_check_integrity;
+
 #define	EQUUID(a,b)	(memcmp(a, b, sizeof(struct uuid)) == 0)
 
 #define	MBRSIZE		512
@@ -460,8 +462,9 @@ gpt_read_hdr(struct g_part_gpt_table *table, struct g_consumer *cp,
 	if (hdr->hdr_lba_self != table->lba[elt])
 		goto fail;
 	hdr->hdr_lba_alt = le64toh(buf->hdr_lba_alt);
-	if (hdr->hdr_lba_alt == hdr->hdr_lba_self ||
-	    hdr->hdr_lba_alt > last)
+	if (hdr->hdr_lba_alt == hdr->hdr_lba_self)
+		goto fail;
+	if (hdr->hdr_lba_alt > last && geom_part_check_integrity)
 		goto fail;
 
 	/* Check the managed area. */