[PATCH] libfdisk: (gpt) fix check for beginning of protective partition

[PATCH] libfdisk: (gpt) fix check for beginning of protective partition

[Michael Marineau]

Search for the protective partition before attempting to validate that
it covers the GPT header. Some hybrid MBRs list partitions out of order.
For example, CoreOS images ship with a hybrid MBR that lists the boot
partition first and the protective partition second in order to maintain
compatibility with old versions of Xen's PV-GRUB.

Signed-off-by: Michael Marineau <michael.marineau@coreos.com>
---
 libfdisk/src/gpt.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/libfdisk/src/gpt.c b/libfdisk/src/gpt.c
index a7766da..989fd75 100644
--- a/libfdisk/src/gpt.c
+++ b/libfdisk/src/gpt.c
@@ -678,11 +678,6 @@ static int valid_pmbr(struct fdisk_context *cxt)
 	if (le16_to_cpu(pmbr->signature) != MSDOS_MBR_SIGNATURE)
 		goto done;
 
-	/* LBA of the GPT partition header */
-	if (pmbr->partition_record[0].starting_lba !=
-	    cpu_to_le32(GPT_PRIMARY_PARTITION_TABLE_LBA))
-		goto done;
-
 	/* seems like a valid MBR was found, check DOS primary partitions */
 	for (i = 0; i < 4; i++) {
 		if (pmbr->partition_record[i].os_type == EFI_PMBR_OSTYPE) {
@@ -693,13 +688,18 @@ static int valid_pmbr(struct fdisk_context *cxt)
 			 */
 			part = i;
 			ret = GPT_MBR_PROTECTIVE;
-			goto check_hybrid;
+			break;
 		}
 	}
 
 	if (ret != GPT_MBR_PROTECTIVE)
 		goto done;
-check_hybrid:
+
+	/* LBA of the GPT partition header */
+	if (pmbr->partition_record[part].starting_lba !=
+	    cpu_to_le32(GPT_PRIMARY_PARTITION_TABLE_LBA))
+		goto done;
+
 	for (i = 0 ; i < 4; i++) {
 		if ((pmbr->partition_record[i].os_type != EFI_PMBR_OSTYPE) &&
 		    (pmbr->partition_record[i].os_type != 0x00))
-- 
2.0.5

Re: [PATCH] libfdisk: (gpt) fix check for beginning of protective partition

[Karel Zak]

On Fri, May 01, 2015 at 09:44:33PM -0700, Michael Marineau wrote:
> Search for the protective partition before attempting to validate that
> it covers the GPT header. Some hybrid MBRs list partitions out of order.
> For example, CoreOS images ship with a hybrid MBR that lists the boot
> partition first and the protective partition second in order to maintain
> compatibility with old versions of Xen's PV-GRUB.

 IMHO is better to say "sorry, this is unsupported use-case", than
 really on bad thing like hybrid GPT.

 Anyway applied, with your patch the code seems more robust, thanks.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com