gpt: preserve first 446 bytes of the PMBR (protective MBR)

* libparted/label/gpt.c (_write_pmbr): Make sure we read and preserve
the first 446 bytes of the device when we are creating the PMBR.

diff --git a/libparted/labels/gpt.c b/libparted/labels/gpt.c
index 536e06a..3afc369 100644 (file)
--- a/libparted/labels/gpt.c
+++ b/libparted/labels/gpt.c
@@ -969,13 +969,23 @@ error:
 }
 
 #ifndef DISCOVER_ONLY
-/* Writes the protective MBR (to keep DOS happy) */
+/* Write the protective MBR (to keep DOS happy) */
 static int
 _write_pmbr (PedDevice * dev)
 {
        LegacyMBR_t pmbr;
 
-       memset(&pmbr, 0, sizeof(pmbr));
+       /* The UEFI spec is not clear about what to do with the following
+          elements of the Protective MBR (pmbr): BootCode (0-440B),
+          UniqueMBRSignature (440B-444B) and Unknown (444B-446B).
+          With this in mind, we try not to modify these elements.  */
+       if (ped_device_read (dev, &pmbr, 0, GPT_PMBR_SECTORS) < GPT_PMBR_SECTORS)
+               memset (&pmbr, 0, sizeof(pmbr));
+
+       /* Zero out all the legacy partitions.
+          There are 4 PartitionRecords.  */
+       memset (pmbr.PartitionRecord, 0, sizeof pmbr.PartitionRecord);
+
        pmbr.Signature = PED_CPU_TO_LE16(MSDOS_MBR_SIGNATURE);
        pmbr.PartitionRecord[0].OSType      = EFI_PMBR_OSTYPE_EFI;
        pmbr.PartitionRecord[0].StartSector = 1;