disk: efi: Correct backing up the MBR boot code

In commit e163a931af34 ("cmd: gpt: backup boot code before writing MBR")
there was added the procedure for storing old boot code when doing "gpt
write". But instead of storing just backup code, the whole MBR was
stored, and only specific fields were replaced further, keeping
everything else intact. That's obviously not what we want.

Fix the code to actually store only old boot code and zero out
everything else. This fixes next testing case:

    => mmc write $loadaddr 0x0 0x7b
    => gpt write mmc 1 $partitions

In case when $loadaddr address and further memory contains 0xff, the
board was bricked (ROM-code probably didn't like partition entries that
were clobbered with 0xff). With this patch applied, commands above don't
brick the board.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Alejandro Hernandez <ajhernandez@ti.com>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/disk/part_efi.c b/disk/part_efi.c
index 72e3997b84..5c1039f013 100644
--- a/disk/part_efi.c
+++ b/disk/part_efi.c
@@ -350,8 +350,6 @@ static int set_protective_mbr(struct blk_desc *dev_desc)
 {
         /* Setup the Protective MBR */
         ALLOC_CACHE_ALIGN_BUFFER_PAD(legacy_mbr, p_mbr, 1, dev_desc->blksz);
-        memset(p_mbr, 0, sizeof(*p_mbr));
-
         if (p_mbr == NULL) {
                 printf("%s: calloc failed!\n", __func__);
                 return -1;
@@ -363,6 +361,10 @@ static int set_protective_mbr(struct blk_desc *dev_desc)
                 return -1;
         }
 
+        /* Clear all data in MBR except of backed up boot code */
+        memset((char *)p_mbr + MSDOS_MBR_BOOT_CODE_SIZE, 0, sizeof(*p_mbr) -
+                        MSDOS_MBR_BOOT_CODE_SIZE);
+
         /* Append signature */
         p_mbr->signature = MSDOS_MBR_SIGNATURE;
         p_mbr->partition_record[0].sys_ind = EFI_PMBR_OSTYPE_EFI_GPT;