[U-Boot] [PATCH] disk: efi: Correct backing up the MBR boot code

[Andy Shevchenko]

+Cc: Ferry, he might be interested in testing this on Intel Edison.

On Tue, May 22, 2018 at 2:04 AM, Sam Protsenko
<semen.protsenko at linaro.org> wrote:
> 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.
>

Thanks for the patch.
It looks like a right thing to do, but let me couple of days to go
through it and recall what and why has been done on Intel Edison WRT
MBR transfering.

> Signed-off-by: Sam Protsenko <semen.protsenko at linaro.org>
> Cc: Alejandro Hernandez <ajhernandez at ti.com>
> ---
>  disk/part_efi.c    | 6 ++++--
>  include/part_efi.h | 3 ++-
>  2 files changed, 6 insertions(+), 3 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;
> diff --git a/include/part_efi.h b/include/part_efi.h
> index 6065c571f3..8525770445 100644
> --- a/include/part_efi.h
> +++ b/include/part_efi.h
> @@ -20,6 +20,7 @@
>  #include <efi.h>
>
>  #define MSDOS_MBR_SIGNATURE 0xAA55
> +#define MSDOS_MBR_BOOT_CODE_SIZE 440
>  #define EFI_PMBR_OSTYPE_EFI 0xEF
>  #define EFI_PMBR_OSTYPE_EFI_GPT 0xEE
>
> @@ -111,7 +112,7 @@ typedef struct _gpt_entry {
>  } __packed gpt_entry;
>
>  typedef struct _legacy_mbr {
> -       u8 boot_code[440];
> +       u8 boot_code[MSDOS_MBR_BOOT_CODE_SIZE];
>         __le32 unique_mbr_signature;
>         __le16 unknown;
>         struct partition partition_record[4];
> --
> 2.17.0
>
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-- 
With Best Regards,
Andy Shevchenko