[U-Boot] [PATCH V2] disk:efi: avoid unaligned access on efi partition

[Hector Palacios]

On 01/28/2014 01:46 PM, Piotr Wilczek wrote:
> This patch fixes part_efi code to avoid unaligned access exception
> on some ARM platforms.
>
> Signed-off-by: Piotr Wilczek <p.wilczek at samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park at samsung.com>
> CC: Tom Rini <trini at ti.com>
> CC: Albert ARIBAUD <albert.u.boot at aribaud.net>
> ---
> Chnages for V2:
>   - used put_unaligned to copy value;
>   - use __aligned to align local array;
>
>   disk/part_efi.c |    7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/disk/part_efi.c b/disk/part_efi.c
> index 9c33ae7..eb2cd57 100644
> --- a/disk/part_efi.c
> +++ b/disk/part_efi.c
> @@ -225,7 +225,7 @@ static int set_protective_mbr(block_dev_desc_t *dev_desc)
>   	p_mbr->signature = MSDOS_MBR_SIGNATURE;
>   	p_mbr->partition_record[0].sys_ind = EFI_PMBR_OSTYPE_EFI_GPT;
>   	p_mbr->partition_record[0].start_sect = 1;
> -	p_mbr->partition_record[0].nr_sects = (u32) dev_desc->lba;
> +	put_unaligned(dev_desc->lba, &p_mbr->partition_record[0].nr_sects);
>
>   	/* Write MBR sector to the MMC device */
>   	if (dev_desc->block_write(dev_desc->dev, 0, 1, p_mbr) != 1) {
> @@ -361,6 +361,8 @@ int gpt_fill_pte(gpt_header *gpt_h, gpt_entry *gpt_e,
>   #ifdef CONFIG_PARTITION_UUIDS
>   	char *str_uuid;
>   #endif
> +	static efi_guid_t basic_guid __aligned(0x04) =
> +		PARTITION_BASIC_DATA_GUID;
>
>   	for (i = 0; i < parts; i++) {
>   		/* partition starting lba */
> @@ -388,8 +390,7 @@ int gpt_fill_pte(gpt_header *gpt_h, gpt_entry *gpt_e,
>   			gpt_e[i].ending_lba = cpu_to_le64(offset - 1);
>
>   		/* partition type GUID */
> -		memcpy(gpt_e[i].partition_type_guid.b,
> -			&PARTITION_BASIC_DATA_GUID, 16);
> +		gpt_e[i].partition_type_guid = basic_guid;
>
>   #ifdef CONFIG_PARTITION_UUIDS
>   		str_uuid = partitions[i].uuid;
>

Sorry, I sent my tested-by on a different thread:
http://patchwork.ozlabs.org/patch/319649/

Tested on i.MX6 (armv7) custom board and it is working fine without
the -mno-unaligned-access flag.

Tested-by: Hector Palacios <hector.palacios at digi.com>

Replying to Tom's question in that other thread, regarding the issue:

 > Can you give me some steps on how to hit this bug?  I believe it's a bug
 > and I believe we need to fix it, I just want to investigate a few things
 > while we've got a trigger case right now.  Thanks!

GPT partition table needs to write a protective MBR to sector 0.
The MBR structure has four partition entries (each occupying 16 bytes) at unaligned 
offsets +1BEh, +1CEh, +1DEh, +1EEh (see [1]). The structure of each partition entry is 
defined at include/part_efi.h

struct partition {
	u8 boot_ind;		/* 0x80 - active */
	u8 head;		/* starting head */
	u8 sector;		/* starting sector */
	u8 cyl;			/* starting cylinder */
	u8 sys_ind;		/* What partition type */
	u8 end_head;		/* end head */
	u8 end_sector;		/* end sector */
	u8 end_cyl;		/* end cylinder */
	__le32 start_sect;	/* starting sector counting from 0 */
	__le32 nr_sects;	/* nr of sectors in partition */
} __packed;

showing eight 1-byte fields and two 4-byte fields. Since the offsets for each 
partition entry are unaligned, the last two fields (which are 32bit) are unaligned as 
well. But it's not an error, it's just the specification of the MBR requires these 
fields to be at those exact offsets. So the usage of unaligned macros for accessing 
those fields is justified.

[1] http://en.wikipedia.org/wiki/Master_boot_record#Sector_layout

Best regards,
--
Hector Palacios