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

[Piotr Wilczek]

> -----Original Message-----
> From: Måns Rullgård [mailto:mans at mansr.com]
> Sent: Saturday, October 12, 2013 1:29 AM
> To: Piotr Wilczek
> Cc: u-boot at lists.denx.de; Tom Rini; Kyungmin Park
> Subject: Re: [PATCH] disk:efi: avoid unaligned access on efi partition
> 
> Piotr Wilczek <p.wilczek at samsung.com> writes:
> 
> > In this patch static variable and memcpy instead of an assignment are
> > used 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>
> > ---
> >  disk/part_efi.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/disk/part_efi.c b/disk/part_efi.c index b7524d6..303b8af
> > 100644
> > --- a/disk/part_efi.c
> > +++ b/disk/part_efi.c
> > @@ -224,7 +224,8 @@ 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;
> > +	memcpy(&p_mbr->partition_record[0].nr_sects, &dev_desc->lba,
> > +	       sizeof(dev_desc->lba));
> 
> Why is this assignment problematic?  Note that the compiler may
> optimise the memcpy() call into a plain assignment including any
> alignment assumptions it was making in the original code.
> 
> The correct fix is either to ensure that pointers are properly aligned
> or that things are annotated as potentially unaligned, whichever is
> more appropriate.
> 
Problem is that the legacy_mbr structure consists 'le16 unknown' field
before 'partition_record' filed and the structure is packed. As a result the
address of 'partition_record' filed is halfword aligned. The compiler uses
str/ldr instructions (address must be 4-byte aligned) to copy u32 'lba' data
thus causing unaligned access exception.

The best option would be to rearrange field in the structure but for other
reasons I cannot do that.
I will use put/get_unaligned as Albert suggested.

Best regards
Piotr Wilczek

> --
> Måns Rullgård
> mans at mansr.com