Piotr Wilczek <[email protected]> writes: >> -----Original Message----- >> From: Måns Rullgård [mailto:[email protected]] >> Sent: Saturday, October 12, 2013 1:29 AM >> To: Piotr Wilczek >> Cc: [email protected]; Tom Rini; Kyungmin Park >> Subject: Re: [PATCH] disk:efi: avoid unaligned access on efi partition >> >> Piotr Wilczek <[email protected]> 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 <[email protected]> >> > Signed-off-by: Kyungmin Park <[email protected]> >> > CC: Tom Rini <[email protected]> >> > --- >> > 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.
If the struct has __attribute__((packed)), gcc should do the right thing already. Note that on ARMv6 and later ldr/str support unaligned addresses unless this is explicitly disabled in the system control register. If you do this, you _MUST_ compile with -mno-unaligned-access. Otherwise you will get problems. -- Måns Rullgård [email protected] _______________________________________________ U-Boot mailing list [email protected] http://lists.denx.de/mailman/listinfo/u-boot
