On Thu, Sep 29 2016, James Clarke wrote:
> Since the MBR layout only has partition records as 2-byte aligned, the 32-bit
> fields in them are not aligned. Thus, they cannot be accessed on some
> architectures (such as SPARC) by using a "struct MBR_part_record *" pointer,
> as the compiler can assume that the pointer is properly aligned. Instead, the
> records must be accessed by going through the MBR struct itself every time.
Weird....
Can you see if adding "__attribute__((packed))" to struct
MBR_part_record also fixes the problem?
It seems strange that the compiler lets you take a pointer, but then
doesn't use it correctly. Maybe it is an inconsistency in the types.
I don't necessarily disagree with your fix, but I'd like to understand
why the current code is wrong.
Thanks,
NeilBrown
>
> Signed-off-by: James Clarke <jrtc27@xxxxxxxxxx>
> ---
> super-mbr.c | 6 ++++++
> util.c | 14 +++++++-------
> 2 files changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/super-mbr.c b/super-mbr.c
> index 62b3f03..303dde4 100644
> --- a/super-mbr.c
> +++ b/super-mbr.c
> @@ -57,6 +57,9 @@ static void examine_mbr(struct supertype *st, char *homehost)
>
> printf(" MBR Magic : %04x\n", sb->magic);
> for (i = 0; i < MBR_PARTITIONS; i++)
> + /* Have to make every access through sb rather than using a pointer to
> + * the partition table (or an entry), since the entries are not
> + * properly aligned. */
> if (sb->parts[i].blocks_num)
> printf("Partition[%d] : %12lu sectors at %12lu (type %02x)\n",
> i,
> @@ -151,6 +154,9 @@ static void getinfo_mbr(struct supertype *st, struct mdinfo *info, char *map)
> info->component_size = 0;
>
> for (i = 0; i < MBR_PARTITIONS ; i++)
> + /* Have to make every access through sb rather than using a pointer to
> + * the partition table (or an entry), since the entries are not
> + * properly aligned. */
> if (sb->parts[i].blocks_num) {
> unsigned long last =
> (unsigned long)__le32_to_cpu(sb->parts[i].blocks_num)
> diff --git a/util.c b/util.c
> index a238a21..08adbd5 100644
> --- a/util.c
> +++ b/util.c
> @@ -1412,7 +1412,6 @@ static int get_gpt_last_partition_end(int fd, unsigned long long *endofpart)
> static int get_last_partition_end(int fd, unsigned long long *endofpart)
> {
> struct MBR boot_sect;
> - struct MBR_part_record *part;
> unsigned long long curr_part_end;
> unsigned part_nr;
> int retval = 0;
> @@ -1429,21 +1428,22 @@ static int get_last_partition_end(int fd, unsigned long long *endofpart)
> if (boot_sect.magic == MBR_SIGNATURE_MAGIC) {
> retval = 1;
> /* found the correct signature */
> - part = boot_sect.parts;
>
> for (part_nr = 0; part_nr < MBR_PARTITIONS; part_nr++) {
> + /* Have to make every access through boot_sect rather than using a
> + * pointer to the partition table (or an entry), since the entries
> + * are not properly aligned. */
> +
> /* check for GPT type */
> - if (part->part_type == MBR_GPT_PARTITION_TYPE) {
> + if (boot_sect.parts[part_nr].part_type == MBR_GPT_PARTITION_TYPE) {
> retval = get_gpt_last_partition_end(fd, endofpart);
> break;
> }
> /* check the last used lba for the current partition */
> - curr_part_end = __le32_to_cpu(part->first_sect_lba) +
> - __le32_to_cpu(part->blocks_num);
> + curr_part_end = __le32_to_cpu(boot_sect.parts[part_nr].first_sect_lba) +
> + __le32_to_cpu(boot_sect.parts[part_nr].blocks_num);
> if (curr_part_end > *endofpart)
> *endofpart = curr_part_end;
> -
> - part++;
> }
> } else {
> /* Unknown partition table */
> --
> 2.10.0
>
> --
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