[PATCH v2 2/7] disk: part_dos: Align dos_partition_t with struct partition

[Ilias Apalodimas]

On Fri, 13 Feb 2026 at 11:52, Javier Martinez Canillas
<javierm at redhat.com> wrote:
>
> Ilias Apalodimas <ilias.apalodimas at linaro.org> writes:
>
> Hello Ilias,
>
> > Hi Javier
> >
> > [...]
> >
> >>         unsigned char end_head;         /* end head                             */
> >>         unsigned char end_sector;       /* end sector                           */
> >>         unsigned char end_cyl;          /* end cylinder                         */
> >> -       unsigned char start4[4];        /* starting sector counting from 0      */
> >> -       unsigned char size4[4];         /* nr of sectors in partition           */
> >> -} dos_partition_t;
> >> +       __le32 start_sect;              /* starting sector counting from 0      */
> >> +       __le32 nr_sects;                /* nr of sectors in partition           */
> >> +} __packed dos_partition_t;
> >
> > This always has to be packed and the part_dos.h definition had this
> > wrong all along?
> >
>
> I wondered exactly the same when working on these patches and did a bit
> of code archaeology, so I'm glad that you asked!
>
> It wasn't wrong because it was using unsigned char arrays for the start
> and number of sectors struct fields, instead of __le32 types.
>
> Given the alignment of a char array is 1-byte (and all the other fields
> are unsigned chars), the structure was naturally aligned.
>
> But I agree that using __le32 and __packed is preferred, and in fact is
> how Linux defines this data structure in include/linux/msdos_partition.h:
>
> struct msdos_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;
>
> The struct_partition in disk/part_efi.c is said to be taken from Linux's
> linux/include/genhd.h, which was the old location before commit 1442f76d4317b
> ("1442f76d4317 block: move struct partition out of genhd.h").
>
> My guess is that the old data structure defined in part_dos.h was copied from
> Linux, and that at the time of the copy it was using unsigned char arrays too.

Ok that makes sense and yes, the alignment is not affected by le32 or
a a 4b char.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
>
> But the struct definition has been like this since the original Linux git
> import, so I couldn't check if my guess was correct.
>
> In any case, now this single data structure definition is aligned with how
> both  the Linux kernel and edk2 define it.
>
> --
> Best regards,
>
> Javier Martinez Canillas
> Core Platforms
> Red Hat
>