[parted-devel] [PATCH] Always allow at least two sectors for extended boot record

[Jim Meyering]

Colin Watson wrote:
> Unless specifically told otherwise, the Linux kernel considers extended
> boot records to be two sectors long, in order to "leave room for LILO".
> When using anything other than cylinder alignment, libparted was only
> allowing one sector in the minimum extended partition geometry, which in
> some situations (e.g. following Phillip Susi's patch to reintroduce
> BLKPG) could confuse the kernel into thinking that the EBR and the first
> logical partition overlapped.
>
> * libparted/labels/dos.c (_get_min_extended_part_geom): Allow at least
> two sectors for the extended boot record.
> ---
>  libparted/labels/dos.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
>
> diff --git a/libparted/labels/dos.c b/libparted/labels/dos.c
> index 6fb4aef..c25d35a 100644
> --- a/libparted/labels/dos.c
> +++ b/libparted/labels/dos.c
> @@ -1747,7 +1747,11 @@ _get_min_extended_part_geom (const PedPartition* ext_part,
>  	min_geom = ped_geometry_duplicate (&walk->geom);
>  	if (!min_geom)
>  		return NULL;
> -	ped_geometry_set_start (min_geom, walk->geom.start - 1 * head_size);
> +	/* We must always allow at least two sectors at the start, to leave
> +	 * room for LILO.  See linux/fs/partitions/msdos.c.
> +	 */
> +	ped_geometry_set_start (min_geom,
> +				walk->geom.start - PED_MAX (1 * head_size, 2));
>
>  	for (walk = ext_part->part_list; walk; walk = walk->next) {
>  		if (!ped_partition_is_active (walk) || walk->num == 5)

Colin,

Here is your patch followed by a quick attempt at exercising it.
As you can see from the FIXME comment, this test appears to be
exercising different code.  Can you suggest how to fix it?