I've been investigating a problem I ran into trying to create partitions
in sector mode and found that the first logical partition can not begin
on the very first sector of the extended partition, immediately
following the EBR.  This apparently is because the kernel creates a dev
node to represent the extended partition and sizes it to two sectors.  I
could have sworn that the kernel did not used to create a device for the
extended partition itself, but I wondered why it was 2 sectors long.  I
found this:

from fs/partitions/msdos.c:

            /* prevent someone doing mkfs or mkswap on an
               extended partition, but leave room for LILO */
            put_partition(state, slot, start, size == 1 ? 1 : 2);

This appears to set the size of the device to 2 sectors, unless the
extended partition is only 1 sector long.  Shouldn't the size be
whatever length there is between the start of the extended partition,
and the first logical partition it contains?  So if there are 63 sectors
there, as is the usual case when using cylinder alignment, then the
device node would expose all of those, but if there is no space, then
the device node should only be 1 sector for the EBR, otherwise it
overlays the first sector of the logical partition.
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This is probably kernel bug. It's really insane that the extended

 Please no. I think the size should not be more than 2 sectors (1024
 bytes). The current concept works for years and we have in userspace 
 /etc/partitions parsers that use "if (blocks <= 1)" to detect
 extended partitions.

 The other problem are mkfs programs, the space used for alignment
 could be 1MiB (or more) -- it's enough many mkfs programs.


 BTW, Linux does not use this policy for the others nested partition
 tables (e.g. Solaris, BSD, Minix, ...). The extended dos partition
 table is exception. The primary partitions for the others nested PT
 are exported to the system with its real size :-)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com
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Could you elaborate a bit on this?  What programs have such tests and

What's wrong with that?  If you REALLY want to, there's no reason you
can't create a tiny fs there.  Then again, I could swear that once upon
a time the kernel simply did not bother creating a dev node for the
extended partition, and this seems to be a hack that was put in to make
it easy for LILO to install to one.  Personally I'd prefer going back to

Indeed, also the hidden space in the logical partitions is also not
exposed, otherwise you would have two dev nodes per logical partition.
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I know about libblkid in e2fsprogs and util-linux-ng. It scans
/proc/partitions to get list of "normal" block devices.


Irrelevant question ;-) It's there for years and it's well know kernel
feature. 

I understand that from a pedantic point of view the current solution is
not perfect, but I don't see any practical reason why we need to change

You have to care about the partition table (EBR).

The current 1024 bytes is completely useless size, if you enlarge the
size of the partition (for example to 1MiB) you will see reports from
people who lost their extended partitions. (I don't believe that all

This is probably better idea than enlarge the size :-)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com
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Good point... mke2fs won't damage it since it leaves the first sector
intact and places its boot sector in sector 1 ( or was it 2? ), but FAT

Aye, I'd prefer the useless device to be removed, but at the very least
it should not use a size of 2 when the second sector is not actually
there because it is the first sector of the logical partition, though we
just patched parted to reserve that second sector anyhow even though you
are using none alignment mode, just to avoid running into this problem.
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