Re: [PATCH] Improve support for mac partition tables with logical sector size != 512

[Peter Jones]

On 12/17/2010 02:32 PM, Jim Meyering wrote:
> Brian C. Lane wrote:
>> From: Peter Jones <address@hidden>
>>
>> On mac partition tables which specify a sector size larger than the
>> physical sector size, we need to reallocate the buffer after we
>> determine the correct sector size.
>>
>> This is a normal condition when CD is used with Apple partitions (for
>> example, the USB rescue stick for the MacBookAir3,1), and then an image
>> (analogous to an .iso image) is created, so also don't raise an exception in
>> _disk_analyse_block_size() when we find that.
>>
>> Also simplify the code in _disk_analyse_block_size() a bit since there's no
>> reason to ever do the byteswapping more than once or convert everything to
>> 512-byte multiples since we neither load nor store it that way.
>> ---
>>  libparted/labels/mac.c |   44 ++++++++++++++++++++++++++------------------
>>  1 files changed, 26 insertions(+), 18 deletions(-)
>>
>> diff --git a/libparted/labels/mac.c b/libparted/labels/mac.c
> 
> Hi Brian,
> 
> Thanks for passing that along.
> Thanks to Peter for writing the patch.
> 
> Is there a bugzilla number for that, or a test case?
> I presume the existing code is causing some sort of failure.
> If so, can you give details, or even outline how to provoke failure?
> I'd really like to get enough information that I can reproduce
> the failure and write a test case.

The test case is:

1) Get a new macbook air
2) find the tiny little USB stick in the box with the restore image on it
3) plug the stick in (it'll be seen as a CD device, so probably sr0 or sr1)
4) dd if=/dev/sr1 of=mac.img
5) losetup /dev/loop0 mac.img
6) parted /dev/loop0 p

at which point it segfaults.  What seems to happen is that ptt_read_sector()
allocates a buffer with the default size of 512.  Then we call 
_disk_analyse_block_size(), which correctly sets the (logical) sector size to
2048, which is what's set in the label's data.  After that, everything expects
the buffer to be 2048 bytes, because that's what the sector size is set at,
but the buffer is still the original 512 byte allocation.

I haven't tested the creation path for this yet, so I have no idea if we can
create a test case simply by using parted to make a mac label on a device with
2kB or 4kB sectors and copying it to a disk image. I don't think I can
/legally/ give you a copy of the mac restore image.

-- 
        Peter