Re: [PATCH,HURD] Fix grub-probe with userland partition support

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 24.04.2012 14:42, Samuel Thibault wrote:
> Vladimir 'φ-coder/phcoder' Serbinenko, le Tue 24 Apr 2012 11:55:33 +0200, a 
> écrit :
>> One could have hd0s1 containing hd0s5 and hd0s6.
> Argl, I forgot that case indeed. It's funny how that contracts the very
> word "partition"...
>
>> In this case we want hd0s5 and not hd0s1. On the other hand a file may
>> reside in partition but be part of a filesystem spanning through the
>> whole disk. This is common for all kinds of hybrid CDROMs, including
>> the ones created by grub-mkrescue. They have iso9660 spanning through
>> the whole disk but all of the disk other than the first sector is
>> in some kind of partition table to avoid it being accidentally
>> overwritten.  So even though the file itself is inside a partition, we
>> want the whole disk.
> The partition itself can not be mounted?
No, since it contains FS structures at wrong offset.
>> Also if file is 0-sized, sparse, compressed or encrypted it won't have
>> blocklist and this approach would fail, possibly even with out-of-range
>> array access.
> Other than 0-sized would work, the eventual storage would still be on
> the partition.
completely sparse just has a size and not backed by any storage.
> 0-sized indeed poses problems.
>
>> I'm surprised that Hurd doesn't offer a way to just ask "What does this
>> filesystem translator consume?"
> Because the whole point of the Hurd is to let the user have access
> to more powerful ways.  A file can reside inside an iso file, which
> is stored in an ext2fs, which is stored in a file,
So much GRUB can handle. We don't handle loopback automatically right
now since it's not clear whether it's a loopback for VM or loopback used
on host.
>  using the user's
> own translator with no constraint on the naming convention, etc. etc.
For these ones we're limited at what GRUB can. grub-probe is used for 2
purposes:
1) Determining the device for root, its FS and UUID
2) Determining the GRUB environment on boot.
The first is used in grub-mkconfig and if Hurd is so much more flexible
10_hurd.in may need to be adjusted accordingly.
As for the second, we're limited to what GRUB can do and so it won't be
possible to have /boot on translator from hyperspace. grub-probe should
error-out if it reaches a translator which can't be handled by GRUB.
> That's why it generally does not make sense to know where a file comes
> from, because you would not know how to parse the result.  The proper
> way would be asking the FS, which may tell you
>
> /hurd/ext2fs --writable --no-inherit-dir-group /dev/hd0s1
>
> or
>
> ext2fs --writable --no-atime --no-inherit-dir-group --store-type=typed 
> device:hd1
>
> But we generally don't want to impose any syntax here, it could actually
> be
>
> /opt/my/own/translator xyz
>
> I guess we'll have to impose some syntax anyway for whatever contains
> /boot, so that grub can open it itself.
There should be a standartised way to get this information for any
conventional FS, otherwise it makes porting programs which use this
information much more difficult and in most cases results in dirty
workarounds.


-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko

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