SELinux inode size gotcha in btrfs.

SELinux inode size gotcha in btrfs.

[Alex]

Hi All,


I've come across the 'gotcha' in XFS where the inode size defaults to 256 [1]
whereas for SELinux the attributes play better when you initialise it at
creation to 512.

>From my reading of the btrfs specs [2] it doesn't look like you'll get caught
with that as the inodes "will not contain embedded file data or extended
attribute data. These things are stored in other item types."

Have I read that right? I've seen xattr bugs patches etc but nothing that would
hit the SE Linux domain.




[1]
http://www.gentoo.org/proj/en/hardened/selinux/selinux-handbook.xml?part=2&chap=1
[2] http://btrfs.ipv5.de/index.php?title=Btrfs_design#Inodes

Re: SELinux inode size gotcha in btrfs.

[Hugo Mills]

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On Mon, Feb 27, 2012 at 10:18:55PM +0000, Alex wrote:
> I've come across the 'gotcha' in XFS where the inode size defaults to 256 [1]
> whereas for SELinux the attributes play better when you initialise it at
> creation to 512.

   A btrfs inode structure is 136 bytes in size. xattrs and any inline
file data are separate from the inode structure, stored with
additional keys in the FS tree (which means that they're quite likely
to appear in the same page, as the inode data, but not guaranteed).

> From my reading of the btrfs specs [2] it doesn't look like you'll get caught
> with that as the inodes "will not contain embedded file data or extended
> attribute data. These things are stored in other item types."
> 
> Have I read that right? I've seen xattr bugs patches etc but nothing that would
> hit the SE Linux domain.

   It's not clear from looking at the gentoo doc what the problem
actually is with different inode sizes... Without some kind of
indication what the issue really is, it's kind of hard to say how this
might affect btrfs.

   Hugo.

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Re: SELinux inode size gotcha in btrfs.

[David Sterba]

On Mon, Feb 27, 2012 at 10:18:55PM +0000, Alex wrote:
> From my reading of the btrfs specs [2] it doesn't look like you'll get caught
> with that as the inodes "will not contain embedded file data or extended
> attribute data. These things are stored in other item types."
>
> Have I read that right? I've seen xattr bugs patches etc but nothing that would
> hit the SE Linux domain.

That's right. Inode represented as btrfs_inode_item does not contain any
xattr fields, they're stored independently as a btrfs_dir_item of type
BTRFS_FT_XATTR . Due to the way the b-tree keys are built, the xattr
item key should be stored near the inode item key, that's for the tree
search side. The xattr data are always stored inline in the b-tree leaf.


david

Re: SELinux inode size gotcha in btrfs.

[Alex]

David Sterba <dave <at> jikos.cz> writes:

> 
> That's right. Inode represented as btrfs_inode_item does not contain any
> xattr fields, they're stored independently as a btrfs_dir_item of type
> BTRFS_FT_XATTR . Due to the way the b-tree keys are built, the xattr
> item key should be stored near the inode item key, that's for the tree
> search side. The xattr data are always stored inline in the b-tree leaf.
> 


Thank you David and Hugo,

My bad on three counts:
1) My courtesy close seems to have gone missing.
 Sorry and thank you for replying.
2) I, perhaps, should have pointed to
https://wiki.debian.org/SELinux/Setup#Prerequisites
_.28kernel_and_filesystems.29
which gives a better explanation.
3) Made it clear that I wasn't attacking
 XFS, It's my fs of (non /boot) choice
for some years now; I'm exploring SELinux
 (after the recent Linux conf au 2012
talk) and know I have used inode size defaults
 for my xfs init back in the day!

Best

Al.

Re: SELinux inode size gotcha in btrfs.

[Hugo Mills]

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On Tue, Feb 28, 2012 at 11:30:26AM +0000, Alex wrote:
> David Sterba <dave <at> jikos.cz> writes:
> 
> > 
> > That's right. Inode represented as btrfs_inode_item does not contain any
> > xattr fields, they're stored independently as a btrfs_dir_item of type
> > BTRFS_FT_XATTR . Due to the way the b-tree keys are built, the xattr
> > item key should be stored near the inode item key, that's for the tree
> > search side. The xattr data are always stored inline in the b-tree leaf.
> > 
> 
> 
> Thank you David and Hugo,
> 
> My bad on three counts:
> 1) My courtesy close seems to have gone missing.
>  Sorry and thank you for replying.
> 2) I, perhaps, should have pointed to
> https://wiki.debian.org/SELinux/Setup#Prerequisites
> _.28kernel_and_filesystems.29
> which gives a better explanation.

   Aaah, OK, that makes sense now. So: we don't have the issue over
xattr sizes at all. The main issue would then seem to be over
atomicity of writing xattrs -- I don't know what the APIs for this
look like, or whether we've implemented them, so I can't comment any
further on btrfs's suitability I'm afraid.

> 3) Made it clear that I wasn't attacking
>  XFS, It's my fs of (non /boot) choice
> for some years now; I'm exploring SELinux
>  (after the recent Linux conf au 2012
> talk) and know I have used inode size defaults
>  for my xfs init back in the day!

   I certainly didn't read it as such.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
   --- In the future, terrorists won't be carrying their ID cards. ---   
                   They'll be carrying yours. -- Henry                   
                         Porter,  Suspect Nation                         

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