linux-kernel - Re: [PATCH 09/10] VFS: Remove read-only checks from dentry

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Message-Id: <E1OtGTx-0000by-Lt@pomaz-ex.szeredi.hu>
Date:	Wed, 08 Sep 2010 10:58:17 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Neil Brown <neilb@...e.de>
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	haveblue@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/10] VFS: Remove read-only checks from
 dentry_permission

On Wed, 8 Sep 2010, Neil Brown wrote:
> On Mon, 06 Sep 2010 21:10:10 +0200
> Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > On Mon, 06 Sep 2010, NeilBrown wrote:
> > > It is not sufficient to depend on the the "filesystem is readonly"
> > > tests in dentry_permission as it does not check if the vfsmnt is
> > > readonly.
> > > All call sites already call mnt_want_write or __mnt_is_readonly which
> > > includes the test on MS_RDONLY.
> > 
> > Last time I checked I found some holes (in nfsd IIRC).  That was a
> > long time ago and things may have changed.
> 
> nfsd looks OK to me.  I didn't do an exhaustive audit but couldn't find
> things that would not still work correctly.

Looks to me as if nfsd_setattr() and some of its callers fail to
acquire a write ref.

> > 
> > That check could be replaced with a
> > 
> > 		if (IS_RDONLY(inode) &&
> > 		    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
> > 			BUG();
> 
> That wouldn't quite work currently.
> sys_faccessat checks __mnt_is_readonly *after* the call to dentry_permission,

Ugh.

> so the above would cause it to BUG.  Possibly the __mnt_is_readonly could be
> checked before dentry_permission.
> 
> However nfsd_permission is a bit more awkward to fix as sometimes it
> deliberately wants to ignore read-only-filesystem issues ... but it might
> still be possible to work around..
> 
> > 
> > which would catch these cases but only if the superblock was marked
> > r/o.  Otherwise it's basically impossible to make sure the callers of
> > the VFS play by the rules.  That was one reason I advocated a
> > path_... interface for the VFS instead of the current dentry based
> > one, but Al didn't like it.
> >
> 
> I guess there comes a point were we just have to document the rules and if
> someone doesn't play by them - that is a bug...

Yeah and that's fine as long as the bug shows up relatively easily and
not result in silent fs corruption once in blue moon instead.

If that MS_RDONLY check is removed the above criterion will not be
met.

Thanks,
Miklos
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