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

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Date:	Wed, 8 Sep 2010 17:47:26 +1000
From:	Neil Brown <neilb@...e.de>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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 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.


> 
> 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,
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...

NeilBrown
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