linux-kernel - Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

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Date:	Thu, 24 Apr 2008 20:47:57 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	dave@...ux.vnet.ibm.com
CC:	miklos@...redi.hu, viro@...IV.linux.org.uk,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	ezk@...sunysb.edu, mhalcrow@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

> > Why are those so important?  Yes, if we have multiple vfs_() calls,
> > surround them with an extra want/drop pair.  We do already do multiple
> > overlapping want/drop pairs with O_TRUNC and O_CREAT (AFAIR).
> 
> The one that I remember is the pair that we take for O_CREAT and the
> nameidata_to_filp() in do_filp_open() that we do on it after the
> creation and hold while the file is open.
> 
> The only reason for this one is that we want to shut down a potential
> race that would allow the file to be created, then still return a -EROFS
> because someone did a r/w->r/o transition between the create an the
> nameidata_to_filp().
> 
> We could pass some information into nameidata_to_filp()->__dentry_open()
> telling it that we already have a write and it doesn't need to take one,
> but I think having the two nested writes is cleaner.

Agreed.

> Is that the case you're thinking of?

Yes.  And the same thing is done a little below for similar reasons
around may_open() which does truncation.

Miklos
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