linux-kernel - Re: [PATCH/RFC 1/2] anon-inodes: Remove fd_install() from anon_inode

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Message-ID: <Pine.LNX.4.64.0802281150060.7660@alien.or.mcafeemobile.com>
Date:	Thu, 28 Feb 2008 12:04:10 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Roland Dreier <rdreier@...co.com>
cc:	Christoph Hellwig <hch@....de>, linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@....linux.org.uk>
Subject: Re: [PATCH/RFC 1/2] anon-inodes: Remove fd_install() from
 anon_inode_getfd()

On Wed, 27 Feb 2008, Roland Dreier wrote:

>  > http://git.kernel.org/?p=linux/kernel/git/viro/vfs-2.6.git;a=commit;h=49be4f8114e6ff0efdab10ebba2493fb67bc3034
> 
> Actually, looking closer at the kvm changes here, I think that
> create_vcpu_fd() needs the same treatment as kvm_dev_ioctl_create_vm()
> gets in the patch because of the race I mentioned in the changelog
> for my patch: otherwise kvm_vcpu_release() could drop the last
> reference to vcpu->kvm->filp before the get_file() gets an extra
> reference.
> 
> I'm beginning to think that moving the fd_install() out of
> anon_inode_getfd() really is worth it to make a safer interface.

If we let the caller call fd_install(), then it may be messed up WRT 
cleanup (fd, file, inode).
How about removing the inode pointer handout altogether, and *doing* 
fd_install() inside anon_inode_getfd() like:

	if (pfile != NULL) {
		get_file(file);
		*pfile = file;
	}
	fd_install(fd, file);

In this way, if the caller want the file* back, he gets the reference 
bumped before fd_install().



- Davide


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