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

[Roland Dreier <rdreier@cisco.com>]

 > If we let the caller call fd_install(), then it may be messed up WRT 
 > cleanup (fd, file, inode).

Yes, that is a tiny bit tricky (need to call put_unused_fd() if you
don't install the fd).

 > 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().

I think that may be a bit cleaner than Al's approach, but it still
leaves the same trap that create_vcpu_fd() falls into.  The current
code is:

static int create_vcpu_fd(struct kvm_vcpu *vcpu)
{
	int fd, r;
	struct inode *inode;
	struct file *file;

	r = anon_inode_getfd(&fd, &inode, &file,
			     "kvm-vcpu", &kvm_vcpu_fops, vcpu);
	if (r)
		return r;
	atomic_inc(&vcpu->kvm->filp->f_count);
	return fd;
}

and with your proposal, the natural way to write that becomes:

static int create_vcpu_fd(struct kvm_vcpu *vcpu)
{
	int fd, r;

	r = anon_inode_getfd(&fd, NULL,
			     "kvm-vcpu", &kvm_vcpu_fops, vcpu);
	if (r)
		return r;
	atomic_inc(&vcpu->kvm->filp->f_count);
	return fd;
}

which still has the same bug.

Maybe a good way to handle this is just to make the get_file() not
optional.  I dunno... I feel like we've spent more discussion on this
point than it deserves, so someone should just make a decision and
I'll adapt the ib_uverbs code to work with whatever it is.

 - R.