Subject: Re: [PATCH net-next] net/smc: Introduce TCP ULP support

From: Al Viro <[email-masked]>
Date: Thu, 4 Aug 2022 04:48:03 +0100

On Thu, Aug 04, 2022 at 03:56:11AM +0100, Al Viro wrote:
> 	Half a year too late, but then it hadn't been posted on fsdevel.
> Which it really should have been, due to
> 
> > +	/* replace tcp socket to smc */
> > +	smcsock->file = tcp->file;
> > +	smcsock->file->private_data = smcsock;
> > +	smcsock->file->f_inode = SOCK_INODE(smcsock); /* replace inode when sock_close */
> > +	smcsock->file->f_path.dentry->d_inode = SOCK_INODE(smcsock); /* dput() in __fput */
> > +	tcp->file = NULL;
> 
> this.  It violates a bunch of rather fundamental assertions about the
> data structures you are playing with, and I'm not even going into the
> lifetime and refcounting issues.
> 
> 	* ->d_inode of a busy positive dentry never changes while refcount
> of dentry remains positive.  A lot of places in VFS rely upon that.
> 	* ->f_inode of a file never changes, period.
> 	* ->private_data of a struct file associated with a socket never
> changes; it can be accessed lockless, with no precautions beyond "make sure
> that refcount of struct file will remain positive".

Consider, BTW, what it does to sockfd_lookup() users.  We grab a reference
to struct file, pick struct socket from its ->private_data, work with that
sucker, then do sockfd_put().  Which does fput(sock->file).

Guess what happens if sockfd_lookup() is given the descriptor of your
TCP socket, just before that tcp->file = NULL?  Right, fput(NULL) as
soon as matching sockfd_put() is called.  And the very first thing fput()
does is this:
        if (atomic_long_dec_and_test(&file->f_count)) {

And that's just one example - a *lot* of places both in VFS and in
net/* rely upon these assertions.  This is really not a workable approach.