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

From: Al Viro <[email-masked]>
Date: Thu, 4 Aug 2022 03:56:11 +0100

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".

PS: more than one thread could be calling methods of that struct socket at the
same time; what's to stop e.g. connect(2) on the same sucker (e.g. called on
the same descriptor from a different thread that happens to share the same
descriptor table) to be sitting there trying to lock the struct sock currently
held locked by caller of tcp_set_ulp()?