Re: [PATCH] fs: export kern_path_locked

[Al Viro]

On Tue, Feb 16, 2021 at 05:31:33PM +0300, Denis Kirjanov wrote:

> We had a change like that:
> Author: WANG Cong <xiyou.wangcong@xxxxxxxxx>
> Date:   Mon Jan 23 11:17:35 2017 -0800
> 
>     af_unix: move unix_mknod() out of bindlock
> 
>     Dmitry reported a deadlock scenario:
> 
>     unix_bind() path:
>     u->bindlock ==> sb_writer
> 
>     do_splice() path:
>     sb_writer ==> pipe->mutex ==> u->bindlock
> 
>     In the unix_bind() code path, unix_mknod() does not have to
>     be done with u->bindlock held, since it is a pure fs operation,
>     so we can just move unix_mknod() out.

*cringe*

I remember now...  Process set:

P1: bind() of AF_UNIX socket to /mnt/sock
P2: splice() from pipe to /mnt/foo
P3: freeze /mnt
P4: splice() from pipe to AF_UNIX socket

P1	grabs ->bindlock
P2	sb_start_write() for what's on /mnt
P2	grabs rwsem shared
P3	blocks in sb_wait_write() trying to grab the same rwsem exclusive
P1	sb_start_write() blocks trying to grab the same rwsem shared
P4	calls ->splice_write(), aka generic_splice_sendpage()
P4 	grabs pipe->mutex
P4	calls ->sendpage(), aka sock_no_sendpage()
P4	calls ->sendmsg(), aka unix_dgram_sendmsg()
P4	calls unix_autobind()
P4	blocks trying to grab ->bindlock
P2	->splice_write(), aka iter_file_splice_write()
P2	blocks trying to grab  pipe->mutex
	DEADLOCK

Sigh...  OK, so we want something like
	user_path_create()
	vfs_mknod()
	created = true
	grab bindlock
	....
	drop bindlock
	if failed && created
		vfs_unlink()
	done_path_create()	
in unix_bind()...  That would push ->bindlock all way down in the hierarchy,
so that should be deadlock-free, but it looks like that'll be fucking ugly ;-/

Let me try and play with that a bit, maybe it can be massaged to something
relatively sane...