Subject: Re: [RFC] apparently bogus logics in unix_find_other() since 2002

From: Solar Designer <[email-masked]>
Date: Mon, 11 Feb 2019 12:21:31 +0100

On Sun, Feb 10, 2019 at 04:24:22AM +0000, Al Viro wrote:
> 	In "net/unix/af_unix.c: Set ATIME on socket inode" (back in
> 2002) we'd grown something rather odd in unix_find_other().  In the
> original patch it was
>                 u=unix_find_socket_byname(sunname, len, type, hash);
> -               if (!u)
> +               if (u) {
> +                       struct dentry *dentry;
> +                       dentry = u->protinfo.af_unix.dentry;
> +                       if (dentry)
> +                               UPDATE_ATIME(dentry->d_inode);
> +               } else
>                         goto fail;

It's this commit:

https://github.com/dmgerman/linux-bitkeeper/commit/80cbc5b9c7393c4456236543ca1e639ea0841c19

There are two hunks in that patch: one after "if (sunname->sun_path[0])"
and the other after "else".  I just did some more digging and found the
private discussion of the time, as well as a previous revision of the
patch (against 2.2.21, whereas the committed one was against 2.4.x of
the same era).  Even the earliest revision I found already has both
hunks.  I couldn't find any discussion as to why the second hunk was
possibly needed.  It is quite possible that I had added it in error.

The original problem this patch addressed was stmpclean deleting sockets
that were still actively used - specifically, PostgreSQL's.  I found
that I also tested the patch on /dev/log and X11 sockets.  However, I
can't find any indication of me ever testing with the first hunk only,
so it's quite possible I wrote both hunks at once and only tested both.

> These days the code is
> 
>                 u = unix_find_socket_byname(net, sunname, len, type, hash);
>                 if (u) {
>                         struct dentry *dentry;
>                         dentry = unix_sk(u)->path.dentry;
>                         if (dentry)
>                                 touch_atime(&unix_sk(u)->path);
>                 } else  
>                         goto fail;
> 
> but the logics is the same.  It's the abstract address case - we have
> '\0' in sunname->sun_path[0].  How in hell could that possibly have
> non-NULL ->path.dentry and what would it be?

This is probably in fact impossible.

I think it'd make sense to drop this logic, reverting to:

		if (!u)
			goto fail;

and then see if atime on an actively used socket in /tmp or on /dev/log
keeps getting updated (due to the first hunk of the above commit).

Alexander