Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

[Eric W. Biederman]

Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:

> On Tue, Apr 05, 2016 at 03:54:25AM +0100, Al Viro wrote:
>
>> That, I take it, is a lookup for .. and buggering off if it fails *or* if
>> we had been in caller's root or something that overmount it?  Not that the
>> latter had been possible - root is a directory and can be overmounted only
>> by another such, and we are called from ->open() of a device node.
>> 
>> > +	/* Remember the result of this permission check for later */
>> > +	ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
>> > +	if (path_pts(&path))
>> > +		goto fail;
>> 
>> Egads, man - you've just introduced a special function for looking up
>> something named "pts" in a given directory!
>> 
>> The reason not to use kern_path() would be what, the fact that it doesn't
>> allow starting at given location?  So let's make a variant that would - and
>> rather than bothering with RCU, just go for something like (completely
>> untested)
>
> Ah...  Right, that would demand exec permissions on the starting point.
> Still, this is incredibly ugly ;-/  I'll try to come up with something
> more tolerable, but this "path_pts" thing is too ugly to live.
> Seriously.

Given that I can think of no other reason than this special case to ever
want to use this code.  I figured having something incredibily special
case and obviously so was the way to go.  Then at least no one would
mistake it for a general purpose facility.

Eric