Re: [PATCH tty-next] devpts: Make each mount of devpts an independent filesystem.

From: Linus Torvalds
Date: Thu Jun 02 2016 - 17:44:11 EST

On Thu, Jun 2, 2016 at 2:23 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> We certainly don't need the permission check.

I agree that we don't need it, but it does simplify the code, and it
doesn't actually harm anything, since the thing we check for
permissions is the same /dev directory, which we already know we had
lookup permissions for for the original ptmx node anyway.

The permission checks I didn't like were the permission checks for the
pts/ and pts/ptmx nodes, because they had nothing to do with the
permission for the original node. In contrast, this is just about a
redundant permission check we already did, and that simplifies the
code a bit.

That said, you're right that it needs to use IS_ERR(path.dentry). And
the whole "lookup_one_len_unlocked()" simplification is just a small
detail that doesn't much matter, so it's not a big deal.

And it is arguable that "d_hash_and_lookup()" that you use is actually
simpler than "lookup_one_len_unlocked()", since we are really only
need to look into the dentry cache, and that's all that
d_hash_and_lookup() does (ie it only ever does a d_lookup(), while
lookup_one_len_unlocked() ends up doing a full lookup in the failuer
case that we don't even care about.

So I don't feel very strongly about it. Your patch is ok by me.