Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags

[Andy Lutomirski]

> On Nov 23, 2018, at 5:10 AM, JÃrg Billeter <j@xxxxxxxxx> wrote:
>
> Hi Aleksa,
>
>> On Tue, 2018-11-13 at 01:26 +1100, Aleksa Sarai wrote:
>> * O_BENEATH: Disallow "escapes" from the starting point of the
>>  filesystem tree during resolution (you must stay "beneath" the
>>  starting point at all times). Currently this is done by disallowing
>>  ".." and absolute paths (either in the given path or found during
>>  symlink resolution) entirely, as well as all "magic link" jumping.
>
> With open_tree(2) and OPEN_TREE_CLONE, will O_BENEATH still be
> necessary?

This discussion reminds me of something Iâm uncomfortable with in the
current patches: currently, most of the O_ flags determine some
property of the returned opened file.  The new O_ flags you're adding
don't -- instead, they affect the lookup of the file.  So O_BENEATH
doesn't return a descriptor that can only be used to loop up files
beneath it -- it just controls whether open(2) succeeds or fails.  It
might be nice for the naming of the flags to reflect this.  I also
don't love that we have some magic AT_ flags that work with some
syscalls and some magic O_ flags that work with others.

In this regard, I liked the AT_ naming better.  Although I don't love
adding AT_ flags because the restrict our ability to usefully use the
high bits of the fd in the future.

--Andy