Re: [PATCH v3 1/3] namei: implement O_BENEATH-style AT_* flags

On 2018-10-13, Al Viro [off-list ref] wrote:

quoted

Pardon me, but... huh?  The reason for your two calls of dirfd_path_init() is,
AFAICS, the combination of absolute pathname with both LOOKUP_XDEV and
LOOKUP_BENEATH at the same time.  That combination is treated as if the pathname
had been relative.  Note that LOOKUP_BENEATH alone is ignored for absolute ones
(and with a good reason - it's a no-op on path_init() level in that case).

What the hell?  It complicates your code and doesn't seem to provide any benefits
whatsoever -- you could bloody well have passed the relative pathname to start with.

IDGI...  Without that kludge it becomes simply "do as we currently do for absolute
pathnames, call dirfd_path_init() for relative ones".  And I would argue that
taking LOOKUP_BENEATH handling out of dirfd_path_init() into path_init() (relative)
case would be a good idea.

As it is, the logics is very hard to follow.

	... and it fails on LOOKUP_BENEATH anyway.  Egads...  So that's for your
LOOKUP_CHROOT ;-/  IMO that's awful, especially with the way you've spread those
LOOKUP_CHROOT cases between these two.

Yeah, the ->root setting in dirfd_path_init() is ugly. :/

	Why not simply have O_THISROOT pick root by dirfd and call file_open_root()?

Wouldn't this require replicating the dirfd_path_init()-like code inside
all of the path_*at() callers which use path_init()? Or is there another
common place we could put it?

And if something wants it for stat(), etc. just have them use it combined with
O_PATH and pass the result to ...at()...

This works for stat and quite a few other things (which is why I only
added openat(2) support for the moment), but I think we'd eventually
need something like this for renameat2(2) as well as a few other choice
*at(2) syscalls. Though I also think that more AT_EMPTY_PATH support
would removed the need for _most_ *at(2) implementations to use this.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>