[PATCH v9 05/10] namei: O_BENEATH-style path resolution flags

[Aleksa Sarai]

On 2019-07-14, Al Viro <viro at zeniv.linux.org.uk> wrote:
> On Sat, Jul 13, 2019 at 03:41:53AM +0100, Al Viro wrote:
> > On Fri, Jul 12, 2019 at 04:00:26PM +0100, Al Viro wrote:
> > > On Fri, Jul 12, 2019 at 02:25:53PM +0100, Al Viro wrote:
> > > 
> > > > 	if (flags & LOOKUP_BENEATH) {
> > > > 		nd->root = nd->path;
> > > > 		if (!(flags & LOOKUP_RCU))
> > > > 			path_get(&nd->root);
> > > > 		else
> > > > 			nd->root_seq = nd->seq;
> > > 
> > > BTW, this assignment is needed for LOOKUP_RCU case.  Without it
> > > you are pretty much guaranteed that lazy pathwalk will fail,
> > > when it comes to complete_walk().
> > > 
> > > Speaking of which, what would happen if LOOKUP_ROOT/LOOKUP_BENEATH
> > > combination would someday get passed?
> > 
> > I don't understand what's going on with ->r_seq in there - your
> > call of path_is_under() is after having (re-)sampled rename_lock,
> > but if that was the only .. in there, who's going to recheck
> > the value?  For that matter, what's to guarantee that the thing
> > won't get moved just as you are returning from handle_dots()?
> > 
> > IOW, what does LOOKUP_IN_ROOT guarantee for caller (openat2())?
> 
> Sigh...  Usual effects of trying to document things:
> 
> 1) LOOKUP_NO_EVAL looks bogus.  It had been introduced by commit 57d4657716ac
> (audit: ignore fcaps on umount) and AFAICS it's crap.  It is set in
> ksys_umount() and nowhere else.  It's ignored by everything except
> filename_mountpoint().  The thing is, call graph for filename_mountpoint()
> is
> 	filename_mountpoint()
> 		<- user_path_mountpoint_at()
> 			<- ksys_umount()
> 		<- kern_path_mountpoint()
> 			<- autofs_dev_ioctl_ismountpoint()
> 			<- find_autofs_mount()
> 				<- autofs_dev_ioctl_open_mountpoint()
> 				<- autofs_dev_ioctl_requester()
> 				<- autofs_dev_ioctl_ismountpoint()
> In other words, that flag is basically "was filename_mountpoint()
> been called by umount(2) or has it come from an autofs ioctl?".
> And looking at the rationale in that commit, autofs ioctls need
> it just as much as umount(2) does.  Why is it not set for those
> as well?  And why is it conditional at all?

In addition, LOOKUP_NO_EVAL == LOOKUP_OPEN (0x100). Is that meant to be
the case? Also I just saw you have a patch in work.namei that fixes this
up -- do you want me to rebase on top of that?

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