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Date:   Sun, 3 Jul 2022 04:59:51 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Alexander Potapenko <glider@google.com>,
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Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to
 step_into()
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On Sat, Jul 02, 2022 at 10:23:16AM -0700, Linus Torvalds wrote:
> On Fri, Jul 1, 2022 at 7:25 AM Alexander Potapenko <glider@google.com> wrote:
> >
> > Under certain circumstances initialization of `unsigned seq` and
> > `struct inode *inode` passed into step_into() may be skipped.
> > In particular, if the call to lookup_fast() in walk_component()
> > returns NULL, and lookup_slow() returns a valid dentry, then the
> > `seq` and `inode` will remain uninitialized until the call to
> > step_into() (see [1] for more info).

> So while I think this needs to be fixed, I think I'd really prefer to
> make the initialization and/or usage rules stricter or at least
> clearer.

Disclaimer: the bits below are nowhere near what I consider a decent
explanation; this might serve as the first approximation, but I really
need to get some sleep before I get it into coherent shape.  4 hours
of sleep today...

The rules are
	* no pathname resolution without successful path_init().
IOW, path_init() failure is an instant fuck off.
	* path_init() success sets nd->inode.  In all cases.
	* nd->inode must be set - LOOKUP_RCU or not, we simply cannot
proceed without it.

	* in non-RCU mode nd->inode must be equal to nd->path.dentry->d_inode.
	* in RCU mode nd->inode must be equal to a value observed in
nd->path.dentry->d_inode while nd->path.dentry->d_seq had been equal to
nd->seq.

	* step_into() gets a dentry/inode/seq triple.  In non-RCU
mode inode and seq are ignored; in RCU mode they must satisfy the
same relationship we have for nd->path.dentry/nd->inode/nd->seq.

> Of course, sometimes the "only get used for LOOKUP_RCU" is very very
> unclear, because even without being an RCU lookup, step_into() will
> save it into nd->inode/seq. So the values were "used", and
> initializing them makes them valid, but then *that* copy must not then
> be used unless RCU was set.

You are misreading that (and I admit that it badly needs documentation).
The whole point of step_into() is to move over to new place.  nd->inode
*MUST* be set on success, no matter what.

>  - I look at that follow_dotdot*() caller case, and think "that looks
> very similar to the lookup_fast() case, but then we have *very*
> different initialization rules".

follow_dotdot() might as well lose inodep and seqp arguments - everything
would've worked just as well without those.  We would've gotten the same
complaints about uninitialized values passed to step_into(), though.

This
                if (unlikely(!parent))
                        error = step_into(nd, WALK_NOFOLLOW,
                                         nd->path.dentry, nd->inode, nd->seq);
in handle_dots() probably contributes to confusion - it's the "we
have stepped on .. in the root, just jump into whatever's mounted on
it" case.  In non-RCU case it looks like a use of nd->seq in non-RCU
mode; however, in that case step_into() will end up ignoring the
last two arguments.

I'll post something more coherent after I get some sleep.  Sorry... ;-/