Re: [PATCH] Re: Possible FS race condition between iterate_dir and d_alloc_parallel

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Sep 14, 2019 at 09:49:21AM -0700, Linus Torvalds wrote:
> On Sat, Sep 14, 2019 at 9:16 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> >         OK, folks, could you try the following?  It survives the local beating
> > so far.
> 
> This looks like the right solution to me. Keep the locking simple,
> take the dentry refcount as long as we keep a ref to it in "*res".

*grumble*

Example of subtleties in the whole mess: this is safe for mainline
now, but only due to "devpts_pty_kill(): don't bother with d_delete()"
already merged.  Without that, we are risking the following fun:

scan_positive() on devpts: finds a dentry, sees it positive, decides
to grab the sucker.  Refcount is currently 1 (will become 2 after
we grab the reference).

devpts_pty_kill(): d_delete(dentry); on that sucker.  Refcount is
currently (still) 1, so we simply make it negative.

scan_positive(): grabs an extra reference to now negative dentry.

devpts_pty_kill(): dput() drops refcount to 1 (what if it got there
before scan_positive() grabbed a reference?  Nothing, really, since
scan_positive() is holding parent's ->d_lock; dput() wouldn't
have progressed through dentry_kill() until it managed to get
that, and it would've rechecked the refcount.  So that's not
a problem)

scan_positive(): returns a reference to negative dentry to
dcache_readdir().  Which proceeds to oops on
                if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
                              d_inode(next)->i_ino, dt_type(d_inode(next))))
since d_inode(next) is NULL.

With the aforementioned commit it *is* safe, since the dentry remains
positive (and unhashed), so we simply act as if dcache_readdir() has
won the race and emitted a record for the sucker killed by devpts_pty_kill().

IOW, backports will either need to bring that commit in as well, or
they'll need to play silly buggers along the lines of

		if (simple_positive(d) && !--count) {
			spin_lock(&d->d_lock);
			if (likely(simple_positive(d)))
				found = dget_dlock(d);
			spin_unlock(&d->d_lock);
			if (found)
				break;
			count = 1;	// it's gone, keep scanning
                }
Probably the latter, since it's less dependent on some other place
doing what devpts used to do...