linux-kernel - Re: [RFC][PATCHSET] iov

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Date:   Mon, 7 Jun 2021 21:07:02 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Sterba <dsterba@...e.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Anton Altaparmakov <anton@...era.com>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [RFC][PATCHSET] iov_iter work

On Mon, Jun 07, 2021 at 04:59:10PM +0100, Christoph Hellwig wrote:
> On Mon, Jun 07, 2021 at 02:43:40PM +0000, Al Viro wrote:
> > > It can't even happen for the legacy architectures, given that the
> > > remaining set_fs() areas are small and never do iov_iter based I/O.
> > 
> > 	Umm...  It's a bit trickier than that - e.g. a kernel thread on
> > a CONFIG_SET_FS target passing a kernel pointer to vfs_read() could've
> > ended up with new_sync_write() hitting iov_iter_init().
> 
> Yes, that is a possbility, but rather unlikely - it would require an
> arch-specific thread using iov_iter_init.  iov_iter_init instances are
> rather fewer, and only very few in arch code.

Doesn't have to be in arch code itself (see above re vfs_read()/vfs_write()),
but AFAICS it doesn't happen.

Anyway, what I'm going to do is
void iov_iter_init(struct iov_iter *i, unsigned int direction,
                        const struct iovec *iov, unsigned long nr_segs,
			size_t count)
{
	WARN_ON(direction & ~(READ | WRITE));

        if (WARN_ON(uaccess_kernel())) {
		// shouldn't be any such callers left...
		iov_iter_kvec(i, direction, (const struct kvec *)iov,
			      nr_segs, count);
		return;
	}
	*i = (struct iov_iter) {
		.iter_type = ITER_IOVEC,
		.data_source = direction,
		.iov = iov,
		.nr_segs = nr_segs,
		.iov_offset = 0,
		.count = count
	};
}

and in a cycle or two replace that if (WARN_ON()) into flat BUG_ON()

Linus, can you live with that variant?  AFAICS, we really should have
no such callers left on any targets.