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

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Message-ID: <YL5CTiR94f5DYPFK@infradead.org>
Date:   Mon, 7 Jun 2021 16:59:10 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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.

> 	AFAICS, we don't have any instances of that, but it's not
> as simple as "we don't do any iov_iter work under set_fs(KERNEL_DS)"

Indeed.