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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 9 Oct 2020 18:03:31 -0700
Message-ID: <CAHk-=whcEzYjkqdpZciHh+iAdUttvfWZYoiHiF67XuTXB1YJLw@mail.gmail.com>
Subject: Re: [PATCH 05/14] fs: don't allow kernel reads and writes without
 iter ops
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@lst.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Fri, Oct 9, 2020 at 3:06 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> It's a bit unintuitive that ppos=NULL means "use pos 0", not "use file->f_pos".

That's not at all what it means.

A NULL ppos means "this has no position at all", and is what we use
for FMODE_STREAM file descriptors (ie sockets, pipes, etc).

It also means that we don't do the locking for position updates.

The fact that "ki_pos" gets set to zero is just because it needs to be
_something_. It shouldn't actually ever be used for stream devices.

                  Linus