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Date:   Fri, 9 Oct 2020 18:19:19 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.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>
Subject: Re: [PATCH 05/14] fs: don't allow kernel reads and writes without
 iter ops
Message-ID: <20201010011919.GC1122@sol.localdomain>
References: <20200903142242.925828-1-hch@lst.de>
 <20200903142242.925828-6-hch@lst.de>
 <20201001223852.GA855@sol.localdomain>
 <20201001224051.GI3421308@ZenIV.linux.org.uk>
 <CAHk-=wgj=mKeN-EfV5tKwJNeHPLG0dybq+R5ZyGuc4WeUnqcmA@mail.gmail.com>
 <20201009220633.GA1122@sol.localdomain>
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On Fri, Oct 09, 2020 at 06:03:31PM -0700, Linus Torvalds wrote:
> 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.
> 

Okay, that makes more sense.  So the patchset from Matthew
https://lkml.kernel.org/linux-fsdevel/20201003025534.21045-1-willy@infradead.org/T/#u
isn't what you had in mind.

- Eric