linux-kernel - Re: [PATCH 05/14] fs: don't allow kernel reads and writes without iter ops

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Date:   Fri, 9 Oct 2020 15:06:33 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 05/14] fs: don't allow kernel reads and writes without
 iter ops

On Fri, Oct 02, 2020 at 09:27:09AM -0700, Linus Torvalds wrote:
> On Thu, Oct 1, 2020 at 3:41 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Better
> >         loff_t dummy = 0;
> > ...
> >                 wr = __kernel_write(file, data, bytes, &dummy);
> 
> No, just fix __kernel_write() to work correctly.
> 
> The fact is, NULL _is_ the right pointer for ppos these days.
> 
> That commit by Christoph is buggy: it replaces new_sync_write() with a
> buggy open-coded version.
> 
> Notice how new_sync_write does
> 
>         kiocb.ki_pos = (ppos ? *ppos : 0);
> ,,,
>         if (ret > 0 && ppos)
>                 *ppos = kiocb.ki_pos;
> 
> but the open-coded version doesn't.
> 
> So just fix that in linux-next. The *last* thing we want is to have
> different semantics for the "same" kernel functions.

It's a bit unintuitive that ppos=NULL means "use pos 0", not "use file->f_pos".

Anyway, it works.  The important thing is, this is still broken in linux-next...

- Eric