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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 2 Oct 2020 09:27:09 -0700
Message-ID: <CAHk-=wgj=mKeN-EfV5tKwJNeHPLG0dybq+R5ZyGuc4WeUnqcmA@mail.gmail.com>
Subject: Re: [PATCH 05/14] fs: don't allow kernel reads and writes without
 iter ops
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Eric Biggers <ebiggers@kernel.org>, 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 Thu, Oct 1, 2020 at 3:41 PM Al Viro <viro@zeniv.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.

               Linus