linux-kernel - Re: [PATCH 03/11] fs: add new read_uptr and write

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Message-ID: <CAHk-=wgC4a9rKrKLTHbH5cA5dyaqqy4Hnsr+re144AiJuNwv9Q@mail.gmail.com>
Date:   Wed, 24 Jun 2020 11:20:26 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 03/11] fs: add new read_uptr and write_uptr file operations

On Wed, Jun 24, 2020 at 11:14 AM Christoph Hellwig <hch@....de> wrote:
>
> So we'd need new user copy functions for just those cases

No. We'd open-code them. They'd look at "oh, I'm supposed to use a
kernel pointer" and just use those.

IOW, basically IN THE CODE that cares (and the whole argument is that
this code is one or two special cases) you do

    /* This has not been converted to the new world order */
    if (get_fs() == KERNEL_DS) memcpy(..) else copy_from_user();

You're overdesigning things. You're making them more complex than they
need to be.

Basically, I do *NOT* want to pollute the VFS layer with new
interfaces that shouldn't exist in the long run. I'd much rather make
the eventual goal be to get rid of 'read/write' entirely in favour of
the 'iter' things, but what I absolutely do *NOT* want to see is to
make a _third_ interface for reading and writing. Quite the reverse.
We should strive to make it a _single_ interface, not add a new one.

And I'd rather have a couple of ugly code details in odd places (that
we can hopefully fix up later) than have new VFS infrastructure that
will then hang around forever more.

                Linus