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

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Date:   Wed, 24 Jun 2020 20:24:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, 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 11:20:26AM -0700, Linus Torvalds wrote:
> 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.

I wish it was so simple.  I really don't like overdesigns, trust me.

But please take a look at setsockopt and all the different instances
(count 90 .setsockopt wireups, and they then branch out into
various subroutines as well).  I really don't want to open code that
there, but we could do helper specific to setsockopt.

Honestly my preference would be to say that no eBPF isn't actually
a user API and just rip out the crap added to it, but I fear that
is not an option.  Because in that case we'd basically be done.

> 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.

Completele agreement on this.  I actually hate the new fops, and only
added them reluctantly as I mis-interpreted what you said.