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Date:   Tue, 8 Oct 2019 21:34:51 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()
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On Tue, Oct 08, 2019 at 08:58:58PM +0100, Al Viro wrote:

> The difference is, they have separate "for read" and "for write" primitives
> and they want the range in their user_access_end() analogue.  Separating
> the read and write isn't a problem for callers (we want them close to
> the actual memory accesses).  Passing the range to user_access_end() just
> might be tolerable, unless it makes you throw up...

NOTE: I'm *NOT* suggesting to bring back the VERIFY_READ/VERIFY_WRITE
argument to access_ok().  We'd gotten rid of it, and for a very good
reason (and decades overdue).

The main difference between access_ok() and user_access_begin() is that
the latter is right next to actual memory access, with user_access_end()
on the other side, also very close.  And most of those guys would be
concentrated in a few functions, where we bloody well know which
direction we are copying.

Even if we try and map ppc allow_..._to_user() on user_access_begin(),
access_ok() remains as it is (and I hope we'll get rid of the majority
of its caller in process).