Re: [PATCH] usercopy: use unsigned long instead of uintptr_t

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org, linux-xfs@vger.kernel.org,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	Uladzislau Rezki <urezki@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Joe Perches <joe@perches.com>
Subject: Re: [PATCH] usercopy: use unsigned long instead of uintptr_t
Date: Thu, 16 Jun 2022 16:51:08 +0200	[thread overview]
Message-ID: <YqtDXPWdFQ/fqgDo@zx2c4.com> (raw)
In-Reply-To: <YqtAShjjo1zC6EgO@casper.infradead.org>

Hi Matthew,

On Thu, Jun 16, 2022 at 03:38:02PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 16, 2022 at 04:36:17PM +0200, Jason A. Donenfeld wrote:
> > A recent commit factored out a series of annoying (unsigned long) casts
> > into a single variable declaration, but made the pointer type a
> > `uintptr_t` rather than the usual `unsigned long`. This patch changes it
> > to be the integer type more typically used by the kernel to represent
> > addresses.
> 
> No.  I did this on purpose.  uintptr_t is the correct type to represent
> a pointer that's being used as an integer.  This dinosaur approach of
> using unsigned long has to stop.

For better or for worse, I've always assumed that the kernel had its
reasons -- legitimate reasons, even -- for preferring `unsigned long` to
a userspace type like `uintptr_t`, so I've always tried to code that
way.

If that's a "dinosaur approach" that "has to stop", it'd certainly be
news to me (and I'm guessing others on the list too). I've never really
seen anybody question the kernel's `unsigned long` usage before.

So hopefully some outcome of this discussion will make it clear, and
then either this patch will go in, or I'll get to work on carefully
adjusting my code that uses `unsigned long` at the moment.

Jason