linux-kernel - Re: [patch 0/4] uaccess: Provide and use helpers for user masked access

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Message-ID: <CAHk-=wjsACUbLM-dAikbHzHBy6RFqyB1TdpHOMAJiGyNYM+FHA@mail.gmail.com>
Date: Sun, 17 Aug 2025 07:00:17 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, 
	André Almeida <andrealmeid@...lia.com>, x86@...nel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [patch 0/4] uaccess: Provide and use helpers for user masked access

On Sun, 17 Aug 2025 at 06:50, David Laight <david.laight.linux@...il.com> wrote:
>
> Linus didn't like it, but I've forgotten why.

I think the reason I didn't love it is that it has a bit subtle
semantics, and I think you just proved my point:

> I'm also not convinced of the name.
> There isn't any 'masking' involved, so it shouldn't be propagated.

Sure there is. Look closer at that patch:

+       if (can_do_masked_user_access())                                \
+               src = masked_user_access_begin(src);                    \

IOW, that macro changes the argument and masks it.

So it's actually really easy to use, but it's also really easy to miss
that it does that.

We've done this before, and I have done it myself. The classic example
is the whole "do_div()" macro that everybody hated because it did
exactly the same thing (we also have "save_flags()" etc that have this
behavior).

So I don't love it - but I can't claim I've not done the same thing,
and honestly, it does make it very easy to use, so when Thomas sent
this series out I didn't speak out against it.

           Linus