linux-kernel - Re: [PATCH v4 10/10] powerpc/uaccess: Implement masked user access

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Message-ID: <20251106123550.GX4067720@noisy.programming.kicks-ass.net>
Date: Thu, 6 Nov 2025 13:35:50 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Andre Almeida <andrealmeid@...lia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, linux-block@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 10/10] powerpc/uaccess: Implement masked user access

On Thu, Nov 06, 2025 at 12:31:28PM +0100, Christophe Leroy wrote:

> On 32 bits it is more tricky. In theory user space can go up to
> 0xbfffffff while kernel will usually start at 0xc0000000. So a gap
> needs to be added in-between. Allthough in theory a single 4k page
> would suffice, it is easier and more efficient to enforce a 128k gap
> below kernel, as it simplifies the masking.

Do we have the requirement that the first access of a masked pointer is
within 4k of the initial address?

Suppose the pointer is to an 16k array, and the memcpy happens to like
going backwards. Then a 4k hole just won't do.