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Miller" , Jakub Kicinski , Simon Horman , Daniel Borkmann , Dave Hansen , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v4 10/10] powerpc/uaccess: Implement masked user access Message-ID: <20251106123550.GX4067720@noisy.programming.kicks-ass.net> References: <5c80dddf8c7b1e75f08b3f42bddde891d6ea3f64.1762427933.git.christophe.leroy@csgroup.eu> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5c80dddf8c7b1e75f08b3f42bddde891d6ea3f64.1762427933.git.christophe.leroy@csgroup.eu> 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.