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Wed, 19 Aug 2026 07:19:42 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: Aqm_jcT6PvEF Date: Wed, 19 Aug 2026 13:19:20 +0200 From: "Arnd Bergmann" To: "Karl Mehltretter" Cc: "Catalin Marinas" , "Will Deacon" , "Mark Rutland" , "Ard Biesheuvel" , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Message-Id: <4078f942-e29a-4f6f-b6fc-780e8c6d6874@app.fastmail.com> In-Reply-To: References: <20260817000231.21311-1-kmehltretter@gmail.com> Subject: Re: [PATCH] arm64: compat: Keep alignment address arithmetic 32-bit Content-Type: text/plain Content-Transfer-Encoding: 7bit On Wed, Aug 19, 2026, at 06:56, Karl Mehltretter wrote: > On Tue, Aug 18, 2026 at 09:31:14AM +0100, Arnd Bergmann wrote: >> On Tue, Aug 18, 2026, at 05:22, Karl Mehltretter wrote: > I now used: > > CONFIG_EXPERT=y > CONFIG_ARM64_64K_PAGES=y > CONFIG_COMPAT=y > CONFIG_KUSER_HELPERS=y > CONFIG_COMPAT_ALIGNMENT_FIXUPS=y > CONFIG_DEFAULT_MMAP_MIN_ADDR=0 > > With 64K pages and KUSER_HELPERS, compat TASK_SIZE is 4 GiB. The last > 64K is the kuser mapping. The test maps the first 64K at zero, so memory > exists on both sides of the wrap. Hopefully nobody actually uses CONFIG_DEFAULT_MMAP_MIN_ADDR=0. I don't see yet why 64K_PAGES makes a difference. I had expected TASK_SIZE to be 4GiB regardless of the page size, and the top page (but not the bottom page) to be reachable from normal userspace. > On native ARM32, the kernel is mapped at the top of the address space. > Userspace cannot reach the 4 GiB wrap. right > So v2 is enough for the original decrementing LDM/STM bug. v1 does not > add a working wrap case by itself. I think the small v2 fix makes more > sense. v1+ would be the theoretically complete version for AArch32 wrap > handling... > > Am I getting this right now, or am I still missing a case? That sounds good, v2 should be all we need then. Arnd