From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 5F0FC2F85B for ; Wed, 4 Feb 2026 00:14:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=63.228.1.57 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770164051; cv=none; b=OLiPh/mIZ1wh42dHxhHa7tzQew8DH6z3UC1ZyNM1OUv1qniz2BrlWiKDrhM3ZbgMzl6q3tSMK/LusL5rFLipBmqEWfycqbiWPTDj4xNQ90HnltizTb/bWm13Arot0sbh6+ZTeFGAtzF3nCaYnKP7rKvBAy9MO47TCLYaP2smBso= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770164051; c=relaxed/simple; bh=bdbt8JN6hQkEks7USoQwkCeqlenQ1RVmHYP/tvhgTgI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kCu6oRFQzmAfZKQxfZRtdhnepXJ2oyOGFUWj2dXIXSVQW1/s9hmsIhcO64ohkngYZxQpdHqPK0K0VATRE7A86Ow5zxCB5dA71rIfl8IaHJnhd4nG7nn+UH27z9dodWHtPWjVU1Mr2vN6WQTCAaUwCdfmjcsWIab2ifJIkHqyFkk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org; spf=pass smtp.mailfrom=kernel.crashing.org; arc=none smtp.client-ip=63.228.1.57 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=kernel.crashing.org Received: from gate.crashing.org (localhost [127.0.0.1]) by gate.crashing.org (8.18.1/8.18.1/Debian-2) with ESMTP id 6140DeeX3681128; Tue, 3 Feb 2026 18:13:40 -0600 Received: (from segher@localhost) by gate.crashing.org (8.18.1/8.18.1/Submit) id 6140DZ1W3681124; Tue, 3 Feb 2026 18:13:35 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Tue, 3 Feb 2026 18:13:35 -0600 From: Segher Boessenkool To: David Laight Cc: "Christophe Leroy (CS GROUP)" , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, llvm@lists.linux.dev, kernel test robot Subject: Re: [PATCH] powerpc/uaccess: Fix inline assembly for clang build on PPC32 Message-ID: References: <8ca3a657a650e497a96bfe7acde2f637dadab344.1770103646.git.chleroy@kernel.org> <20260203221939.059bb903@pumpkin> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260203221939.059bb903@pumpkin> Hi! On Tue, Feb 03, 2026 at 10:19:39PM +0000, David Laight wrote: > On Tue, 3 Feb 2026 08:30:41 +0100 > "Christophe Leroy (CS GROUP)" wrote: > > > Test robot reports the following error with clang-16.0.6: > > > > In file included from kernel/rseq.c:75: > > include/linux/rseq_entry.h:141:3: error: invalid operand for instruction > > unsafe_get_user(offset, &ucs->post_commit_offset, efault); > > ^ > > include/linux/uaccess.h:608:2: note: expanded from macro 'unsafe_get_user' > > arch_unsafe_get_user(x, ptr, local_label); \ > > ^ > > arch/powerpc/include/asm/uaccess.h:518:2: note: expanded from macro 'arch_unsafe_get_user' > > __get_user_size_goto(__gu_val, __gu_addr, sizeof(*(p)), e); \ > > ^ > > arch/powerpc/include/asm/uaccess.h:284:2: note: expanded from macro '__get_user_size_goto' > > __get_user_size_allowed(x, ptr, size, __gus_retval); \ > > ^ > > arch/powerpc/include/asm/uaccess.h:275:10: note: expanded from macro '__get_user_size_allowed' > > case 8: __get_user_asm2(x, (u64 __user *)ptr, retval); break; \ > > ^ > > arch/powerpc/include/asm/uaccess.h:258:4: note: expanded from macro '__get_user_asm2' > > " li %1+1,0\n" \ > > ^ > > :7:5: note: instantiated into assembly here > > li 31+1,0 > > ^ > > 1 error generated. > > > > On PPC32, for 64 bits vars a pair of registers is used. Usually the > > lower register in the pair is the high part and the higher register is > > the low part. GCC uses r3/r4 ... r11/r12 ... r14/r15 ... r30/r31 > > > > In older kernel code inline assembly was using %1 and %1+1 to represent > > 64 bits values. However here it looks like clang uses r31 as high part, > > allthough r32 doesn't exist hence the error. > > > > Allthoug %1+1 should work, most places now use %L1 instead of %1+1, so > > let's do the same here. > > > > With that change, the build doesn't fail anymore and a disassembly shows > > clang uses r17/r18 and r31/r14 pair when GCC would have used r16/r17 and > > r30/r31: > > Isn't it all horribly worse than that? > It only failed because clang picked r31, but if can pick two non-adjacent > registers might it not pick any pair. > In which case there could easily be a 64bit get_user() that reads an incorrect > value and corrupts another register. > Find one and you might have a privilege escalation. I don't think LLVM is that broken, it only has problems for some edge cases. Yes, I might expect too much. But without proof to the contrary let's assume things are okay :-) And, worrying. But what can we do against it! Other than never ever use LLVM for anything serious, of course. Segher