From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4A814C27C4F for ; Wed, 26 Jun 2024 13:33:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=QBjcVA6BS1/Pbjb8BHPPISR7hOxaqaa6zds+89MMBNU=; b=GpeBl8jOY7YJYW 1auXdXUFIhBh98e4Nzv9mF0a+xN/kQ5zCe+r9HzoJKdWkaRuun6u2N04bdw3/iaGbqTThpa8u7pZR Y6HVh95Da6dUgp0mzX7x6LtDpJxGeqlL8S+dkYClGsZWRM6db+jiZ2WZv3b5kgcypUaCoOqgaeqhT 6KarNSOgaY3DIqXiN2wf/TETIemKoKxmKmTY2wyinLaTCMlZEqDNHibHZOtmpIx0ANP/fC7jxz2Yn hndqsWIKwrLxotT/jGBN1XVpvD9+AzJXWS6Dz7ppayZcSngPo6rbpKUIKmK2+4XwS5Y94qHwgTLaw puvL2pqjMuZxEj1zgAGQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1sMSlf-00000006ymM-09fq; Wed, 26 Jun 2024 13:32:59 +0000 Received: from sin.source.kernel.org ([2604:1380:40e1:4800::1]) by bombadil.infradead.org with esmtps (Exim 4.97.1 #2 (Red Hat Linux)) id 1sMSlZ-00000006yiQ-2o37 for linux-riscv@lists.infradead.org; Wed, 26 Jun 2024 13:32:57 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sin.source.kernel.org (Postfix) with ESMTP id E6CA1CE21B8; Wed, 26 Jun 2024 13:32:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3AA85C2BD10; Wed, 26 Jun 2024 13:32:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1719408760; bh=BpC4jkRO/Y4dva9jy2LNSSoyaFURbYQjZeqc2cVtz0E=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=f4fRYrbwf1JT5YZst1PiAi4aFzq22Q10NLheMwylJUhz8LLva7P4Bpi+kywhJYXEt RNLKDvyL1TZTvSIsQLImQo1uEkeHfZHbiLKYXYL1BcZ/phfTW2Wa/cPeJG/qI2BwSM UtDfChLs+W09T/UjLqPD2xIZamMQy8JNF5c2szq9PfN433dsIGAOOsrulb+GPIPhmW gseBbYOQ2uFIGizssSl38u7UCrGhVKB7Bd1FEGA/mC3j/bFwcJEho0QaXYIy3AGVKu nf6Sna6aiAlnUZm6Si+We7A3BbVPN5ThytXCD0+2v+QtCbdRrRzyowKZp3igLB2lJJ p6tgEk9LWOI/Q== Date: Wed, 26 Jun 2024 21:18:34 +0800 From: Jisheng Zhang To: Arnd Bergmann Cc: Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user Message-ID: References: <20240625040500.1788-1-jszhang@kernel.org> <20240625040500.1788-3-jszhang@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240626_063254_561701_5FCF1529 X-CRM114-Status: GOOD ( 42.71 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Wed, Jun 26, 2024 at 08:49:59PM +0800, Jisheng Zhang wrote: > On Wed, Jun 26, 2024 at 08:32:38PM +0800, Jisheng Zhang wrote: > > On Tue, Jun 25, 2024 at 07:54:30AM +0200, Arnd Bergmann wrote: > > > On Tue, Jun 25, 2024, at 06:04, Jisheng Zhang wrote: > > > > I believe the output constraints "=m" is not necessary, because > > > > the instruction itself is "write", we don't need the compiler > > > > to "write" for us. So tell compiler we read from memory instead > > > > of writing. > > > > > > > > Signed-off-by: Jisheng Zhang > > > > > > I think this is a bit too confusing: clearly there is no > > > read access from the __user pointer, so what you add in here > > > is not correct. There also needs to be a code comment about > > > > Here is my understanding: the __put_user is implemented with > > sd(or its less wider variant, sw etc.), w/o considering the > > ex_table, the previous code can be simplified as below: > > > > __asm__ __volatile__ ( > > "sw %z2, %1\n" > > : "+r" (err), "=m" (*(ptr)) > > : "rJ" (__x)); > > > > Here ptr is really an input, just tells gcc where to store, > > And the "store" action is from the "sw" instruction, I don't > > need the gcc generates "store" instruction for me. so IMHO, > > there's no need to use output constraints here. so I changed > > it to > > > > __asm__ __volatile__ ( > > "sw %z1, %2\n" > > : "+r" (err) > > : "rJ" (__x), "m"(*(ptr))); > > > > The key here: is this correct? > > > > > > Here is the put_user piece code and comments from x86 > > > > /* > > * Tell gcc we read from memory instead of writing: this is because > > * we do not write to any memory gcc knows about, so there are no > > * aliasing issues. > > */ > > #define __put_user_goto(x, addr, itype, ltype, label) \ > > asm goto("\n" \ > > "1: mov"itype" %0,%1\n" \ > > _ASM_EXTABLE_UA(1b, %l2) \ > > : : ltype(x), "m" (__m(addr)) \ > > : : label) > > Here is the simplified put_user piece code of arm64: > > #define __put_mem_asm(store, reg, x, addr, err, type) \ > asm volatile( \ > "1: " store " " reg "1, [%2]\n" \ > "2:\n" \ > _ASM_EXTABLE_##type##ACCESS_ERR(1b, 2b, %w0) \ > : "+r" (err) \ > : "rZ" (x), "r" (addr)) > > no output constraints either. It just uses "r" input constraints to tell make it accurate: by this I mean the "addr" of __put_user() isn't in the output constraints. > gcc to read the store address into one proper GP reg. > > > > > > > As can be seen, x86 also doesn't put the (addr) in output constraints, > > I think x86 version did similar modification in history, but when I tried > > to searh the git history, the comment is there from the git first day. > > > > Any hint or suggestion is appreciated! > > > > > why you do it this way, as it's not clear that this is > > > a workaround for old compilers without > > > CONFIG_CC_HAS_ASM_GOTO_OUTPUT. > > > > > > > index 09d4ca37522c..84b084e388a7 100644 > > > > --- a/arch/riscv/include/asm/uaccess.h > > > > +++ b/arch/riscv/include/asm/uaccess.h > > > > @@ -186,11 +186,11 @@ do { \ > > > > __typeof__(*(ptr)) __x = x; \ > > > > __asm__ __volatile__ ( \ > > > > "1:\n" \ > > > > - " " insn " %z2, %1\n" \ > > > > + " " insn " %z1, %2\n" \ > > > > "2:\n" \ > > > > _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %0) \ > > > > - : "+r" (err), "=m" (*(ptr)) \ > > > > - : "rJ" (__x)); \ > > > > + : "+r" (err) \ > > > > + : "rJ" (__x), "m"(*(ptr))); \ > > > > } while (0) > > > > > > > > > > I suspect this could just be a "r" constraint instead of > > > "m", treating the __user pointer as a plain integer. > > > > I tried "r", the generated code is not as good as "m" > > > > for example > > __put_user(0x12, &frame->uc.uc_flags); > > > > with "m", the generated code will be > > > > ... > > csrs sstatus,a5 > > li a4,18 > > sd a4,128(s1) > > csrc sstatus,a5 > > ... > > > > > > with "r", the generated code will be > > > > ... > > csrs sstatus,a5 > > li a4,18 > > addi s1,s1,128 > > sd a4,0(s1) > > csrc sstatus,a5 > > ... > > > > As can be seen, "m" can make use of the 'offset' of > > sd, so save one instruction. > > > > > > > > For kernel pointers, using "m" and "=m" constraints > > > correctly is necessary since gcc will often access the > > > same data from C code as well. For __user pointers, we > > > can probably get away without it since no C code is > > > ever allowed to just dereference them. If you do that, > > > you may want to have the same thing in the __get_user > > > side. > > > > > > Arnd _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv