From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3E972181CFF for ; Wed, 26 Jun 2024 13:03:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719407037; cv=none; b=D3T1po0sO1dtNXQ/Vy4/FdfsXcaspLip8dgsJXOog5nI6TnEWS0kFCvcq3uTjmDvzSP+9cQuMy8XGmEjKSHL8AWVpeyw+2VAy8bj43wx1c8dNIEBNWHNkIptUOgB1jYJ2GQpFuTZ0onjtHo0hfT2W72V+hAN3ryzYwSHGG/quMM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719407037; c=relaxed/simple; bh=Uta7Ar4IuN68vM/aeq1mQnasHGnsAdafsPir9UrleIA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Nn6DHeQbweqLBOHJtEJGXUxCdw+nEhigdA9eTVR29EVVVqxLzvvkX5nsgQUDs5PNh0JQjapVkKK15nDkM9LgB84W73FcNKcKaS+iOQM/ni0Fe4uJbNwvz+Tw+FMkMKp0s+SrM0Kx50e8CkZEC91CvQKQT2luChiILBgqUsfKuK8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Pd/i5zHI; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Pd/i5zHI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 91FF3C2BD10; Wed, 26 Jun 2024 13:03:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1719407036; bh=Uta7Ar4IuN68vM/aeq1mQnasHGnsAdafsPir9UrleIA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Pd/i5zHIn/AQu7DnXglDjXVDP9Kqs1JHUD2n7nhTlLi8yjm2IKz+JBZx+4Yr+VtVE e7yHGcUPsRbgcbw2P9+0iAa3eQIuPxdxCvD4zQ0061JLX7KAItn2oMa+/oo7u6EDKu InxF+MFcm1V/lYROQX9e8N+GxCkVpWhXIeDRiX1glTL0WRzxgq4B6Rtu/igGkMkHjJ kKbqnRgp6CKAu2f9hX6M/ujgXDcLOn2TSFJ7iTZJAYpwPGfOf02rZC1mJD1CS20bqy b3G+YD1AUQjI6Cbskf2xxrUstH05FMPGqFtukjg1/JATbW2sStWEYgcPcvdhb0UfS+ VYyYxbL50Koew== Date: Wed, 26 Jun 2024 20:49:50 +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> 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=utf-8 Content-Disposition: inline In-Reply-To: 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 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