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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0AD0DC3A5A5 for ; Thu, 5 Sep 2019 09:43:17 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id CA7AC20825 for ; Thu, 5 Sep 2019 09:43:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="gSxTZCnV" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CA7AC20825 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject: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=2aumRHmyjJZFpM181b0w2fFRV8PtM+iW6sOcb0RB02w=; b=gSxTZCnV1g1Vhp xk6knhglPonT0C6g395ZkusCe6nnJRSROCeatyrwlSbbk0H/bEMdvWqgZeaCgzuBMVESdgPzoL44N zE9/qkizpGSFsRWwDIJzBhi7EpVkyWuLrKVc3Y/jT/dxZs1R4RtvEMoZ3qJFfKOyMqVD7AvVL4A18 3AkyRVPn+4MnANkE4PN7ynvW7Wfu1ur17aNCOvk9jpW+0zO7b95FfXg/bOT93pJD/nko6BSedqKqN YqmJKw19NwfKFjtTa8rGQ9YVeBccb4cjifVj6qEFYhCS3b5B9iesgm+Ulw8VVhOqy69xm4Drw9IRA Pv/01b7wqvNFITV2ZD/A==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i5oIN-0008I1-6T; Thu, 05 Sep 2019 09:43:15 +0000 Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i5oII-0008Hm-Ap; Thu, 05 Sep 2019 09:43:11 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id DE6893011DF; Thu, 5 Sep 2019 11:42:28 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id B5FA420D8627E; Thu, 5 Sep 2019 11:43:05 +0200 (CEST) Date: Thu, 5 Sep 2019 11:43:05 +0200 From: Peter Zijlstra To: Aleksa Sarai Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Message-ID: <20190905094305.GJ2349@hirez.programming.kicks-ass.net> References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-2-cyphar@cyphar.com> <20190905073205.GY2332@hirez.programming.kicks-ass.net> <20190905092622.tlb6nn3uisssdfbu@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190905092622.tlb6nn3uisssdfbu@yavin.dot.cyphar.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Alexander Shishkin , Rasmus Villemoes , Alexei Starovoitov , linux-kernel@vger.kernel.org, David Howells , linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org, Jiri Olsa , linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Tycho Andersen , Aleksa Sarai , Shuah Khan , Ingo Molnar , linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, linux-xtensa@linux-xtensa.org, Kees Cook , Arnd Bergmann , Jann Horn , linuxppc-dev@lists.ozlabs.org, linux-m68k@lists.linux-m68k.org, Al Viro , Andy Lutomirski , Shuah Khan , Namhyung Kim , David Drysdale , Christian Brauner , "J. Bruce Fields" , linux-parisc@vger.kernel.org, linux-api@vger.kernel.org, Chanho Min , Jeff Layton , Oleg Nesterov , Eric Biederman , linux-alpha@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Linus Torvalds , containers@lists.linux-foundation.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote: > On 2019-09-05, Peter Zijlstra wrote: > > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote: > > > +/** > > > + * copy_struct_to_user: copy a struct to user space > > > + * @dst: Destination address, in user space. > > > + * @usize: Size of @dst struct. > > > + * @src: Source address, in kernel space. > > > + * @ksize: Size of @src struct. > > > + * > > > + * Copies a struct from kernel space to user space, in a way that guarantees > > > + * backwards-compatibility for struct syscall arguments (as long as future > > > + * struct extensions are made such that all new fields are *appended* to the > > > + * old struct, and zeroed-out new fields have the same meaning as the old > > > + * struct). > > > + * > > > + * @ksize is just sizeof(*dst), and @usize should've been passed by user space. > > > + * The recommended usage is something like the following: > > > + * > > > + * SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize) > > > + * { > > > + * int err; > > > + * struct foo karg = {}; > > > + * > > > + * // do something with karg > > > + * > > > + * err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg)); > > > + * if (err) > > > + * return err; > > > + * > > > + * // ... > > > + * } > > > + * > > > + * There are three cases to consider: > > > + * * If @usize == @ksize, then it's copied verbatim. > > > + * * If @usize < @ksize, then kernel space is "returning" a newer struct to an > > > + * older user space. In order to avoid user space getting incomplete > > > + * information (new fields might be important), all trailing bytes in @src > > > + * (@ksize - @usize) must be zerored > > > > s/zerored/zero/, right? > > It should've been "zeroed". That reads wrong to me; that way it reads like this function must take that action and zero out the 'rest'; which is just wrong. This function must verify those bytes are zero, not make them zero. > > > , otherwise -EFBIG is returned. > > > > 'Funny' that, copy_struct_from_user() below seems to use E2BIG. > > This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for > a "too big" struct passed to the kernel, and EFBIG for a "too big" > struct passed to user-space. I would personally have preferred EMSGSIZE > instead of EFBIG, but felt using the existing error codes would be less > confusing. Sadly a recent commit: 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code") Made the situation even 'worse'. > > > + if (unlikely(!access_ok(src, usize))) > > > + return -EFAULT; > > > + > > > + /* Deal with trailing bytes. */ > > > + if (usize < ksize) > > > + memset(dst + size, 0, rest); > > > + else if (usize > ksize) { > > > + const void __user *addr = src + size; > > > + char buffer[BUFFER_SIZE] = {}; > > > > Isn't that too big for on-stack? > > Is a 64-byte buffer too big? I picked the number "at random" to be the > size of a cache line, but I could shrink it down to 32 bytes if the size > is an issue (I wanted to avoid needless allocations -- hence it being > on-stack). Ah, my ctags gave me a definition of BUFFER_SIZE that was 512. I suppose 64 should be OK. > > > + > > > + while (rest > 0) { > > > + size_t bufsize = min(rest, sizeof(buffer)); > > > + > > > + if (__copy_from_user(buffer, addr, bufsize)) > > > + return -EFAULT; > > > + if (memchr_inv(buffer, 0, bufsize)) > > > + return -E2BIG; > > > + > > > + addr += bufsize; > > > + rest -= bufsize; > > > + } > > > > The perf implementation uses get_user(); but if that is too slow, surely > > we can do something with uaccess_try() here? > > Is there a non-x86-specific way to do that (unless I'm mistaken only x86 > has uaccess_try() or the other *_try() wrappers)? The main "performance > improvement" (if you can even call it that) is that we use memchr_inv() > which finds non-matching characters more efficiently than just doing a > loop. Oh, you're right, that's x86 only :/ _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel