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Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i5pSc-00065U-Lb; Thu, 05 Sep 2019 10:57:55 +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 C8326303121; Thu, 5 Sep 2019 12:57:12 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id B4C5529E3BB8C; Thu, 5 Sep 2019 12:57:49 +0200 (CEST) Date: Thu, 5 Sep 2019 12:57:49 +0200 From: Peter Zijlstra To: Aleksa Sarai Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Message-ID: <20190905105749.GW2386@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> <20190905094305.GJ2349@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190905094305.GJ2349@hirez.programming.kicks-ass.net> 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 , patrick.bellasi@arm.com, 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 11:43:05AM +0200, Peter Zijlstra wrote: > 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'. And thinking more about things; I'm not convinced the above patch is actually right. Do we really want to simply truncate all the attributes of the task? And should we not at least set sched_flags when there are non-default clamp values applied? See; that is I think the primary bug that had chrt failing; we tried to publish the default clamp values as !0. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel