From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH] Syscall arguments are unsigned long (full registers) Date: Mon, 04 Jul 2016 16:59:35 +0200 Message-ID: <3421998.D1ABSO4c36@wuerfel> References: <20160704142802.GL25121@tassilo.jf.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Return-path: In-Reply-To: Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Tautschnig, Michael" Cc: Andi Kleen , "x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org" , "linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Jaswinder Singh List-Id: linux-api@vger.kernel.org On Monday, July 4, 2016 2:47:10 PM CEST Tautschnig, Michael wrote: > Thanks a lot for the immediate feedback. > > > On 4 Jul 2016, at 16:28, Andi Kleen wrote: > > > > On Mon, Jul 04, 2016 at 01:52:58PM +0000, Tautschnig, Michael wrote: > >> All syscall arguments are passed in as types of the same byte size as > >> unsigned long (width of full registers). Using a smaller type without a > >> cast may result in losing bits of information. In all other instances > >> apart from the ones fixed by the patch the code explicitly introduces > >> type casts (using, e.g., SYSCALL_DEFINE1). > >> > >> While goto-cc reported these problems at build time, it is noteworthy > >> that the calling conventions specified in the System V AMD64 ABI do > >> ensure that parameters 1-6 are passed via registers, thus there is no > >> implied risk of misaligned stack access. > > > > Does this actually fix anything? > > > > It will ensure the behaviour on 32 and 64-bit systems is consistent, i.e., > no truncation occurs. This is to ensure that future uses of these syscalls > do not face surprises. > > It looks to me like you are introducing a truncation, not removing one as your comment suggests: long do_arch_prctl(struct task_struct *task, int code, unsigned long addr); -long sys_arch_prctl(int code, unsigned long addr) +long sys_arch_prctl(unsigned long code, unsigned long addr) { return do_arch_prctl(current, code, addr); } This is the same truncation that we do with SYSCALL_DEFINE2(), clearing the top 32 bits of the 'code' parameter to ensure that user space doesn't pass data unexpectedly. That change seems reasonable, but why not just use SYSCALL_DEFINE2() directly for consistency with the other syscalls? Arnd