From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S267674AbUHEN05 (ORCPT ); Thu, 5 Aug 2004 09:26:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S267677AbUHEN04 (ORCPT ); Thu, 5 Aug 2004 09:26:56 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.103]:2468 "EHLO e3.ny.us.ibm.com") by vger.kernel.org with ESMTP id S267674AbUHENYv (ORCPT ); Thu, 5 Aug 2004 09:24:51 -0400 Date: Thu, 5 Aug 2004 19:03:48 +0530 From: Suparna Bhattacharya To: Andi Kleen Cc: prasanna@in.ibm.com, linux-kernel@vger.kernel.org, torvalds@osdl.org, akpm@osdl.org Subject: Re: [1/3] kprobes-func-args-268-rc3.patch Message-ID: <20040805133348.GA4471@in.ibm.com> Reply-To: suparna@in.ibm.com References: <2pMJz-13N-9@gated-at.bofh.it> <20040805122431.GA4411@in.ibm.com> <20040805125423.GA63682@muc.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20040805125423.GA63682@muc.de> User-Agent: Mutt/1.4i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 05, 2004 at 02:54:23PM +0200, Andi Kleen wrote: > On Thu, Aug 05, 2004 at 05:54:31PM +0530, Suparna Bhattacharya wrote: > > > I think you misunderstood Linus' suggestion. The problem with > > > modifying arguments on the stack frame is always there because the C > > > ABI allows it. One suggested solution was to use a second function > > > > I did realise that it is the ABI which allows this, but I thought > > that the only situation in which we know gcc to actually clobber > > arguments from the callee in practice is for tailcall optimization. > > It just breaks the most common workaround. Just curious, do you know if other cases/optimizations where the callee clobbers arguments on stack ? > > > I'm not sure if that can be guaranteed and yes saving bytes from > > stack would avoid the problem totally (hence the comment) and make > > it less tied to expected innards of the compiler. The only issue > > with that is deciding the maximum number of arguments so it is > > generic enough. > > 64bytes, aka 16 arguments seem far enough. OK, is there is consensus on this ? We'd have to make the code check for stack boundary etc and probably compare and copy back only if there has been a change. > > > > call that passes the arguments again to get a private copy. But the > > > compiler's tail call optimization could sabotate that when you a > > > are not careful. > > > > > > That's all quite hackish and compiler dependent. I would suggest an > > > assembly wrapper that copies the arguments when !CONFIG_REGPARM. > > > Just assume the function doesn't have more than a fixed number > > > of arguments, that should be good enough. > > > > > > This way you avoid any subtle compiler dependencies. > > > With CONFIG_REGPARM it's enough to just save/restore pt_regs, > > > which kprobes will do anyways. > > > > > > > > Even with CONFIG_REGPARM, if you have a large > > number of arguments for example, is spill over into stack > > a possibility ? > > Yes. For more than three (Linux uses -mregparm=3) > Also varargs arguments will be always on the stack I think. Right, so making the copy dependent on !CONFIG_REGPARM wouldn't make sense would it ? Regards Suparna -- Suparna Bhattacharya (suparna@in.ibm.com) Linux Technology Center IBM Software Lab, India