From owner-freebsd-arch Mon Oct 29 13:44: 4 2001 Delivered-To: freebsd-arch@freebsd.org Received: from mail12.speakeasy.net (mail12.speakeasy.net [216.254.0.212]) by hub.freebsd.org (Postfix) with ESMTP id 3C4DF37B405 for ; Mon, 29 Oct 2001 13:44:00 -0800 (PST) Received: (qmail 30085 invoked from network); 29 Oct 2001 21:43:59 -0000 Received: from unknown (HELO laptop.baldwin.cx) ([64.81.54.73]) (envelope-sender ) by mail12.speakeasy.net (qmail-ldap-1.03) with SMTP for ; 29 Oct 2001 21:43:59 -0000 Message-ID: X-Mailer: XFMail 1.4.0 on FreeBSD X-Priority: 3 (Normal) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit MIME-Version: 1.0 In-Reply-To: <20011029151912.D14748@locore.ca> Date: Mon, 29 Oct 2001 13:43:48 -0800 (PST) From: John Baldwin To: Jake Burkholder Subject: Re: syscall() ABI questions Cc: arch@FreeBSD.ORG Sender: owner-freebsd-arch@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG On 29-Oct-01 Jake Burkholder wrote: > Apparently, On Mon, Oct 29, 2001 at 11:59:01AM -0800, > John Baldwin said words to the effect of; > > [...] > >> >> > >> >> > I think you're right about fork and rfork being able to use the MIASM >> >> > code. rfork with RFMEM is special but it can';t be safely called from >> >> > C anyway. The vfork wrapper needs to stay on x86 at least because both >> >> > processes return to the same stack; if the retunr address is not saved >> >> > in >> >> > a register the child may clobber the parent's when it "rets" and pops >> >> > the stack. >> >> >> >> Same kernel stack? The register is set in the trapframe which means it >> >> is >> >> saved on the kernel stack. Is that shared in the vfork case? >> > >> > Same user stack. The trapframe is copied to the child's kernel stack. >> >> Then the vfork case should be fine, b/c we fixup %eax in the child's kernel >> stack so that when it returns from the syscall, %eax already has the right >> value. > > Sorry, maybe I wasn;t clear. The problem is that the parent's return > address can get clobbered unless its saved in a register and copied > through the trapframe. Otherwise if the child returns from the kernel > and immediately executes a "ret", the parent's return address will be > below %esp on the shared stack and could get clobbered. It uses > jmp *%ecx now to return, which works fine because both parent and > child have a _copy_ of the value. > > I hope this makes sense :) Oh, doh, yes. For some reason I had forgotten vfork had more to it than fork does. I amend to mean that we remove the setting of %eax which means fork and rfork go back to being standard (aside from the rfork_thread() case to handle RFMEM) -- John Baldwin -- http://www.FreeBSD.org/~jhb/ PGP Key: http://www.baldwin.cx/~john/pgpkey.asc "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message