Date: Mon, 29 Oct 2001 13:43:48 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Jake Burkholder <jake@locore.ca> Cc: arch@FreeBSD.ORG Subject: Re: syscall() ABI questions Message-ID: <XFMail.011029134348.jhb@FreeBSD.org> In-Reply-To: <20011029151912.D14748@locore.ca>
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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 <jhb@FreeBSD.org> -- 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
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