Date: Mon, 29 Oct 2001 15:19:12 -0500 From: Jake Burkholder <jake@locore.ca> To: John Baldwin <jhb@FreeBSD.ORG> Cc: arch@FreeBSD.ORG Subject: Re: syscall() ABI questions Message-ID: <20011029151912.D14748@locore.ca> In-Reply-To: <XFMail.011029115901.jhb@FreeBSD.org>; from jhb@FreeBSD.ORG on Mon, Oct 29, 2001 at 11:59:01AM -0800 References: <20011029145458.C14748@locore.ca> <XFMail.011029115901.jhb@FreeBSD.org>
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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 :) > > -- > > 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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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