Date: Mon, 29 Oct 2001 12:58:20 -0800 (PST) From: Julian Elischer <julian@elischer.org> To: Jake Burkholder <jake@locore.ca> Cc: John Baldwin <jhb@FreeBSD.ORG>, arch@FreeBSD.ORG Subject: Re: syscall() ABI questions Message-ID: <Pine.BSF.4.21.0110291255300.26174-100000@InterJet.elischer.org> In-Reply-To: <20011029151912.D14748@locore.ca>
next in thread | previous in thread | raw e-mail | index | archive | help
But the API for vfork forbids the child from doing a 'ret' after the vfork returns.. if it does that, all bets are off.... you can do: vfork(); exec(); and that 's about all that's ok... I'm not even sure about vfork(); exit(); On Mon, 29 Oct 2001, 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 :) > > > > > -- > > > > 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 > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.BSF.4.21.0110291255300.26174-100000>