Date: Sun, 21 Mar 1999 18:57:01 +0000 (GMT) From: Brian Feldman <green@unixhelp.org> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Alfred Perlstein <bright@rush.net>, "John S. Dyson" <dyson@iquest.net>, samit@usa.ltindia.com, commiters@FreeBSD.ORG, freebsd-current@FreeBSD.ORG Subject: Re: rfork() Message-ID: <Pine.BSF.4.05.9903211855540.3128-100000@zone.syracuse.net> In-Reply-To: <199903211805.KAA13805@apollo.backplane.com>
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On Sun, 21 Mar 1999, Matthew Dillon wrote: > :> :the assembly wouldn't be needed. Hmm... actually... if one were to mmap() a > :> :stack and as soon as the rfork() returned movl newstack,%esp and whatnot, > :> :wouldn't this be a pretty simple solution? > :> > :> No, because one of the processes may overrun the stack before the other > :> one managed to return from rfork(). The child process cannot use the > :> old stack at all. > : > :Why would a simple movl be using the stack? > > If you are making a subroutine *call* to the rfork() routine, where > do you think the return PC address is stored? On the stack. The > rfork() routine is going to 'ret' *after* doing the rfork syscall. > 'ret' pops the stack. While this in itself is not modifying the stack, > you can still wind up with the situation where process A returns from > the rfork and then does something else which overwrites the stack before > process B has a chance to return from the rfork(). Why does it matter if something munges the stack in proc A though before proc B returns since proc B is going to immediately switch over to a new stack? > > This is why, in my assembly example, I was forced to make the syscall > manually rather then call the rfork() library function. > > :> > : Brian Feldman _ __ ___ ___ ___ > > -Matt > > Matthew Dillon > <dillon@backplane.com> > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-current" in the body of the message > Brian Feldman _ __ ___ ___ ___ green@unixhelp.org _ __ ___ | _ ) __| \ http://www.freebsd.org/ _ __ ___ ____ | _ \__ \ |) | FreeBSD: The Power to Serve! _ __ ___ ____ _____ |___/___/___/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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