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
> <dil...@backplane.com>
>
>
> To Unsubscribe: send mail to majord...@freebsd.org
> with "unsubscribe freebsd-current" in the body of the message
>
Brian Feldman _ __ ___ ___ ___
gr...@unixhelp.org _ __ ___ | _ ) __| \
http://www.freebsd.org/ _ __ ___ ____ | _ \__ \ |) |
FreeBSD: The Power to Serve! _ __ ___ ____ _____ |___/___/___/
To Unsubscribe: send mail to majord...@freebsd.org
with "unsubscribe freebsd-current" in the body of the message
