Re: usermode linux on BSD?

On Wed, 2004-03-10 at 14:13, Robert Watson wrote:
> On Wed, 10 Mar 2004, David Gilbert wrote:
> 
> > Has anyone made an attempt to run usermode linux on FreeBSD?  Is the
> > issue-list long? 
> 
> There was a neat paper at BSDCon 2003 discussing running usermode FreeBSD
> on Linux, and it talked about what would be necessary to make usermode
> FreeBSD run on FreeBSD.  You can find the paper off the USENIX web site,
> or perhaps via Google.  I think it was a relatively small set of changes.

I have been toying with user mode BSD... and maybe even run user-mode
Linux on FreeBSD.... And one of the requirements is to be able to alter
syscalls in flight.

I think something line the following patch would help...

The concept goes something like this...

The User-mode-unix would rfork and set up ptrace,
The parent monitors the child process for syscalls...

  child = rfork_thread(RFPROC|RFMEM, stack, child_func, NULL);
  waitpid(child, &status, WUNTRACED);
  ptrace(PT_TO_SCE, child, (caddr_t)1, 0);
  for (;;) {
    waitpid(child, &status, WUNTRACED);
    if (WSTOPSIG(status) == SIGTRAP) {
      struct reg r;
      ptrace(PT_GETREGS, child, (caddr_t)&r, 0);
      save_syscall= r.r_eax;
      r.r_eax = SYS_getpid;
      ptrace(PT_SETREGS, child, (caddr_t)&r, 0);
      ptrace(PT_TO_SCX, child, (caddr_t)1, 0);
      waitpid(child, &status, WUNTRACED);
      ptrace(PT_GETREGS, child, (caddr_t)&r, 0);
      r.r_eax = save_syscall;
      simulate_syscall(&r);
      ptrace(PT_SETREGS, child, (caddr_t)&r, 0);
    }
    ptrace(PT_TO_SCE, child, (caddr_t)1, 0);
  }    

User mode Linux works with a similar mechanism.

BTW, I haven't yet tested the patch below - just want some wiser, more
experienced eyes to look at it first.

--- trap.c.orig	Thu Mar 11 15:22:17 2004
+++ trap.c	Thu Mar 11 15:50:06 2004
_at__at_ -936,6 +936,7 _at__at_
 		cred_update_thread(td);
 	if (p->p_flag & P_SA)
 		thread_user_enter(p, td);
+	redo_syscall:
 	params = (caddr_t)frame.tf_esp + sizeof(int);
 	code = frame.tf_eax;
 	orig_tf_eflags = frame.tf_eflags;
_at__at_ -1003,7 +1004,17 _at__at_
 
 		STOPEVENT(p, S_SCE, narg);
 
-		PTRACESTOP_SC(p, td, S_PT_SCE);
+
+		if ((p->p_flag & P_TRACED) && (p->p_stops & S_PT_SCE)) {
+			PROC_LOCK(p);
+			ptracestop(td, SIGTRAP);
+		   	if (!((p->p_flag & P_TRACED) &&
+			      (p->p_stops & S_PT_SCE))) {
+				if ((callp->sy_narg & SYF_MPSAFE) == 0)
+					mtx_unlock(&Giant);
+				goto redo_syscall;
+			}
+		}
 
 		error = (*callp->sy_call)(td, args);
 	}



> Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
> robert_at_fledge.watson.org      Senior Research Scientist, McAfee Research
> 
> 
> > 
> > Dave.
> > 
> > -- 
> > ============================================================================
> > |David Gilbert, Independent Contractor.       | Two things can only be     |
> > |Mail:       dave_at_daveg.ca                    |  equal if and only if they |
> > |http://daveg.ca                              |   are precisely opposite.  |
> > =========================================================GLO================
> > _______________________________________________
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> > 
> 
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-- 
Antony T Curtis BSc     Unix Analyst Programmer
http://homepage.ntlworld.com/antony.t.curtis/