Weird truss output

[Dan Nelson]

In the last episode (Dec 03), Vlad GALU said:
> I'm running a statically linked binary, which I've built inside a
> jail. The jail's libc & co are in sync with the host's. Truss then
> shows this:
> 
> -- cut here --
> -- UNKNOWN SYSCALL 1048532 --
> -- UNKNOWN SYSCALL 1048532 --

Is this a threaded app that you attached truss to after it was started?
The method that truss uses to catch syscall enter/exit events doesn't
indicate whether the event is an enter or an exit, so if you attach
while a syscall is active, truss handles the exit event as if it were a
syscall entry event, and never gets back in synch.  It gets worse with
threaded apps because each thread is another chance to get out of
synch.  Try this patch:

Index: i386-fbsd.c
===================================================================
RCS file: /home/ncvs/src/usr.bin/truss/i386-fbsd.c,v
retrieving revision 1.29
diff -u -p -r1.29 i386-fbsd.c
--- i386-fbsd.c	28 Jul 2007 23:15:04 -0000	1.29
+++ i386-fbsd.c	3 Dec 2008 15:20:09 -0000
@@ -149,7 +149,14 @@ i386_syscall_entry(struct trussinfo *tru
   fsc.name =
     (syscall_num < 0 || syscall_num > nsyscalls) ? NULL : syscallnames[syscall_num];
   if (!fsc.name) {
-    fprintf(trussinfo->outfile, "-- UNKNOWN SYSCALL %d --\n", syscall_num);
+    fprintf(trussinfo->outfile, "-- UNKNOWN SYSCALL %u (0x%08x) --\n", syscall_num, syscall_num);
+    if ((unsigned int)syscall_num > 0x1000) {
+      /* When attaching to a running process, we have a 50-50 chance
+         of attaching to a process waiting in a syscall, which means
+         our first trap is an exit instead of an entry and we're out
+         of synch. Reset our flag */
+      trussinfo->curthread->in_syscall = 0;
+    }
   }
 
   if (fsc.name && (trussinfo->flags & FOLLOWFORKS)


-- 
	Dan Nelson
	dnelson at allantgroup.com