kern_exec: check p_tracecred instead of p_cred

[Andriy Gapon]

Currently even if root ktraces an unprivileged process that execs a sguid
executable, then tracing aborted at that point.  I think that that happens
because wrong credentials are checked at that point.

What do you think about the following patch?

commit 956a80783bc39162b1d64383188c5037f9767413
Author: Andriy Gapon <avg at icyb.net.ua>
Date:   Sat Sep 22 18:17:46 2012 +0300

    kern_exec: check p_tracecred instead of p_cred

    ... when deciding whether to continue tracing across suid/sgid exec

diff --git a/sys/kern/kern_exec.c b/sys/kern/kern_exec.c
index 90f7311..8d62c1e 100644
--- a/sys/kern/kern_exec.c
+++ b/sys/kern/kern_exec.c
@@ -694,7 +694,8 @@ interpret:
 		setsugid(p);

 #ifdef KTRACE
-		if (priv_check_cred(oldcred, PRIV_DEBUG_DIFFCRED, 0))
+		if (p->p_tracecred != NULL &&
+		    priv_check_cred(p->p_tracecred, PRIV_DEBUG_DIFFCRED, 0))
 			ktrprocexec(p, &tracecred, &tracevp);
 #endif
 		/*

-- 
Andriy Gapon