[src/trunk]: src/sys/kern Add a paranoid racy lock check in child_return()

[kamil <kamil%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/a02b1e9bd8e1
branches:  trunk
changeset: 455610:a02b1e9bd8e1
user:      kamil <kamil%NetBSD.org@localhost>
date:      Sun Apr 07 14:50:41 2019 +0000

description:
Add a paranoid racy lock check in child_return()

In theory a child could be detached for some reason or another during
the time window between checking for PSL_TRACED and acquiring proc_lock.

Acquire the proc_lock mutex and recheck for PSL_TRACED before emitting
SIGTRAP. sigswitch() must acquite it internally anyway so this does not
have a negative impact and adds an extra sanity check.

For !PSL_TRACED case there is no impact.

diffstat:

 sys/kern/kern_fork.c |  14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diffs (44 lines):

diff -r cf618f15b7e4 -r a02b1e9bd8e1 sys/kern/kern_fork.c
--- a/sys/kern/kern_fork.c      Sun Apr 07 14:44:51 2019 +0000
+++ b/sys/kern/kern_fork.c      Sun Apr 07 14:50:41 2019 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_fork.c,v 1.208 2019/04/06 11:54:21 kamil Exp $    */
+/*     $NetBSD: kern_fork.c,v 1.209 2019/04/07 14:50:41 kamil Exp $    */
 
 /*-
  * Copyright (c) 1999, 2001, 2004, 2006, 2007, 2008 The NetBSD Foundation, Inc.
@@ -67,7 +67,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_fork.c,v 1.208 2019/04/06 11:54:21 kamil Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_fork.c,v 1.209 2019/04/07 14:50:41 kamil Exp $");
 
 #include "opt_ktrace.h"
 #include "opt_dtrace.h"
@@ -619,16 +619,24 @@
        struct proc *p = l->l_proc;
 
        if (p->p_slflag & PSL_TRACED) {
+               /* Paranoid check */
+               mutex_enter(proc_lock);
+               if (!(p->p_slflag & PSL_TRACED)) {
+                       mutex_exit(proc_lock);
+                       goto my_tracer_is_gone;
+               }
+
                mutex_enter(p->p_lock);
                p->p_xsig = SIGTRAP;
                p->p_sigctx.ps_faked = true; // XXX
                p->p_sigctx.ps_info._signo = p->p_xsig;
                p->p_sigctx.ps_info._code = TRAP_CHLD;
-               sigswitch(0, SIGTRAP, true);
+               sigswitch(0, SIGTRAP, false);
                // XXX ktrpoint(KTR_PSIG)
                mutex_exit(p->p_lock);
        }
 
+my_tracer_is_gone:
        md_child_return(l);
 
        /*