Date: Mon, 30 Jun 2003 15:00:57 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: freebsd-stable@freebsd.org Subject: sysctl_out_proc() race in stable w/ patch Message-ID: <200306302200.h5UM0vg2025972@apollo.backplane.com>
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There is a rather serious race with copyout() and process termination
in -stable. sysctl_kern_proc() loops through the allproc list writing
the results to user memory. If it stalls during the copyout (e.g. the
user memory has to take a vm_fault) and the process is ripped out from
under it it will go looping into never never land.
The solution is very simple, simply PHOLD()/PRELE() the process during
the copyout() and in exit1() wait for the lock count to go to 0.
The patch below is a hack and cannot be directly applied due to other
changes in my tree, but if someone would like to tackle this for
4.x I've provided you with a good start.
The problem doesn't appear to be an issue in -current with the PROC
lock held, though I am unsure if lock reentrancy is possible when
a vm_fault is taken.
-Matt
Index: kern/kern_exit.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_exit.c,v
retrieving revision 1.12
diff -u -r1.12 kern_exit.c
--- kern/kern_exit.c 30 Jun 2003 19:50:31 -0000 1.12
+++ kern/kern_exit.c 30 Jun 2003 21:44:32 -0000
@@ -451,7 +451,19 @@
...
- KASSERT(p->p_lock == 0, ("p_lock not 0! %p", p));
+
+ /*
+ * Other kernel threads may be in the middle of
+ * accessing the proc. For example, kern/kern_proc.c
+ * could be blocked writing proc data to a sysctl.
+ * At the moment, if this occurs, we are not woken
+ * up and rely on a one-second retry.
+ */
+ if (p->p_lock) {
+ printf("Diagnostic: waiting for p_lock\n");
+ while (p->p_lock)
+ tsleep(p, PWAIT, "reap2", hz);
+ }
/* charge childs scheduling cpu usage to parent */
if (curproc->p_pid != 1) {
Index: kern/kern_proc.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_proc.c,v
retrieving revision 1.7
diff -u -r1.7 kern_proc.c
--- kern/kern_proc.c 28 Jun 2003 02:36:43 -0000 1.7
+++ kern/kern_proc.c 30 Jun 2003 21:46:25 -0000
@@ -529,8 +529,9 @@
if (!PRISON_CHECK(cr1, p->p_ucred))
continue;
-
+ PHOLD(p);
error = sysctl_out_proc(p, req, doingzomb);
+ PRELE(p);
if (error)
return (error);
}
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