[src/trunk]: src/sys/dev/pad pad(4): Explain what's wrong with using device p...

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

details:   https://anonhg.NetBSD.org/src/rev/d910f8935247
branches:  trunk
changeset: 1021721:d910f8935247
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Mon Jun 14 18:44:53 2021 +0000

description:
pad(4): Explain what's wrong with using device pointers like this.

...and why the kernel lock is not enough.

diffstat:

 sys/dev/pad/pad.c |  34 ++++++++++++++++++++++++++++++++--
 1 files changed, 32 insertions(+), 2 deletions(-)

diffs (55 lines):

diff -r 39123708e787 -r d910f8935247 sys/dev/pad/pad.c
--- a/sys/dev/pad/pad.c Mon Jun 14 18:44:45 2021 +0000
+++ b/sys/dev/pad/pad.c Mon Jun 14 18:44:53 2021 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: pad.c,v 1.74 2021/06/14 18:44:45 riastradh Exp $ */
+/* $NetBSD: pad.c,v 1.75 2021/06/14 18:44:53 riastradh Exp $ */
 
 /*-
  * Copyright (c) 2007 Jared D. McNeill <jmcneill%invisible.ca@localhost>
@@ -27,7 +27,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: pad.c,v 1.74 2021/06/14 18:44:45 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: pad.c,v 1.75 2021/06/14 18:44:53 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/types.h>
@@ -386,6 +386,36 @@
        device_t self = sc->sc_dev;
        cfdata_t cf = device_cfdata(self);
 
+       /*
+        * XXX This is not quite enough to prevent racing with drvctl
+        * detach.  What can happen:
+        *
+        *      cpu0                            cpu1
+        *
+        *      pad_close
+        *      take kernel lock
+        *      sc->sc_open = 0
+        *      drop kernel lock
+        *      wait for config_misc_lock
+        *                                      drvctl detach
+        *                                      take kernel lock
+        *                                      drop kernel lock
+        *                                      wait for config_misc_lock
+        *                                      retake kernel lock
+        *                                      drop config_misc_lock
+        *      take config_misc_lock
+        *      wait for kernel lock
+        *                                      pad_detach (sc_open=0 already)
+        *                                      free device
+        *                                      drop kernel lock
+        *      use device after free
+        *
+        * We need a way to grab a reference to the device so it won't
+        * be freed until we're done -- it's OK if we config_detach
+        * twice as long as it's idempotent, but not OK if the first
+        * config_detach frees the struct device before the second one
+        * has finished handling it.
+        */
        KERNEL_LOCK(1, NULL);
        KASSERT(sc->sc_open);
        sc->sc_open = 0;