Trap SIGBUS to handle truncated shared memory segments

If a client passes a section of memory via file descriptor and then
subsequently truncates that file, the underlying pages will be freed
and the addresses invalidated. Subsequent accesses to the page will
fail with a SIGBUS error.

Trap that SIGBUS, figure out which segment was causing the error and
then allocate new pages to fill in for that region. Mark the offending
shared segment as invalid and free the resource ID so that the client
will be able to tell when subsequently attempting to use the segment.

Signed-off-by: Keith Packard <keithp@keithp.com>

v2: Use MAP_FIXED to simplify the recovery logic (Mark Kettenis)
v3: Also catch errors in ShmCreateSegment

Conflicts:
	include/dix-config.h.in
	include/xorg-config.h.in

3 files changed, 52 insertions, 0 deletions

diff --git a/include/Makefile.am b/include/Makefile.am
index 5cdea1d15..13d91e22c 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -2,6 +2,7 @@ if XORG
 sdk_HEADERS =		\
 	XIstubs.h	\
 	Xprintf.h	\
+	busfault.h	\
 	callback.h	\
 	client.h	\
 	closestr.h	\
diff --git a/include/busfault.h b/include/busfault.h
new file mode 100644
index 000000000..3b668818d
--- /dev/null
+++ b/include/busfault.h
@@ -0,0 +1,48 @@
+/*
+ * Copyright © 2013 Keith Packard
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting documentation, and
+ * that the name of the copyright holders not be used in advertising or
+ * publicity pertaining to distribution of the software without specific,
+ * written prior permission.  The copyright holders make no representations
+ * about the suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
+ * OF THIS SOFTWARE.
+ */
+
+#ifndef _BUSFAULT_H_
+#define _BUSFAULT_H_
+
+#include <dix-config.h>
+
+#ifdef BUSFAULT
+
+#include <sys/types.h>
+
+typedef void (*busfault_notify_ptr) (void *context);
+
+struct busfault *
+busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void *context);
+
+void
+busfault_unregister(struct busfault *busfault);
+
+void
+busfault_check(void);
+
+Bool
+busfault_init(void);
+
+#endif
+
+#endif /* _BUSFAULT_H_ */
diff --git a/include/dix-config.h.in b/include/dix-config.h.in
index e4eea8d01..9bdbe53c1 100644
--- a/include/dix-config.h.in
+++ b/include/dix-config.h.in
@@ -455,4 +455,7 @@
 /* Use XTrans FD passing support */
 #undef XTRANS_SEND_FDS
 
+/* Wrap SIGBUS to catch MIT-SHM faults */
+#undef BUSFAULT
+
 #endif /* _DIX_CONFIG_H_ */