[PATCH 4/5] Provide facility to catch unhandled page faults in user mode.

Add a "fault_callback" to the thread structure which is called when a unhandled
page fault happens in user space. A SIGSEGV will only be sent if the callback
returns "true".
---
 headers/private/kernel/thread_types.h |    8 +++++++-
 src/system/kernel/vm/vm.cpp           |    8 ++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/headers/private/kernel/thread_types.h b/headers/private/kernel/thread_types.h
index 2088f8c..61227c3 100644
--- a/headers/private/kernel/thread_types.h
+++ b/headers/private/kernel/thread_types.h
@@ -224,6 +224,9 @@ struct team {
 
 typedef int32 (*thread_entry_func)(thread_func, void *);
 
+typedef bool (*page_fault_callback)(addr_t address, addr_t faultAddress,
+	bool isWrite);
+
 struct thread {
 	int32			flags;			// summary of events relevant in interrupt
 									// handlers (signals pending, user debugging
@@ -275,7 +278,10 @@ struct thread {
 		cbuf		*buffer;
 	} msg;
 
-	addr_t			fault_handler;
+	union {
+		addr_t              fault_handler;
+		page_fault_callback fault_callback;
+	};
 	int32			page_faults_allowed;
 		/* this field may only stay in debug builds in the future */
 
diff --git a/src/system/kernel/vm/vm.cpp b/src/system/kernel/vm/vm.cpp
index b6e8a5f..73df1d7 100644
--- a/src/system/kernel/vm/vm.cpp
+++ b/src/system/kernel/vm/vm.cpp
@@ -3864,8 +3864,12 @@ vm_page_fault(addr_t address, addr_t faultAddress, bool isWrite, bool isUser,
 			release_sem_etc(addressSpace->sem, READ_COUNT, 0);
 			vm_put_address_space(addressSpace);
 #endif
-			if (user_debug_exception_occurred(B_SEGMENT_VIOLATION, SIGSEGV))
-				send_signal(team_get_current_team_id(), SIGSEGV);
+			struct thread *thread = thread_get_current_thread();
+			if (thread->fault_callback == NULL || thread->fault_callback(
+				address, faultAddress, isWrite)) {
+				if (user_debug_exception_occurred(B_SEGMENT_VIOLATION, SIGSEGV))
+					send_signal(team_get_current_team_id(), SIGSEGV);
+			}
 		}
 	}
 
-- 
1.5.4.2


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