Add user-space stub for task entry.

This will prevent kernel space from needing to dereference user-space
addresses for starting a task, which is safer and is easier for VMM.

Change-Id: Icad3b832550cedbf291ed8b032840f4049fba18e
Reviewed-on: http://gfw160.austin.ibm.com:8080/gerrit/202
Tested-by: Jenkins Server
Reviewed-by: Douglas R. Gilbert <dgilbert@us.ibm.com>
Reviewed-by: MATTHEW S. BARTH <msbarth@us.ibm.com>

2 files changed, 30 insertions, 6 deletions

diff --git a/src/kernel/start.S b/src/kernel/start.S
index ac57926b4..381e33bde 100644
--- a/src/kernel/start.S
+++ b/src/kernel/start.S
@@ -360,6 +360,29 @@ system_call_fast_path:
 1:
     rfid                        ;// Return from interrupt.
 
+
+    ;// @fn userspace_task_entry
+    ;// Stub to load the function address and TOC base from userspace and
+    ;// jump to task entry point.  Used so the kernel doesn't need to
+    ;// dereference userspace addresses (which could be bad).
+    ;//
+    ;// Requires: 
+    ;//     * GPR4 -> Function pointer.
+    ;//     * LR -> task_end stub.
+    ;//     * GPR3 -> Task argument.
+    ;//     * GPR1 -> Task stack pointer.
+    ;// Results:
+    ;//     * TOC base -> GPR2
+    ;//     * Function Address -> CTR
+    ;//     * GPR1, GPR3 preserved.
+    ;//     * Branch to CTR (no link).
+.global userspace_task_entry
+userspace_task_entry:
+    ld r5, 0(r4)
+    mtctr r5
+    ld r2, 8(r4)
+    bctr
+
 .section .data
     .balign 1024
 kernel_stack:
diff --git a/src/kernel/taskmgr.C b/src/kernel/taskmgr.C
index 0573bab7d..8073c0b48 100644
--- a/src/kernel/taskmgr.C
+++ b/src/kernel/taskmgr.C
@@ -8,6 +8,8 @@
 #include <string.h>
 #include <limits.h>
 
+extern "C" void userspace_task_entry();
+
 void TaskManager::idleTaskLoop(void* unused)
 {
     while(1)
@@ -55,12 +57,11 @@ task_t* TaskManager::_createTask(TaskManager::task_fn_t t,
     memset(task, '\0', sizeof(task_t));
 
     task->tid = this->getNextTid();
-
-    // Function pointer 't' is actually a TOC entry.  
-    //     TOC[0] = function address
-    //     TOC[1] = TOC base -> r2
-    task->context.nip = (void*) ((uint64_t*) t)[0];
-    task->context.gprs[2] = ((uint64_t*)t)[1];
+    
+    // Set NIP to be userspace_task_entry stub and GPR3 to be the 
+    // function pointer for the desired task entry point.
+    task->context.nip = reinterpret_cast<void*>(&userspace_task_entry);
+    task->context.gprs[4] = reinterpret_cast<uint64_t>(t);
 
     // Set up LR to be the entry point for task_end in case a task 
     // 'returns' from its entry point.  By the Power ABI, the entry