libmthread: Fix guard page mapping.

Edited by David van Moolenbroek to deallocate the guard page as well.
Note that while the new approach is better in theory (previously, the
hole could end up being filled by another allocated page), guard page
protection is now broken in practice, because VM does not support
setting specific page permissions (in this case, PROT_NONE).

Change-Id: I882624f5d152d3ebe82fca649cbad85aa4931780

diff --git a/minix/lib/libmthread/allocate.c b/minix/lib/libmthread/allocate.c
index 776525414165776b8ec73cc450db616355260b47..9d2c458408c1becf2e57b70be573c31e20caa6fc 100644 (file)
--- a/minix/lib/libmthread/allocate.c
+++ b/minix/lib/libmthread/allocate.c
@@ -422,8 +422,10 @@ void *arg;
 # error "Unsupported platform"
 #endif
        stacksize = guarded_stacksize;
-       if (munmap(guard_start, MTHREAD_GUARDSIZE) != 0)
-               mthread_panic("unable to unmap stack space for guard");
+       /* Mere unmapping could allow a later allocation to fill the gap. */
+        if (mmap(guard_start, MTHREAD_GUARDSIZE, PROT_NONE,
+               MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0) != guard_start)
+               mthread_panic("unable to overmap stack space for guard");
        tcb->m_context.uc_stack.ss_sp = guard_end;
   } else
        tcb->m_context.uc_stack.ss_sp = stackaddr;
@@ -444,6 +446,9 @@ mthread_thread_t thread;
 /* Reset the thread to its default values. Free the allocated stack space. */
 
   mthread_tcb_t *rt;
+  size_t stacksize;
+  char *stackaddr;
+
   if (!isokthreadid(thread)) mthread_panic("Invalid thread id"); 
 
   rt = mthread_find_tcb(thread);
@@ -456,8 +461,15 @@ mthread_thread_t thread;
   rt->m_cond = NULL;
   if (rt->m_attr.ma_stackaddr == NULL) { /* We allocated stack space */
        if (rt->m_context.uc_stack.ss_sp) {
-               if (munmap(rt->m_context.uc_stack.ss_sp,
-                                rt->m_context.uc_stack.ss_size) != 0) {
+               stacksize = rt->m_context.uc_stack.ss_size;
+               stackaddr = rt->m_context.uc_stack.ss_sp;
+#if defined(__i386__) || defined(__arm__)
+               stacksize += MTHREAD_GUARDSIZE;
+               stackaddr -= MTHREAD_GUARDSIZE;
+#else
+# error "Unsupported platform"
+#endif
+               if (munmap(stackaddr, stacksize) != 0) {
                        mthread_panic("unable to unmap memory");
                }
        }