From: Cristiano Giuffrida <giuffrida@cs.vu.nl>
Date: Fri, 2 Jan 2015 15:51:29 +0000 (+0100)
Subject: libmthread: Fix guard page mapping.
X-Git-Url: http://zhaoyanbai.com/repos/%22http:/www.isc.org/icons/zpipe.c?a=commitdiff_plain;h=75206e2f3ee37c1d2c7c801cb8e11d207d81190a;p=minix.git

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 776525414..9d2c45840 100644
--- 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");
 		}
 	}