s390/mmap: take stack_guard_gap into account for mmap_base

The s390 version of the mmap_base function is ignorant of stack_guard_gap
which can lead to a placement of the stack vs. the mmap base that does not
leave enough space for the stack rlimit.

Add the stack_guard_gap to the calculation and while we are at it the
check for gap+pad overflows as well.
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/mmap.c

@@ -29,14 +29,6 @@ static unsigned long stack_maxrandom_size(void)
 	return STACK_RND_MASK << PAGE_SHIFT;
 }
 
-/*
- * Top of mmap area (just below the process stack).
- *
- * Leave at least a ~32 MB hole.
- */
-#define MIN_GAP (32*1024*1024)
-#define MAX_GAP (STACK_TOP/6*5)
-
 static inline int mmap_is_legacy(struct rlimit *rlim_stack)
 {
 	if (current->personality & ADDR_COMPAT_LAYOUT)
@@ -60,13 +52,26 @@ static inline unsigned long mmap_base(unsigned long rnd,
 				      struct rlimit *rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
+	unsigned long pad = stack_maxrandom_size() + stack_guard_gap;
+	unsigned long gap_min, gap_max;
+
+	/* Values close to RLIM_INFINITY can overflow. */
+	if (gap + pad > gap)
+		gap += pad;
+
+	/*
+	 * Top of mmap area (just below the process stack).
+	 * Leave at least a ~32 MB hole.
+	 */
+	gap_min = 32 * 1024 * 1024UL;
+	gap_max = (STACK_TOP / 6) * 5;
+
+	if (gap < gap_min)
+		gap = gap_min;
+	else if (gap > gap_max)
+		gap = gap_max;
 
-	if (gap < MIN_GAP)
-		gap = MIN_GAP;
-	else if (gap > MAX_GAP)
-		gap = MAX_GAP;
-	gap &= PAGE_MASK;
-	return STACK_TOP - stack_maxrandom_size() - rnd - gap;
+	return PAGE_ALIGN(STACK_TOP - gap - rnd);
 }
 
 unsigned long