don't let mmap allocate down to zero

[Rik van Riel]

With some programs the 2.6 kernel can end up allocating memory
at address zero, for a non-MAP_FIXED mmap call!  This causes
problems with some programs and is generally rude to do. This
simple patch fixes the problem in my tests.

Make sure that we don't allocate memory all the way down to zero,
so the NULL pointer never gets covered up with anonymous memory
and we don't end up violating the C standard.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>

--- linux-2.6.9/mm/mmap.c.nullmmap	2005-01-25 18:00:26.000000000 -0500
+++ linux-2.6.9/mm/mmap.c	2005-01-26 08:48:03.438701673 -0500
@@ -1114,6 +1114,8 @@ void arch_unmap_area(struct vm_area_stru
 		area->vm_mm->free_area_cache = area->vm_start;
 }

+#define SHLIB_BASE             0x00111000
+
 /*
  * This mmap-allocator allocates new areas top-down from below the
  * stack's low limit (the base):
@@ -1162,6 +1164,13 @@ try_again:
 			return addr;

 		/*
+		 * Make sure we don't allocate all the way down to
+		 * zero, which would break NULL pointer detection.
+		 */
+		if (addr < SHLIB_BASE)
+			goto fail;
+
+		/*
 		 * new region fits between prev_vma->vm_end and
 		 * vma->vm_start, use it:
 		 */
@@ -1258,8 +1267,6 @@ get_unmapped_area_prot(struct file *file
 EXPORT_SYMBOL(get_unmapped_area_prot);


-#define SHLIB_BASE             0x00111000
-
 unsigned long arch_get_unmapped_exec_area(struct file *filp, unsigned long 
addr0,
 		unsigned long len0, unsigned long pgoff, unsigned long flags)
 {
-
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