Fix ptrace and fork interaction

mm/memory.c

@@ -213,7 +213,7 @@ int copy_page_range(struct mm_struct *dst, struct mm_struct *src,
 	pgd_t * src_pgd, * dst_pgd;
 	unsigned long address = vma->vm_start;
 	unsigned long end = vma->vm_end;
-	unsigned long cow = (vma->vm_flags & (VM_SHARED | VM_WRITE)) == VM_WRITE;
+	unsigned long cow = (vma->vm_flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
 
 	src_pgd = pgd_offset(src, address)-1;
 	dst_pgd = pgd_offset(dst, address)-1;
@@ -280,7 +280,7 @@ skip_copy_pte_range:		address = (address + PMD_SIZE) & PMD_MASK;
 					goto cont_copy_pte_range;
 
 				/* If it's a COW mapping, write protect it both in the parent and the child */
-				if (cow) {
+				if (cow && pte_write(pte)) {
 					ptep_set_wrprotect(src_pte);
 					pte = *src_pte;
 				}
@@ -492,20 +492,25 @@ static inline struct page * get_page_map(struct page *page)
 int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, unsigned long start,
 		int len, int write, int force, struct page **pages, struct vm_area_struct **vmas)
 {
-	int i = 0;
+	int i;
+	unsigned int flags;
+
+	/* 
+	 * Require read or write permissions.
+	 * If 'force' is set, we only require the "MAY" flags.
+	 */
+	flags = write ? (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
+	flags &= force ? (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
+	i = 0;
+	
 
 	do {
 		struct vm_area_struct *	vma;
 
 		vma = find_extend_vma(mm, start);
 
-		if ( !vma ||
-		    (!force &&
-		     	((write && (!(vma->vm_flags & VM_WRITE))) ||
-		    	 (!write && (!(vma->vm_flags & VM_READ))) ) )) {
-			if (i) return i;
-			return -EFAULT;
-		}
+		if ( !vma || !(flags & vma->vm_flags) )
+			return i ? : -EFAULT;
 
 		spin_lock(&mm->page_table_lock);
 		do {