exec: factor out duplicate mmap code

Anonymous and file-backed RAM allocation are now almost exactly the same.

Reduce code duplication by moving RAM mmap code out of oslib-posix.c and
exec.c.

Reported-by: Marc-André Lureau <mlureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Tested-by: Thibaut Collet <thibaut.collet@6wind.com>

util/mmap-alloc.c

@@ -0,0 +1,71 @@
+/*
+ * Support for RAM backed by mmaped host memory.
+ *
+ * Copyright (c) 2015 Red Hat, Inc.
+ *
+ * Authors:
+ *  Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+#include <qemu/mmap-alloc.h>
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <assert.h>
+
+void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
+{
+    /*
+     * Note: this always allocates at least one extra page of virtual address
+     * space, even if size is already aligned.
+     */
+    size_t total = size + align;
+    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
+    void *ptr1;
+
+    if (ptr == MAP_FAILED) {
+        return NULL;
+    }
+
+    /* Make sure align is a power of 2 */
+    assert(!(align & (align - 1)));
+    /* Always align to host page size */
+    assert(align >= getpagesize());
+
+    ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
+                MAP_FIXED |
+                (fd == -1 ? MAP_ANONYMOUS : 0) |
+                (shared ? MAP_SHARED : MAP_PRIVATE),
+                fd, 0);
+    if (ptr1 == MAP_FAILED) {
+        munmap(ptr, total);
+        return NULL;
+    }
+
+    ptr += offset;
+    total -= offset;
+
+    if (offset > 0) {
+        munmap(ptr - offset, offset);
+    }
+
+    /*
+     * Leave a single PROT_NONE page allocated after the RAM block, to serve as
+     * a guard page guarding against potential buffer overflows.
+     */
+    if (total > size + getpagesize()) {
+        munmap(ptr + size + getpagesize(), total - size - getpagesize());
+    }
+
+    return ptr;
+}
+
+void qemu_ram_munmap(void *ptr, size_t size)
+{
+    if (ptr) {
+        /* Unmap both the RAM block and the guard page */
+        munmap(ptr, size + getpagesize());
+    }
+}