[Qemu-devel] [PATCH] util/mmap-alloc: add comments, assertions

["Michael S. Tsirkin" <mst@redhat.com>]

Document RAM guard page logic within mmap-alloc.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Paolo, can you pls confirm this is what you had in mind?

 util/mmap-alloc.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 05c8b4b..d978399 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -12,9 +12,14 @@
 #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)
 {
+    /*
+     * 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;
@@ -24,6 +29,11 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align)
         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 | MAP_ANONYMOUS | MAP_PRIVATE, fd, 0);
     if (ptr1 == MAP_FAILED) {
@@ -37,6 +47,11 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align)
     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());
     }
@@ -47,6 +62,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align)
 void qemu_ram_munmap(void *ptr, size_t size)
 {
     if (ptr) {
+        /* Unmap both the RAM block and the guard page */
         munmap(ptr, size + getpagesize());
     }
 }
-- 
MST