util: add fallback for qemu_memfd_alloc()

Add an open/unlink/mmap fallback for system that do not support
memfd (only available since 3.17, ~1y ago).

This patch may require additional SELinux policies to work for enforced
systems, but should fail gracefully in this case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Thibaut Collet <thibaut.collet@6wind.com>

util/memfd.c

@@ -97,10 +97,26 @@ void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
             return NULL;
         }
     } else {
-        perror("memfd");
+        const char *tmpdir = g_get_tmp_dir();
+        gchar *fname;
+
+        fname = g_strdup_printf("%s/memfd-XXXXXX", tmpdir);
+        mfd = mkstemp(fname);
+        unlink(fname);
+        g_free(fname);
+
+        if (mfd == -1) {
+            perror("mkstemp");
             return NULL;
         }
 
+        if (ftruncate(mfd, size) == -1) {
+            perror("ftruncate");
+            close(mfd);
+            return NULL;
+        }
+    }
+
     ptr = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, mfd, 0);
     if (ptr == MAP_FAILED) {
         perror("mmap");