Wayland: Use memfd_create() and seals if available

This allows the compositor to avoid having to setup and teardown a
SIGBUS signal handler whenever it needs to read from this surface, as it
knows we won’t be able to shrink the file and so doesn’t have to protect
against that.

This codepath will only be used on Linux ≥ 3.17 with glibc ≥ 2.27, and
possibly other kernels and libc.  The former code will continue to be
used as a fallback, either if memfd_create() fails or if it isn’t
available.

CMakeLists.txt

@@ -273,7 +273,9 @@ if (_GLFW_WAYLAND)
     list(APPEND glfw_INCLUDE_DIRS "${XKBCOMMON_INCLUDE_DIRS}")
 
     include(CheckIncludeFiles)
+    include(CheckFunctionExists)
     check_include_files(xkbcommon/xkbcommon-compose.h HAVE_XKBCOMMON_COMPOSE_H)
+    check_function_exists(memfd_create HAVE_MEMFD_CREATE)
 
     if (NOT ("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux"))
         find_package(EpollShim)

src/glfw_config.h.in

@@ -57,4 +57,6 @@
 
 // Define this to 1 if xkbcommon supports the compose key
 #cmakedefine HAVE_XKBCOMMON_COMPOSE_H
+// Define this to 1 if the libc supports memfd_create()
+#cmakedefine HAVE_MEMFD_CREATE
 

src/wl_window.c

@@ -144,23 +144,37 @@ static int createAnonymousFile(off_t size)
     int fd;
     int ret;
 
-    path = getenv("XDG_RUNTIME_DIR");
-    if (!path)
+#ifdef HAVE_MEMFD_CREATE
+    fd = memfd_create("glfw-shared", MFD_CLOEXEC | MFD_ALLOW_SEALING);
+    if (fd >= 0)
     {
-        errno = ENOENT;
-        return -1;
+        // We can add this seal before calling posix_fallocate(), as the file
+        // is currently zero-sized anyway.
+        //
+        // There is also no need to check for the return value, we couldn’t do
+        // anything with it anyway.
+        fcntl(fd, F_ADD_SEALS, F_SEAL_SHRINK | F_SEAL_SEAL);
+    }
+    else
+#endif
+    {
+        path = getenv("XDG_RUNTIME_DIR");
+        if (!path)
+        {
+            errno = ENOENT;
+            return -1;
+        }
+
+        name = calloc(strlen(path) + sizeof(template), 1);
+        strcpy(name, path);
+        strcat(name, template);
+
+        fd = createTmpfileCloexec(name);
+        free(name);
+        if (fd < 0)
+            return -1;
     }
 
-    name = calloc(strlen(path) + sizeof(template), 1);
-    strcpy(name, path);
-    strcat(name, template);
-
-    fd = createTmpfileCloexec(name);
-
-    free(name);
-
-    if (fd < 0)
-        return -1;
     ret = posix_fallocate(fd, 0, size);
     if (ret != 0)
     {