qemu_fw_cfg: Make fw_cfg_rev_attr a proper kobj_attribute

fw_cfg_showrev() is called by an indirect call in kobj_attr_show(),
which violates clang's CFI checking because fw_cfg_showrev()'s second
parameter is 'struct attribute', whereas the ->show() member of 'struct
kobj_structure' expects the second parameter to be of type 'struct
kobj_attribute'.

$ cat /sys/firmware/qemu_fw_cfg/rev
3

$ dmesg | grep "CFI failure"
[   26.016832] CFI failure (target: fw_cfg_showrev+0x0/0x8):

Fix this by converting fw_cfg_rev_attr to 'struct kobj_attribute' where
this would have been caught automatically by the incompatible pointer
types compiler warning. Update fw_cfg_showrev() accordingly.

Fixes: 75f3e8e ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Link: ClangBuiltLinux#1299
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Sami Tolvanen <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Reviewed-by: Sami Tolvanen <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

Author: nathanchance

drivers/firmware/qemu_fw_cfg.c

@@ -299,15 +299,13 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static ssize_t fw_cfg_showrev(struct kobject *k, struct attribute *a, char *buf)
+static ssize_t fw_cfg_showrev(struct kobject *k, struct kobj_attribute *a,
+			      char *buf)
 {
 	return sprintf(buf, "%u\n", fw_cfg_rev);
 }
 
-static const struct {
-	struct attribute attr;
-	ssize_t (*show)(struct kobject *k, struct attribute *a, char *buf);
-} fw_cfg_rev_attr = {
+static const struct kobj_attribute fw_cfg_rev_attr = {
 	.attr = { .name = "rev", .mode = S_IRUSR },
 	.show = fw_cfg_showrev,
 };