qemu_fw_cfg: Make fw_cfg_rev_attr a proper kobj_attribute

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: 75f3e8e47f38 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Link: https://github.com/ClangBuiltLinux/linux/issues/1299
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210211194258.4137998-1-nathan@kernel.org