Re: [PATCH 1/2] test: add minimal module for verification testing

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  3 Dec 2013 13:27:33 -0800 Kees Cook <keescook@chromium.org> wrote:

> When doing module loading verification tests (for example, with module
> singing, or LSM hooks), it is very handy to have a module that can be
> built on all systems under test, isn't auto-loaded at boot, and has
> no device or similar dependencies. This creates the "test_module.ko"
> module for that purpose, which only reports its load and unload to printk.
> 
> --- /dev/null
> +++ b/kernel/test_module.c
> @@ -0,0 +1,26 @@
> +/*
> + * "hello world" kernel module
> + */
> +
> +#define pr_fmt(fmt) "test_module: " fmt
> +
> +#include <linux/module.h>

It really should have a whole bunch more includes, rather than relying
on winning the nested include lottery.

> +static int __init test_module_init(void)
> +{
> +	pr_info("Hello, world\n");

pr_info is maybe too low a facility level?  Pretty much the whole point
of the module is to print this message, so let's ensure that it is visible.

> +	return 0;
> +}
> +
> +module_init(test_module_init);
> +
> +static void __exit test_module_exit(void)
> +{
> +	pr_info("Goodbye\n");
> +}
> +
> +module_exit(test_module_exit);
> +
> +MODULE_AUTHOR("Kees Cook <keescook@chromium.org>");
> +MODULE_LICENSE("GPL");
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index db25707aa41b..20abc92032e0 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1578,6 +1578,20 @@ config DMA_API_DEBUG
>  	  This option causes a performance degredation.  Use only if you want
>  	  to debug device drivers. If unsure, say N.
>  
> +config TEST_MODULE
> +	tristate "Test module loading with 'hello world' module"
> +	default n
> +	depends on m
> +	help
> +	  This builds the "test_module" module that emits "Hello, world"
> +	  on printk when loaded. It is designed to be used for basic
> +	  evaluation of the module loading subsystem (for example when
> +	  validating module verification). It lacks any extra dependencies,
> +	  and will not normally be loaded by the system unless explicitly
> +	  request by name.

Would be useful to have a comment along these lines in test_module.c,
so readers know why it exists.