Re: [PATCH v11 18/19] module: fix types of device tables aliases

[Rusty Russell]

Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
> MODULE_DEVICE_TABLE() macro used to create aliases to device tables.
> Normally alias should have the same type as aliased symbol.
>
> Device tables are arrays, so they have 'struct type##_device_id[x]'
> types. Alias created by MODULE_DEVICE_TABLE() will have non-array type -
> 	'struct type##_device_id'.
>
> This inconsistency confuses compiler, it could make a wrong
> assumption about variable's size which leads KASan to
> produce a false positive report about out of bounds access.

Hmm, as Andrew Morton points out, this breaks some usage; if we just
fix the type (struct type##_device_id[]) will that work instead?

I'm guessing not, since typeof(x) will presumably preserve sizing
information?

Cheers,
Rusty.

>
> For every global variable compiler calls __asan_register_globals()
> passing information about global variable (address, size, size with
> redzone, name ...) __asan_register_globals() poison symbols
> redzone to detect possible out of bounds accesses.
>
> When symbol has an alias __asan_register_globals() will be called
> as for symbol so for alias. Compiler determines size of variable by
> size of variable's type. Alias and symbol have the same address,
> so if alias have the wrong size part of memory that actually belongs
> to the symbol could be poisoned as redzone of alias symbol.
>
> By fixing type of alias symbol we will fix size of it, so
> __asan_register_globals() will not poison valid memory.
>
> Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
> ---
>  include/linux/module.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index b653d7c..42999fe 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -135,7 +135,7 @@ void trim_init_extable(struct module *m);
>  #ifdef MODULE
>  /* Creates an alias so file2alias.c can find device table. */
>  #define MODULE_DEVICE_TABLE(type, name)					\
> -  extern const struct type##_device_id __mod_##type##__##name##_device_table \
> +extern const typeof(name) __mod_##type##__##name##_device_table		\
>    __attribute__ ((unused, alias(__stringify(name))))
>  #else  /* !MODULE */
>  #define MODULE_DEVICE_TABLE(type, name)
> -- 
> 2.2.2
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