Subject: Re: [PATCH v6 16/25] init: lto: fix PREL32 relocations

From: Kees Cook <[email-masked]>
Date: Wed, 14 Oct 2020 15:53:59 -0700

On Mon, Oct 12, 2020 at 05:31:54PM -0700, Sami Tolvanen wrote:
> With LTO, the compiler can rename static functions to avoid global
> naming collisions. As initcall functions are typically static,
> renaming can break references to them in inline assembly. This
> change adds a global stub with a stable name for each initcall to
> fix the issue when PREL32 relocations are used.
> 
> Signed-off-by: Sami Tolvanen <[email-masked]>
> Reviewed-by: Kees Cook <[email-masked]>

This is another independent improvement... this could land before the
other portions of the series.

-Kees

> ---
>  include/linux/init.h | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/init.h b/include/linux/init.h
> index af638cd6dd52..cea63f7e7705 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -209,26 +209,49 @@ extern bool initcall_debug;
>   */
>  #define __initcall_section(__sec, __iid)			\
>  	#__sec ".init.." #__iid
> +
> +/*
> + * With LTO, the compiler can rename static functions to avoid
> + * global naming collisions. We use a global stub function for
> + * initcalls to create a stable symbol name whose address can be
> + * taken in inline assembly when PREL32 relocations are used.
> + */
> +#define __initcall_stub(fn, __iid, id)				\
> +	__initcall_name(initstub, __iid, id)
> +
> +#define __define_initcall_stub(__stub, fn)			\
> +	int __init __stub(void);				\
> +	int __init __stub(void)					\
> +	{ 							\
> +		return fn();					\
> +	}							\
> +	__ADDRESSABLE(__stub)
>  #else
>  #define __initcall_section(__sec, __iid)			\
>  	#__sec ".init"
> +
> +#define __initcall_stub(fn, __iid, id)	fn
> +
> +#define __define_initcall_stub(__stub, fn)			\
> +	__ADDRESSABLE(fn)
>  #endif
>  
>  #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> -#define ____define_initcall(fn, __name, __sec)			\
> -	__ADDRESSABLE(fn)					\
> +#define ____define_initcall(fn, __stub, __name, __sec)		\
> +	__define_initcall_stub(__stub, fn)			\
>  	asm(".section	\"" __sec "\", \"a\"		\n"	\
>  	    __stringify(__name) ":			\n"	\
> -	    ".long	" #fn " - .			\n"	\
> +	    ".long	" __stringify(__stub) " - .	\n"	\
>  	    ".previous					\n");
>  #else
> -#define ____define_initcall(fn, __name, __sec)			\
> +#define ____define_initcall(fn, __unused, __name, __sec)	\
>  	static initcall_t __name __used 			\
>  		__attribute__((__section__(__sec))) = fn;
>  #endif
>  
>  #define __unique_initcall(fn, id, __sec, __iid)			\
>  	____define_initcall(fn,					\
> +		__initcall_stub(fn, __iid, id),			\
>  		__initcall_name(initcall, __iid, id),		\
>  		__initcall_section(__sec, __iid))
>  
> -- 
> 2.28.0.1011.ga647a8990f-goog
> 

-- 
Kees Cook

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