Re: [PATCH 4/8] kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS

From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: linux-kbuild@vger.kernel.org, Sam Ravnborg <sam@ravnborg.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	linux-kernel@vger.kernel.org,
	Michal Marek <michal.lkml@markovi.net>,
	Will Deacon <will.deacon@arm.com>, Ingo Molnar <mingo@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: [PATCH 4/8] kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS
Date: Fri, 16 Nov 2018 00:13:10 -0500 (EST)	[thread overview]
Message-ID: <nycvar.YSQ.7.76.1811152346510.1993@knanqh.ubzr> (raw)
In-Reply-To: <1542270435-11181-5-git-send-email-yamada.masahiro@socionext.com>

On Thu, 15 Nov 2018, Masahiro Yamada wrote:

> My main motivation of this commit is to clean up scripts/Kbuild.include
> and scripts/Makefile.build.
> 
> Currently, CONFIG_TRIM_UNUSED_KSYMS works with a tricky gimmick;
> possibly exported symbols are detected by letting $(CPP) replace
> EXPORT_SYMBOL* with a special string '=== __KSYM_*===', which is
> post-processed by sed, and passed to fixdep. The extra preprocessing
> is costly, and hacking cmd_and_fixdep is ugly.
> 
> I came up with a new way to find exported symbols; insert a dummy
> symbol __ksym_marker_* to each potentially exported symbol. Those
> dummy symbols are picked up by $(NM), post-processed by sed, then
> appended to .*.cmd files. I collected the post-process part to a
> new shell script scripts/gen_ksymdeps.sh for readability. The dummy
> symbols are put into the .discard.* section so that the linker
> script rips them off the final vmlinux or modules.

Brilliant!  I really like it.

Minor comments below.

> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> index 4d73e6e..294d6ae 100644
> --- a/include/asm-generic/export.h
> +++ b/include/asm-generic/export.h
> @@ -59,16 +59,19 @@ __kcrctab_\name:
>  .endm
>  #undef __put
>  
> -#if defined(__KSYM_DEPS__)
> -
> -#define __EXPORT_SYMBOL(sym, val, sec)	=== __KSYM_##sym ===
> -
> -#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
> +#if defined(CONFIG_TRIM_UNUSED_KSYMS)
>  
>  #include <linux/kconfig.h>
>  #include <generated/autoksyms.h>
>  
> +.macro __ksym_marker sym
> +	.section ".discard.ksym","a"
> +__ksym_marker_\sym:
> +	 .previous

Does this work as intended? I have vague memories about having problems 
with sections being discarded when they don't allocate any space.

> +.endm
> +
>  #define __EXPORT_SYMBOL(sym, val, sec)				\
> +	__ksym_marker sym;					\
>  	__cond_export_sym(sym, val, sec, __is_defined(__KSYM_##sym))
>  #define __cond_export_sym(sym, val, sec, conf)			\
>  	___cond_export_sym(sym, val, sec, conf)
> diff --git a/include/linux/export.h b/include/linux/export.h
> index ce764a5..0413a3d 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -92,22 +92,22 @@ struct kernel_symbol {
>   */
>  #define __EXPORT_SYMBOL(sym, sec)
>  
> -#elif defined(__KSYM_DEPS__)
> +#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
> +
> +#include <generated/autoksyms.h>
>  
>  /*
>   * For fine grained build dependencies, we want to tell the build system
>   * about each possible exported symbol even if they're not actually exported.
> - * We use a string pattern that is unlikely to be valid code that the build
> - * system filters out from the preprocessor output (see ksym_dep_filter
> - * in scripts/Kbuild.include).
> + * We use a symbol pattern __ksym_marker_<symbol> that the build system filters
> + * from the $(NM) output (see scripts/gen_ksymdep.sh). These symbols are
> + * discarded in the final link stage.
>   */
> -#define __EXPORT_SYMBOL(sym, sec)	=== __KSYM_##sym ===
> -
> -#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
> -
> -#include <generated/autoksyms.h>
> +#define __ksym_marker(sym)	\
> +	static int __ksym_marker_##sym[0] __section(".discard.ksym") __used

Even if this is discarded during the final link, maybe this could save 
a tiny amount of disk space by using a char instead?

> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 7f3ca6e..e5ba9b1 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -254,9 +254,18 @@ objtool_dep = $(objtool_obj)					\
>  	      $(wildcard include/config/orc/unwinder.h		\
>  			 include/config/stack/validation.h)
>  
> +ifdef CONFIG_TRIM_UNUSED_KSYMS
> +cmd_gen_ksymdeps = \
> +	$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ > $(dot-target).tmp; \
> +	cat $(dot-target).tmp >> $(dot-target).cmd; \
> +	rm -f $(dot-target).tmp;

Why don't you append to $(dot-target).cmd directly?

Nicolas