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
