Re: [PATCH v9 04/15] arch: introduce ASM function sections

Borislav Petkov
2022-01-17T21:08:06Z

On Thu, Dec 23, 2021 at 01:21:58AM +0100, Alexander Lobakin wrote:
> Sometimes it is useful to create a separate section for every
> function (symbol in general) to be able then to selectively merge
> them back into on or several others. This is how DCE and a part of
^^

"one"

DCE == Dead Code Elimination?

In any case, write it out first please and then use the abbreviation.

> LTO work.

I would've said that too but that one at least has a Kconfig entry which
explains what it is so no need.

/me looks further

Aha there is LD_DEAD_CODE_DATA_ELIMINATION. So connect the two pls.

> Currently, only C functions are in scope

You mean, currently this is done only for C functions? The "in scope"
formulation sounds weird.

> and the compilers are able to do this automatically when
> `-ffunction-section` is specified.

-ffunction-sections, plural.

> Add a basic infra for supporting ASM function sections. If any of

yah s/ASM/asm/g. It's not like it is an acronym or so.

and also, you should explain that "asm function sections" means "put a
function symbol defined in asm, into a separate section".

> the required build options (DCE, LTO, FG-KASLR later) is on and
> the target architecture claims it supports them, all ASM functions
> and "code" will be placed into separate named sections by default.
> This is achieved using --sectname-subst GAS flag which will then
> substitute "%S" in a .pushsection or .section directive with the

Thanks for explaining this. The gas manpage is very, hm, verbose
<sarcarstic eyeroll> ;-\:

" --sectname-subst
Honor substitution sequences in section names.
"

...

> diff --git a/include/linux/linkage.h b/include/linux/linkage.h
> index dbf8506decca..0c0ddf4429dc 100644
> --- a/include/linux/linkage.h
> +++ b/include/linux/linkage.h
> @@ -73,6 +73,37 @@
> #define __ALIGN_STR ".align 4,0x90"
> #endif
>
> +/*
> + * Allow ASM symbols to have their own unique sections if they are being
> + * generated by the compiler for C functions (DCE, LTO).
> + */
> +#if defined(CONFIG_HAVE_ASM_FUNCTION_SECTIONS) && \
> + ((defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) && !defined(MODULE)) || \
> + (defined(CONFIG_LTO_CLANG)))
> +
> +#define SYM_PUSH_SECTION(name) \
> + .pushsection %S.name, "ax"
> +
> +#define SYM_POP_SECTION() \
> + .popsection
> +
> +#define __ASM_PUSH_SECTION(name) \
> + ".pushsection %S." name ", \"ax\""
> +
> +#else /* Just .text */

Just .text?

> +
> +#define SYM_PUSH_SECTION(name)
> +#define SYM_POP_SECTION()
> +#define __ASM_PUSH_SECTION(name)
> +
> +#endif /* Just .text */
> +
> +#define ASM_PUSH_SECTION(name) \
> + __ASM_PUSH_SECTION(__stringify(name))
> +
> +#define ASM_POP_SECTION() \
> + __stringify(SYM_POP_SECTION())
> +
> #ifdef __ASSEMBLY__
>
> /* SYM_T_FUNC -- type used by assembler to mark functions */
> @@ -209,6 +240,15 @@
> SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN)
> #endif
>
> +/*
> + * SYM_FUNC_START_WEAK -- use where there are two global names for one

SYM_FUNC_START_WEAK_ALIAS

> + * function, and one of them is weak
> + */
> +#ifndef SYM_FUNC_START_WEAK_ALIAS
> +#define SYM_FUNC_START_WEAK_ALIAS(name) \
> + SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN)
> +#endif
> +
> /*
> * SYM_FUNC_START_ALIAS -- use where there are two global names for one
> * function
> @@ -225,12 +265,24 @@
> * later.
> */
> #define SYM_FUNC_START(name) \
> + SYM_PUSH_SECTION(name) ASM_NL \
> + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)
> +#endif
> +
> +/*
> + * SYM_FUNC_START_SECT -- use for global functions, will be conditionally
> + * placed into a section specified in the second argument
> + */
> +#ifndef SYM_FUNC_START_SECT
> +#define SYM_FUNC_START_SECT(name, to) \

(name, sect)

"to" reads kinda unclear what it is supposed to mean.

> + SYM_PUSH_SECTION(to) ASM_NL \
> SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)
> #endif
>
> /* SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment */
> #ifndef SYM_FUNC_START_NOALIGN
> #define SYM_FUNC_START_NOALIGN(name) \
> + SYM_PUSH_SECTION(name) ASM_NL \
> SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE)
> #endif
>
> @@ -238,24 +290,38 @@
> #ifndef SYM_FUNC_START_LOCAL
> /* the same as SYM_FUNC_START_LOCAL_ALIAS, see comment near SYM_FUNC_START */
> #define SYM_FUNC_START_LOCAL(name) \
> + SYM_PUSH_SECTION(name) ASM_NL \
> SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN)
> #endif
>
> /* SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o alignment */
> #ifndef SYM_FUNC_START_LOCAL_NOALIGN
> #define SYM_FUNC_START_LOCAL_NOALIGN(name) \
> + SYM_PUSH_SECTION(name) ASM_NL \
> + SYM_START(name, SYM_L_LOCAL, SYM_A_NONE)
> +#endif
> +
> +/*
> + * SYM_FUNC_START_LOCAL_NOALIGN_SECT -- use for local functions, w/o alignment,
> + * will be conditionally placed into a section specified in the second argument
> + */
> +#ifndef SYM_FUNC_START_LOCAL_NOALIGN_SECT
> +#define SYM_FUNC_START_LOCAL_NOALIGN_SECT(name, to) \

Ditto. And so on below.

...

> diff --git a/init/Kconfig b/init/Kconfig
> index 37926d19a74a..3babc0aeac61 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1386,6 +1386,17 @@ config CC_OPTIMIZE_FOR_SIZE
>
> endchoice
>
> +config HAVE_ASM_FUNCTION_SECTIONS
> + depends on ARCH_SUPPORTS_ASM_FUNCTION_SECTIONS
> + depends on $(cc-option,-Wa$(comma)--sectname-subst)
> + def_bool y
> + help
> + This enables ASM function sections if both architecture
> + and toolchain supports that. It allows creating a separate

"... support it."

> + .text section for each ASM function in order to improve

s/.text // - the section name is specified by the macro arg.

> + DCE and LTO (works the same way as -ffunction-sections for
> + C code).
> +

Thx.

--
Regards/Gruss,
Boris.