Messages in this thread | | | Date | Mon, 17 Jan 2022 22:08:00 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v9 04/15] arch: introduce ASM function sections |
| |
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.
https://people.kernel.org/tglx/notes-about-netiquette
| |