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Commit ed13fc33 authored by Matthias Maennich's avatar Matthias Maennich Committed by Jessica Yu
Browse files

export: explicitly align struct kernel_symbol 



This change allows growing struct kernel_symbol without wasting bytes to
alignment. It also concretized the alignment of ksymtab entries if
relative references are used for ksymtab entries.

struct kernel_symbol was already implicitly being aligned to the word
size, except on x86_64 and m68k, where it is aligned to 16 and 2 bytes,
respectively.

As far as I can tell there is no requirement for aligning struct
kernel_symbol to 16 bytes on x86_64, but gcc aligns structs to their
size, and the linker aligns the custom __ksymtab sections to the largest
data type contained within, so setting KSYM_ALIGN to 16 was necessary to
stay consistent with the code generated for non-ASM EXPORT_SYMBOL(). Now
that non-ASM EXPORT_SYMBOL() explicitly aligns to word size (8),
KSYM_ALIGN is no longer necessary.

In case of relative references, the alignment has been changed
accordingly to not waste space when adding new struct members.

As for m68k, struct kernel_symbol is aligned to 2 bytes even though the
structure itself is 8 bytes; using a 4-byte alignment shouldn't hurt.

I manually verified the output of the __ksymtab sections didn't change
on x86, x86_64, arm, arm64 and m68k. As expected, the section contents
didn't change, and the ELF section alignment only changed on x86_64 and
m68k. Feedback from other archs more than welcome.

Co-developed-by: default avatarMartijn Coenen <maco@android.com>
Signed-off-by: default avatarMartijn Coenen <maco@android.com>
Reviewed-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarMatthias Maennich <maennich@google.com>
Signed-off-by: default avatarJessica Yu <jeyu@kernel.org>
parent c5e4a062

arch/m68k/include/asm/export.h

+0 −1
Original line number Diff line number Diff line 
#define KSYM_ALIGN 2

#define KCRC_ALIGN 2

#include <asm-generic/export.h>

include/asm-generic/export.h

+3 −5
Original line number Diff line number Diff line
@@ -4,15 +4,13 @@ 
#ifndef KSYM_FUNC

#define KSYM_FUNC(x) x

#endif

#ifdef CONFIG_64BIT

#ifndef KSYM_ALIGN

#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS

#define KSYM_ALIGN 4

#elif defined(CONFIG_64BIT)

#define KSYM_ALIGN 8

#endif

#else

#ifndef KSYM_ALIGN

#define KSYM_ALIGN 4

#endif

#endif

#ifndef KCRC_ALIGN

#define KCRC_ALIGN 4

#endif

include/linux/export.h

+2 −1
Original line number Diff line number Diff line
@@ -52,7 +52,7 @@ extern struct module __this_module; 
#define __KSYMTAB_ENTRY(sym, sec)					\

	__ADDRESSABLE(sym)						\

	asm("	.section \"___ksymtab" sec "+" #sym "\", \"a\"	\n"	\

	    "	.balign	8					\n"	\

	    "	.balign 4					\n"	\

	    "__ksymtab_" #sym ":				\n"	\

	    "	.long	" #sym "- .				\n"	\

	    "	.long	__kstrtab_" #sym "- .			\n"	\
@@ -66,6 +66,7 @@ struct kernel_symbol { 
#define __KSYMTAB_ENTRY(sym, sec)					\

	static const struct kernel_symbol __ksymtab_##sym		\

	__attribute__((section("___ksymtab" sec "+" #sym), used))	\

	__aligned(sizeof(void *))					\

	= { (unsigned long)&sym, __kstrtab_##sym }



struct kernel_symbol {