Commit ed13fc33 authored Sep 06, 2019 by Matthias Maennich Committed by Jessica Yu Sep 10, 2019

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: Martijn Coenen <maco@android.com>
Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Jessica 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 {

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