[v4,02/12] export: explicitly align struct kernel_symbol
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Message ID 20190903150638.242049-3-maennich@google.com
State New
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Commit Message

Matthias Maennich Sept. 3, 2019, 3:06 p.m. UTC
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>
---
 arch/m68k/include/asm/export.h | 1 -
 include/asm-generic/export.h   | 8 +++-----
 include/linux/export.h         | 3 ++-
 3 files changed, 5 insertions(+), 7 deletions(-)

Patch
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diff --git a/arch/m68k/include/asm/export.h b/arch/m68k/include/asm/export.h
index 0af20f48bd07..b53008b67ce1 100644
--- a/arch/m68k/include/asm/export.h
+++ b/arch/m68k/include/asm/export.h
@@ -1,3 +1,2 @@ 
-#define KSYM_ALIGN 2
 #define KCRC_ALIGN 2
 #include <asm-generic/export.h>
diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 294d6ae785d4..63f54907317b 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -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
diff --git a/include/linux/export.h b/include/linux/export.h
index fd8711ed9ac4..28a4d2150689 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -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 {