linux-kernel - [PATCH 1/6] export: explicitly align struct kernel

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Message-Id: <20180716122125.175792-2-maco@android.com>
Date:   Mon, 16 Jul 2018 14:21:20 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     linux-kernel@...r.kernel.org
Cc:     Martijn Coenen <maco@...roid.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver Neukum <oneukum@...e.com>,
        Arnd Bergmann <arnd@...db.de>, Jessica Yu <jeyu@...nel.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Sam Ravnborg <sam@...nborg.org>, linux-kbuild@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-usb@...r.kernel.org,
        usb-storage@...ts.one-eyed-alien.net, linux-scsi@...r.kernel.org,
        linux-arch@...r.kernel.org, maco@...gle.com, sspatil@...gle.com,
        malchev@...gle.com, joelaf@...gle.com
Subject: [PATCH 1/6] export: explicitly align struct kernel_symbol.

This change allows growing struct kernel_symbol without wasting bytes to
alignment.

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.

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.

Signed-off-by: Martijn Coenen <maco@...roid.com>
---
 arch/m68k/include/asm/export.h | 1 -
 arch/x86/include/asm/Kbuild    | 1 +
 arch/x86/include/asm/export.h  | 5 -----
 include/linux/export.h         | 1 +
 4 files changed, 2 insertions(+), 6 deletions(-)
 delete mode 100644 arch/x86/include/asm/export.h

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/arch/x86/include/asm/Kbuild b/arch/x86/include/asm/Kbuild
index de690c2d2e33..a0ab9ab61c75 100644
--- a/arch/x86/include/asm/Kbuild
+++ b/arch/x86/include/asm/Kbuild
@@ -8,5 +8,6 @@ generated-y += xen-hypercalls.h
 
 generic-y += dma-contiguous.h
 generic-y += early_ioremap.h
+generic-y += export.h
 generic-y += mcs_spinlock.h
 generic-y += mm-arch-hooks.h
diff --git a/arch/x86/include/asm/export.h b/arch/x86/include/asm/export.h
deleted file mode 100644
index 2a51d66689c5..000000000000
--- a/arch/x86/include/asm/export.h
+++ /dev/null
@@ -1,5 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-#ifdef CONFIG_64BIT
-#define KSYM_ALIGN 16
-#endif
-#include <asm-generic/export.h>
diff --git a/include/linux/export.h b/include/linux/export.h
index b768d6dd3c90..ad6b8e697b27 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -64,6 +64,7 @@ extern struct module __this_module;
 	static const struct kernel_symbol __ksymtab_##sym		\
 	__used								\
 	__attribute__((section("___ksymtab" sec "+" #sym), used))	\
+	__attribute__((aligned(sizeof(void *))))                        \
 	= { (unsigned long)&sym, __kstrtab_##sym }
 
 #if defined(__KSYM_DEPS__)
-- 
2.18.0.203.gfac676dfb9-goog