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>

diff --git a/arch/m68k/include/asm/export.h b/arch/m68k/include/asm/export.h
index 0af20f48bd076047b8a91e6b181b6f7ea6bb03f2..b53008b67ce1c5c1b1305378581ec8c9d05242b4 100644 (file)
--- 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 294d6ae785d4e9fed609ceb51d69975c0b51ed85..63f54907317b088d1f8a183e89311eb024a42aed 100644 (file)
--- 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 fd8711ed9ac41de61efae43232c6e61983df93bc..28a4d2150689bb726dd52b53b56a65c42bccf9c3 100644 (file)
--- 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 {