kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS

include/{linux,asm-generic}/export.h defines a weak symbol, __crc_*
as a placeholder.

Genksyms writes the version CRCs into the linker script, which will be
used for filling the __crc_* symbols. The linker script format depends
on CONFIG_MODULE_REL_CRCS. If it is enabled, __crc_* holds the offset
to the reference of CRC.

It is time to get rid of this complexity.

Now that modpost parses text files (.*.cmd) to collect all the CRCs,
it can generate C code that will be linked to the vmlinux or modules.

Generate a new C file, .vmlinux.export.c, which contains the CRCs of
symbols exported by vmlinux. It is compiled and linked to vmlinux in
scripts/link-vmlinux.sh.

Put the CRCs of symbols exported by modules into the existing *.mod.c
files. No additional build step is needed for modules. As before,
*.mod.c are compiled and linked to *.ko in scripts/Makefile.modfinal.

No linker magic is used here. The new C implementation works in the
same way, whether CONFIG_RELOCATABLE is enabled or not.
CONFIG_MODULE_REL_CRCS is no longer needed.

Previously, Kbuild invoked additional $(LD) to update the CRCs in
objects, but this step is unneeded too.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)

3 files changed, 33 insertions, 36 deletions

diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 07a36a874dca..5e4b1f2369d2 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -2,6 +2,14 @@
 #ifndef __ASM_GENERIC_EXPORT_H
 #define __ASM_GENERIC_EXPORT_H
 
+/*
+ * This comment block is used by fixdep. Please do not remove.
+ *
+ * When CONFIG_MODVERSIONS is changed from n to y, all source files having
+ * EXPORT_SYMBOL variants must be re-compiled because genksyms is run as a
+ * side effect of the *.o build rule.
+ */
+
 #ifndef KSYM_FUNC
 #define KSYM_FUNC(x) x
 #endif
@@ -12,9 +20,6 @@
 #else
 #define KSYM_ALIGN 4
 #endif
-#ifndef KCRC_ALIGN
-#define KCRC_ALIGN 4
-#endif
 
 .macro __put, val, name
 #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
@@ -43,17 +48,6 @@ __ksymtab_\name:
 __kstrtab_\name:
 	.asciz "\name"
 	.previous
-#ifdef CONFIG_MODVERSIONS
-	.section ___kcrctab\sec+\name,"a"
-	.balign KCRC_ALIGN
-#if defined(CONFIG_MODULE_REL_CRCS)
-	.long __crc_\name - .
-#else
-	.long __crc_\name
-#endif
-	.weak __crc_\name
-	.previous
-#endif
 #endif
 .endm
 
diff --git a/include/linux/export-internal.h b/include/linux/export-internal.h
new file mode 100644
index 000000000000..c2b1d4fd5987
--- /dev/null
+++ b/include/linux/export-internal.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Please do not include this explicitly.
+ * This is used by C files generated by modpost.
+ */
+
+#ifndef __LINUX_EXPORT_INTERNAL_H__
+#define __LINUX_EXPORT_INTERNAL_H__
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+
+/* __used is needed to keep __crc_* for LTO */
+#define SYMBOL_CRC(sym, crc, sec)   \
+	u32 __section("___kcrctab" sec "+" #sym) __used __crc_##sym = crc
+
+#endif /* __LINUX_EXPORT_INTERNAL_H__ */
diff --git a/include/linux/export.h b/include/linux/export.h
index 27d848712b90..565c5ffcb26f 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -11,6 +11,14 @@
  * hackers place grumpy comments in header files.
  */
 
+/*
+ * This comment block is used by fixdep. Please do not remove.
+ *
+ * When CONFIG_MODVERSIONS is changed from n to y, all source files having
+ * EXPORT_SYMBOL variants must be re-compiled because genksyms is run as a
+ * side effect of the *.o build rule.
+ */
+
 #ifndef __ASSEMBLY__
 #ifdef MODULE
 extern struct module __this_module;
@@ -19,26 +27,6 @@ extern struct module __this_module;
 #define THIS_MODULE ((struct module *)0)
 #endif
 
-#ifdef CONFIG_MODVERSIONS
-/* Mark the CRC weak since genksyms apparently decides not to
- * generate a checksums for some symbols */
-#if defined(CONFIG_MODULE_REL_CRCS)
-#define __CRC_SYMBOL(sym, sec)						\
-	asm("	.section \"___kcrctab" sec "+" #sym "\", \"a\"	\n"	\
-	    "	.weak	__crc_" #sym "				\n"	\
-	    "	.long	__crc_" #sym " - .			\n"	\
-	    "	.previous					\n")
-#else
-#define __CRC_SYMBOL(sym, sec)						\
-	asm("	.section \"___kcrctab" sec "+" #sym "\", \"a\"	\n"	\
-	    "	.weak	__crc_" #sym "				\n"	\
-	    "	.long	__crc_" #sym "				\n"	\
-	    "	.previous					\n")
-#endif
-#else
-#define __CRC_SYMBOL(sym, sec)
-#endif
-
 #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
 #include <linux/compiler.h>
 /*
@@ -85,7 +73,6 @@ struct kernel_symbol {
 /*
  * For every exported symbol, do the following:
  *
- * - If applicable, place a CRC entry in the __kcrctab section.
  * - Put the name of the symbol and namespace (empty string "" for none) in
  *   __ksymtab_strings.
  * - Place a struct kernel_symbol entry in the __ksymtab section.
@@ -98,7 +85,6 @@ struct kernel_symbol {
 	extern typeof(sym) sym;							\
 	extern const char __kstrtab_##sym[];					\
 	extern const char __kstrtabns_##sym[];					\
-	__CRC_SYMBOL(sym, sec);							\
 	asm("	.section \"__ksymtab_strings\",\"aMS\",%progbits,1	\n"	\
 	    "__kstrtab_" #sym ":					\n"	\
 	    "	.asciz 	\"" #sym "\"					\n"	\