modpost: generate vmlinux.symvers and reuse it for the second modpost

The full build runs modpost twice, first for vmlinux.o and second for
modules.

The first pass dumps all the vmlinux symbols into Module.symvers, but
the second pass parses vmlinux again instead of reusing the dump file,
presumably because it needs to avoid accumulating stale symbols.

Loading symbol info from a dump file is faster than parsing an ELF object.
Besides, modpost deals with various issues to parse vmlinux in the second
pass.

A solution is to make the first pass dumps symbols into a separate file,
vmlinux.symvers. The second pass reads it, and parses module .o files.
The merged symbol information is dumped into Module.symvers in the same
way as before.

This makes further modpost cleanups possible.

Also, it fixes the problem of 'make vmlinux', which previously overwrote
Module.symvers, throwing away module symbols.

I slightly touched scripts/link-vmlinux.sh so that vmlinux is re-linked
when you cross this commit. Otherwise, vmlinux.symvers would not be
generated.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitignore

@@ -56,6 +56,7 @@ modules.order
 /linux
 /vmlinux
 /vmlinux.32
+/vmlinux.symvers
 /vmlinux-gdb.py
 /vmlinuz
 /System.map

Documentation/dontdiff

@@ -251,6 +251,7 @@ vmlinux-*
 vmlinux.aout
 vmlinux.bin.all
 vmlinux.lds
+vmlinux.symvers
 vmlinuz
 voffset.h
 vsyscall.lds

Makefile

@@ -1416,7 +1416,7 @@ endif # CONFIG_MODULES
 # make distclean Remove editor backup files, patch leftover files and the like
 
 # Directories & files removed with 'make clean'
-CLEAN_FILES += include/ksym \
+CLEAN_FILES += include/ksym vmlinux.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps
 
 # Directories & files removed with 'make mrproper'

scripts/Makefile.modpost

@@ -55,10 +55,10 @@ ifdef MODPOST_VMLINUX
 quiet_cmd_modpost = MODPOST $@
       cmd_modpost = $(MODPOST) $<
 
-Module.symvers: vmlinux.o
+vmlinux.symvers: vmlinux.o
 	$(call cmd,modpost)
 
-__modpost: Module.symvers
+__modpost: vmlinux.symvers
 
 else
 
@@ -66,7 +66,8 @@ MODPOST += -s \
 	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS))
 
 ifeq ($(KBUILD_EXTMOD),)
-MODPOST += $(wildcard vmlinux)
+
+input-symdump := vmlinux.symvers
 output-symdump := Module.symvers
 
 else

scripts/link-vmlinux.sh

@@ -218,8 +218,6 @@ on_signals()
 }
 trap on_signals HUP INT QUIT TERM
 
-#
-#
 # Use "make V=1" to debug this script
 case "${KBUILD_VERBOSE}" in
 *1*)