LINUX: Symlink src/libafs/AFS_cvn.c

Currently, make_kbuild_makefile.pl generates a Makefile for building our
Linux kernel module by listing our various objects in 'openafs-objs',
and symlinking the relevant source file from src/foo/bar.c into
MODLOAD-*/bar.c. For example, src/libafs/MODLOAD-*/afs_init.c is a
symlink to src/afs/afs_init.c.

We determine where each source file lives by looking at our Makefile
rules. This works for all of our source files, except
AFS_component_version_number.c, which has no single location in the
tree, but is built inside every subsystem. For
AFS_component_version_number.c, we don't make a symlink, but instead
copy the rules from Makefile.version so that
AFS_component_version_number.c is generated locally, and does not use a
symlink like our other source files.

The rules in Makefile.version look like this:

AFS_component_version_number.o: AFS_component_version_number.c
AFS_component_version_number.c: [...]/src/config/Makefile.version
	[logic to generate AFS_component_version_number.c]

But for the Linux build before Linux 6.13, that doesn't work as-is
because the Linux buildsystem is not running in our MODLOAD directory,
but inside the Linux source tree. So, to make this work,
make_kbuild_makefile.pl modifies the rules so they look like this:

/path/to/src/libafs/MODLOAD-x.y.z/AFS_component_version_number.o: /path/to/src/libafs/MODLOAD-x.y.z/AFS_component_version_number.c
/path/to/src/libafs/MODLOAD-x.y.z/AFS_component_version_number.c: [...]/src/config/Makefile.version
	[logic to generate AFS_component_version_number.c]

Which works, before Linux 6.13. After the build runs, our source files
look like this, for example:

  $ ls -l src/libafs/MODLOAD-*/AFS_component_version_number.c src/libafs/MODLOAD-*/afs_init.c
  -rw-r--r-- 1 [...] src/libafs/MODLOAD-x.y.z/AFS_component_version_number.c
  lrwxrwxrwx 1 [...] src/libafs/MODLOAD-x.y.z/afs_init.c -> /path/to/src/afs/afs_init.c

With Linux 6.13, Linux changed how the kbuild process builds external
kernel modules with this commit:

 'kbuild: change working directory to external module directory with M='
 (13b25489b6f8)

which is followed by additional follow-up changes within the merge
commit:

 Merge tag 'kbuild-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild

 Pull Kbuild updates from Masahiro Yamada:' (6a34dfa15d6e)

With these changes, our working directory is now src/libafs/MODLOAD-*
when building the openafs kernel module, and we try to build
AFS_component_version_number.o instead of
/path/to/.../AFS_component_version_number.o. As a result, 'make' doesn't
know how to generate AFS_component_version_number.c, and the build
fails:

  make[8]: *** No rule to make target 'AFS_component_version_number.o', needed by 'openafs.o'.  Stop.
  make[7]: *** [/usr/src/linux-6.13/Makefile:1989: .] Error 2
  make[6]: *** [/usr/src/linux-6.13/Makefile:251: __sub-make] Error 2
  make[5]: Leaving directory '/usr/src/linux-6.13/'
  FAILURE: make exit code 2
  make[4]: Leaving directory '/home/../openafs/src/libafs/MODLOAD-6.13.0-rc2-SP'
  make[4]: *** [Makefile.afs:283: openafs.ko] Error 1

To fix this, change make_kbuild_makefile.pl to create a symlink for
AFS_component_version_number.c just like it does for all other source
files. Use a target of src/libafs/AFS_component_version_number.c, and
make sure that AFS_component_version_number.c is generated for the
'setup' libafs target.

We have to hard-code this special case for
AFS_component_version_number.c, since none of our Makefile rules specify
a full path to AFS_component_version_number.c as a dependency for
AFS_component_version_number.o. But otherwise,
AFS_component_version_number.c is now treated the same as all other
source files.

With this commit, our source files now look like this:

  $ ls -l src/libafs/MODLOAD-*/AFS_component_version_number.c src/libafs/MODLOAD-*/afs_init.c
  lrwxrwxrwx 1 [...] src/libafs/MODLOAD-x.y.z/AFS_component_version_number.c -> /path/to/src/libafs/AFS_component_version_number.c
  lrwxrwxrwx 1 [...] src/libafs/MODLOAD-x.y.z/afs_init.c -> /path/to/src/afs/afs_init.c

Remove the make_kbuild_makefile.pl code that copies the Makefile.version
rules, since AFS_component_version_number.c is not generated locally
anymore.

Written in collaboration with cwills@sinenomine.net.

Reviewed-on: https://gerrit.openafs.org/16034
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
(cherry picked from commit 5b01ee836d)

Change-Id: Ic94d525a24b53195f010d3530d6f15cb4ad1e430
Reviewed-on: https://gerrit.openafs.org/16063
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/libafs/MakefileProto.LINUX.in

@@ -151,7 +151,7 @@ LINUX_MODULE_EXT=o
 LINUX_MODULE_NAME=
 LOCAL_SMP_DEF=
 
-setup:
+setup: AFS_component_version_number.c
 	-$(RM) -f h net netinet sys rpc
 	-ln -fs rx rpc
 	for m in ${MPS} ; do \

src/libafs/make_kbuild_makefile.pl

@@ -61,19 +61,19 @@ foreach $mf (@Makefiles) {
   $F->close();
 }
 
+# AFS_component_version_number.c is a special case. There is no single location
+# where it normally exists; usually it's generated in every subsystem, so our
+# makefiles don't specify an absolute path to the source file (they just say to
+# use the AFS_component_version_number.c from the current directory). Here, to
+# make it more like our other source files, we say we want to use
+# AFS_component_version_number.c from src/libafs.
+$deps{'AFS_component_version_number.o'} = "$vars{TOP_OBJDIR}/src/libafs/AFS_component_version_number.c";
+
 
 $KDIR = "$vars{TOP_OBJDIR}/src/libafs/$KDIR";
 @libafs_objs = (split(' ', $vars{AFSAOBJS}), split(' ', $vars{AFSNFSOBJS}));
 @afspag_objs = (split(' ', $vars{AFSPAGOBJS}));
 
-$MV = new IO::File("$vars{TOP_OBJDIR}/src/config/Makefile.version", O_RDONLY)
-        or die "$vars{TOP_OBJDIR}/src/config/Makefile.version: $!\n";
-while (<$MV>) {
-  s#AFS_component_version_number#$KDIR/AFS_component_version_number#g;
-  $MakefileVersion .= $_;
-}
-$MV->close();
-
 if (! -d $KDIR) {
   mkdir($KDIR, 0777) or die "$KDIR: $!\n";
 }
@@ -170,6 +170,5 @@ if ($vars{LINUX_KBUILD_CFLAGS_VAR} ne "") {
 print $F "obj-m := $TARG.o afspag.o\n";
 print $F "$TARG-objs := ", join("\\\n $_", @libafs_objs), "\n";
 print $F "afspag-objs := ", join("\\\n $_", @afspag_objs), "\n";
-print $F "\n$MakefileVersion\n";
 $F->close() or die "$KDIR/Makefile: $!\n";