module: allow UNUSED_KSYMS_WHITELIST to be relative against objtree.

If UNUSED_KSYMS_WHITELIST is a file generated
before Kbuild runs, and the source tree is in
a read-only filesystem, the developer must put
the file somewhere and specify an absolute
path to UNUSED_KSYMS_WHITELIST. This worked,
but if IKCONFIG=y, an absolute path is embedded
into .config and eventually into vmlinux, causing
the build to be less reproducible when building
on a different machine.

This patch makes the handling of
UNUSED_KSYMS_WHITELIST to be similar to
MODULE_SIG_KEY.

First, check if UNUSED_KSYMS_WHITELIST is an
absolute path, just as before this patch. If so,
use the path as is.

If it is a relative path, use wildcard to check
the existence of the file below objtree first.
If it does not exist, fall back to the original
behavior of adding $(srctree)/ before the value.

After this patch, the developer can put the generated
file in objtree, then use a relative path against
objtree in .config, eradicating any absolute paths
that may be evaluated differently on different machines.

Signed-off-by: Yifan Hong <[e-mail]>
Reviewed-by: Elliot Berman <[e-mail]>
Signed-off-by: Luis Chamberlain <[e-mail]>

kernel/module/Kconfig

···
 	  exported at all times, even in absence of in-tree users. The value to
 	  set here is the path to a text file containing the list of symbols,
 	  one per line. The path can be absolute, or relative to the kernel
-	  source tree.
+	  source or obj tree.
 
 config MODULES_TREE_LOOKUP
 	def_bool y

scripts/Makefile.modpost

···
 
 ifdef CONFIG_TRIM_UNUSED_KSYMS
 ksym-wl := $(CONFIG_UNUSED_KSYMS_WHITELIST)
-ksym-wl := $(if $(filter-out /%, $(ksym-wl)),$(srctree)/)$(ksym-wl)
+ksym-wl := $(if $(filter-out /%, $(ksym-wl)),$(if $(wildcard $(ksym-wl)),,$(srctree)/))$(ksym-wl)
 modpost-args += -t $(addprefix -u , $(ksym-wl))
 modpost-deps += $(ksym-wl)
 endif