linux: Fix out-of-src builds

The linux kernel modules haven't been building successfully when the
build occurs in a separate directory than the source code, which is a
common build pattern in Linux. Was not able to determine the root cause,
but the %.o targets in subdirectories are no longer being matched by the
pattern targets in the Linux Kbuild system. This change fixes the issue
by dynamically creating the missing ones inside our Kbuild.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes openzfs#17517

Author: ckane

module/Kbuild.in

@@ -494,3 +494,34 @@ UBSAN_SANITIZE_zfs/sa.o := n
 ifeq ($(CONFIG_ALTIVEC),y)
 $(obj)/zfs/vdev_raidz_math_powerpc_altivec.o : c_flags += -maltivec
 endif
+
+# The following recipes attempt to fix out of src-tree builds, where $(src) != $(obj), so that the
+# subdir %.c/%.S -> %.o targets will work as expected. The in-kernel pattern targets do not seem to
+# be working on subdirs since about ~6.10
+zobjdirs = $(dir $(zfs-objs)) $(dir $(spl-objs))                                             \
+  $(dir $(zfs-$(CONFIG_X86))) $(dir $(zfs-$(CONFIG_UML_X86))) $(dir $(zfs-$(CONFIG_ARM64)))  \
+  $(dir $(zfs-$(CONFIG_PPC64))) $(dir $(zfs-$(CONFIG_PPC)))
+
+z_cdirs = $(sort $(filter-out lua/setjmp/ $(addprefix icp/asm-aarch64/, aes/ blake3/ modes/ sha2/) \
+  $(addprefix icp/asm-x86_64/, aes/ blake3/ modes/ sha2/)                                          \
+  $(addprefix icp/asm-ppc/, aes/ blake3/ modes/ sha2/)                                             \
+  $(addprefix icp/asm-ppc64/, aes/ blake3/ modes/ sha2/), $(zobjdirs)))
+z_sdirs = $(sort $(filter lua/setjmp/ $(addprefix icp/asm-aarch64/, aes/ blake3/ modes/ sha2/)     \
+  $(addprefix icp/asm-x86_64/, aes/ blake3/ modes/ sha2/)                                          \
+  $(addprefix icp/asm-ppc/, aes/ blake3/ modes/ sha2/)                                             \
+  $(addprefix icp/asm-ppc64/, aes/ blake3/ modes/ sha2/), $(zobjdirs)))
+
+define ZKMOD_C_O_MAKE_TARGET
+$1%.o: $(src)/$1%.c FORCE
+	$$(call if_changed_rule,cc_o_c)
+	$$(call cmd,force_checksrc)
+endef
+
+define ZKMOD_S_O_MAKE_TARGET
+$1%.o: $(src)/$1%.S FORCE
+	$$(call if_changed_rule,as_o_S)
+	$$(call cmd,force_checksrc)
+endef
+
+$(foreach target,$(z_cdirs), $(eval $(call ZKMOD_C_O_MAKE_TARGET,$(target))))
+$(foreach target,$(z_sdirs), $(eval $(call ZKMOD_S_O_MAKE_TARGET,$(target))))