scoutfs: add git describe to built module

It's handy to quickly find the git commit that built a given module.  We
add a MOD_INFO() tag for it so we can see it in modinfo on the built
module.  We add a ELF note that the kernel tracks in
/sys/modules/$m/notes/ when the module is loaded.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/Makefile

@@ -12,7 +12,12 @@ else
 SP = @:
 endif
 
-SCOUTFS_ARGS := CONFIG_SCOUTFS_FS=m -C $(SK_KSRC) M=$(CURDIR)/src
+SCOUTFS_GIT_DESCRIBE := \
+	$(shell git describe --all --abbrev=6 --long 2>/dev/null || \
+		echo not-in-a-git-repository)
+
+SCOUTFS_ARGS := SCOUTFS_GIT_DESCRIBE=$(SCOUTFS_GIT_DESCRIBE) \
+		CONFIG_SCOUTFS_FS=m -C $(SK_KSRC) M=$(CURDIR)/src
 
 all: module
 

kmod/src/Makefile

@@ -1,5 +1,7 @@
 obj-$(CONFIG_SCOUTFS_FS) := scoutfs.o
 
+CFLAGS_super.o = -DSCOUTFS_GIT_DESCRIBE=\"$(SCOUTFS_GIT_DESCRIBE)\"
+
 CFLAGS_scoutfs_trace.o = -I$(src) # define_trace.h double include
  
 scoutfs-y += alloc.o bio.o btree.o compact.o counters.o data.o dir.o kvec.o \

kmod/src/super.c

@@ -324,6 +324,15 @@ static int __init scoutfs_module_init(void)
 {
 	int ret;
 
+	/*
+	 * gcc only recently learned to let __attribute__(section) add
+	 * SHT_NOTE notes.  But the assembler always could.
+	 */
+	__asm__ __volatile__ (
+		".section	.note.git_describe,\"a\"\n"
+		".string	\""SCOUTFS_GIT_DESCRIBE"\\n\"\n"
+		".previous\n");
+
 	scoutfs_init_counters();
 
 	scoutfs_kset = kset_create_and_add("scoutfs", NULL, fs_kobj);
@@ -349,3 +358,4 @@ module_exit(scoutfs_module_exit)
 
 MODULE_AUTHOR("Zach Brown <zab@versity.com>");
 MODULE_LICENSE("GPL");
+MODULE_INFO(git_describe, SCOUTFS_GIT_DESCRIBE);