[PATCH] Improve support for exporting btrfs subvolumes.

[Neil Brown <neilb@suse.de>]

If you export two subvolumes of a btrfs filesystem, they will both be
given the same uuid so lookups will be confused.
blkid cannot differentiate the two, so we must use the fsid from
statfs64 to identify the filesystem.

We cannot tell if blkid or statfs is best without knowing internal
details of the filesystem in question, so we need to encode specific
knowledge of btrfs in mountd.  This is unfortunate.

To ensure smooth handling of this and possible future changes in uuid
generation, we add infrastructure for multiple different uuids to be
recognised on old filehandles, but only the preferred on is used on
new filehandles.

Signed-off-by: NeilBrown <neilb@suse.de>

--
This is a substantially revised version of a patch I posted a while
ago.
I tried to find a way to do it would hard coding knowledge of btrfs in
nfs-utils, but it isn't possible.  For some filesystems, f_fsid is
best, for some it is worst.  No way to tell the difference.

This patch add infrastructure so that if we find a better way to get a
good uuid (e.g. a new syscall), we can slot it in for new filehandles,
but old filehandles using the old uuid will still work.

I believe this is ready for inclusion upstream.
Thanks,
NeilBrown


diff --git a/utils/mountd/cache.c b/utils/mountd/cache.c
index caef5b2..85cd829 100644
--- a/utils/mountd/cache.c
+++ b/utils/mountd/cache.c
@@ -170,13 +170,16 @@ void auth_unix_gid(FILE *f)
 #if USE_BLKID
 static const char *get_uuid_blkdev(char *path)
 {
+	/* We set *safe if we know that we need the
+	 * fsid from statfs too.
+	 */
 	static blkid_cache cache = NULL;
 	struct stat stb;
 	char *devname;
 	blkid_tag_iterate iter;
 	blkid_dev dev;
 	const char *type;
-	const char *val = NULL;
+	const char *val, *uuid = NULL;
 
 	if (cache == NULL)
 		blkid_get_cache(&cache, NULL);
@@ -193,42 +196,29 @@ static const char *get_uuid_blkdev(char *path)
 	iter = blkid_tag_iterate_begin(dev);
 	if (!iter)
 		return NULL;
-	while (blkid_tag_next(iter, &type, &val) == 0)
+	while (blkid_tag_next(iter, &type, &val) == 0) {
 		if (strcmp(type, "UUID") == 0)
+			uuid = val;
+		if (strcmp(type, "TYPE") == 0 &&
+		    strcmp(val, "btrfs") == 0) {
+			uuid = NULL;
 			break;
+		}
+	}
 	blkid_tag_iterate_end(iter);
-	return val;
+	return uuid;
 }
 #else
 #define get_uuid_blkdev(path) (NULL)
 #endif
 
-int get_uuid(char *path, char *uuid, int uuidlen, char *u)
+int get_uuid(const char *val, int uuidlen, char *u)
 {
 	/* extract hex digits from uuidstr and compose a uuid
 	 * of the given length (max 16), xoring bytes to make
-	 * a smaller uuid.  Then compare with uuid
+	 * a smaller uuid.
 	 */
 	int i = 0;
-	const char *val = NULL;
-	char fsid_val[17];
-
-	if (path) {
-		val = get_uuid_blkdev(path);
-		if (!val) {
-			struct statfs64 st;
-
-			if (statfs64(path, &st))
-				return 0;
-			if (!st.f_fsid.__val[0] && !st.f_fsid.__val[1])
-				return 0;
-			snprintf(fsid_val, 17, "%08x%08x",
-				 st.f_fsid.__val[0], st.f_fsid.__val[1]);
-			val = fsid_val;
-		}
-	} else {
-		val = uuid;
-	}
 	
 	memset(u, 0, uuidlen);
 	for ( ; *val ; val++) {
@@ -252,6 +242,60 @@ int get_uuid(char *path, char *uuid, int uuidlen, char *u)
 	return 1;
 }
 
+int uuid_by_path(char *path, int type, int uuidlen, char *uuid)
+{
+	/* get a uuid for the filesystem found at 'path'.
+	 * There are several possible ways of generating the
+	 * uuids (types).
+	 * Type 0 is used for new filehandles, while other types
+	 * may be used to interpret old filehandle - to ensure smooth
+	 * forward migration.
+	 * We return 1 if a uuid was found (and it might be worth 
+	 * trying the next type) or 0 if no more uuid types can be
+	 * extracted.
+	 */
+
+	/* Possible sources of uuid are
+	 * - blkid uuid
+	 * - statfs64 uuid
+	 *
+	 * On some filesystems (e.g. vfat) the statfs64 uuid is simply an
+	 * encoding of the device that the filesystem is mounted from, so
+	 * it we be very bad to use that (as device numbers change).  blkid
+	 * must be preferred.
+	 * On other filesystems (e.g. btrfs) the statfs64 uuid contains
+	 * important info that the blkid uuid cannot contain:  This happens
+	 * when multiple subvolumes are exported (they have the same
+	 * blkid uuid but different statfs64 uuids).
+	 * We rely on get_uuid_blkdev *knowing* which is which and not returning
+	 * a uuid for filesystems where the statfs64 uuid is better.
+	 *
+	 */
+	struct statfs64 st;
+	char fsid_val[17];
+	const char *blkid_val;
+	const char *val;
+
+	blkid_val = get_uuid_blkdev(path);
+
+	if (statfs64(path, &st) == 0 &&
+	    (st.f_fsid.__val[0] || st.f_fsid.__val[1]))
+		snprintf(fsid_val, 17, "%08x%08x",
+			 st.f_fsid.__val[0], st.f_fsid.__val[1]);
+	else
+		fsid_val[0] = 0;
+
+	if (blkid_val && (type--) == 0)
+		val = blkid_val;
+	else if (fsid_val[0] && (type--) == 0)
+		val = fsid_val;
+	else
+		return 0;
+
+	get_uuid(val, uuidlen, uuid);
+	return 1;
+}
+
 /* Iterate through /etc/mtab, finding mountpoints
  * at or below a given path
  */
@@ -398,6 +442,7 @@ void nfsd_fh(FILE *f)
 			struct stat stb;
 			char u[16];
 			char *path;
+			int type;
 
 			if (exp->m_export.e_flags & NFSEXP_CROSSMOUNT) {
 				static nfs_export *prev = NULL;
@@ -461,10 +506,14 @@ void nfsd_fh(FILE *f)
 					continue;
 			check_uuid:
 				if (exp->m_export.e_uuid)
-					get_uuid(NULL, exp->m_export.e_uuid,
+					get_uuid(exp->m_export.e_uuid,
 						 uuidlen, u);
-				else if (get_uuid(path, NULL, uuidlen, u) == 0)
-					continue;
+				else
+					for (type = 0;
+					     uuid_by_path(path, type, uuidlen, u);
+					     type++)
+						if (memcmp(u, fhuuid, uuidlen) != 0)
+							break;
 
 				if (memcmp(u, fhuuid, uuidlen) != 0)
 					continue;
@@ -600,13 +649,13 @@ static int dump_to_cache(FILE *f, char *domain, char *path, struct exportent *ex
 		write_secinfo(f, exp, flag_mask);
  		if (exp->e_uuid == NULL || different_fs) {
  			char u[16];
- 			if (get_uuid(path, NULL, 16, u)) {
+ 			if (uuid_by_path(path, 0, 16, u)) {
  				qword_print(f, "uuid");
  				qword_printhex(f, u, 16);
  			}
  		} else {
  			char u[16];
- 			get_uuid(NULL, exp->e_uuid, 16, u);
+ 			get_uuid(exp->e_uuid, 16, u);
  			qword_print(f, "uuid");
  			qword_printhex(f, u, 16);
  		}