[PATCH] export: Fix rootdir corner case in next_mnt()

[Richard Weinberger <richard@nod.at>]

Currently the following setup causes failure:

1. /etc/exports:
/ *(rw,crossmnt,no_subtree_check,fsid=root)

2. /etc/nfs.conf:
[exports]
 rootdir=/nfs_srv

3. Mounts:
/root/fs1.ext4 on /nfs_srv type ext4 (rw,relatime)
/root/fs2.ext4 on /nfs_srv/fs2 type ext4 (rw,relatime)

4. On the client:
$ ls /nfs_client/fs2
ls: cannot open directory '/nfs_client/fs2': Stale file handle

The problem is that next_mnt() misses the corner case that
every mount is a sub-mount of "/".
So it fails to see that /nfs_srv/fs2 is a mountpoint when the
client asks for fs2 it and as consequence the crossmnt mechanism
fails.

Signed-off-by: Richard Weinberger <richard@nod.at>
---
 support/export/cache.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/support/export/cache.c b/support/export/cache.c
index 2497d4f48df3..1c526277d3c6 100644
--- a/support/export/cache.c
+++ b/support/export/cache.c
@@ -410,12 +410,16 @@ static char *next_mnt(void **v, char *p)
 		*v = f;
 	} else
 		f = *v;
-	while ((me = getmntent(f)) != NULL && l > 1) {
+	while ((me = getmntent(f)) != NULL && l >= 1) {
 		char *mnt_dir = nfsd_path_strip_root(me->mnt_dir);
 
 		if (!mnt_dir)
 			continue;
 
+		/* Everything below "/" is a proper sub-mount */
+		if (strcmp(p, "/") == 0)
+			return mnt_dir;
+
 		if (strncmp(mnt_dir, p, l) == 0 && mnt_dir[l] == '/')
 			return mnt_dir;
 	}
-- 
2.26.2