hugetlbfs: prevent filesystem stacking of hugetlbfs

syzbot found issues with having hugetlbfs on a union/overlay as reported
in [1].  Due to the limitations (no write) and special functionality of
hugetlbfs, it does not work well in filesystem stacking.  There are no
know use cases for hugetlbfs stacking.  Rather than making modifications
to get hugetlbfs working in such environments, simply prevent stacking.

[1] https://lore.kernel.org/linux-mm/000000000000b4684e05a2968ca6@google.com/

Reported-by: syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com
Suggested-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Colin Walters <walters@verbum.org>
Link: http://lkml.kernel.org/r/80f869aa-810d-ef6c-8888-b46cee135907@oracle.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 523954d00dff6209fe04f5a45ed1c03a2acdce5b..b5c109703daaf9ff2f4384c1f53e15ee1c5f3e55 100644 (file)
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1364,6 +1364,12 @@ hugetlbfs_fill_super(struct super_block *sb, struct fs_context *fc)
        sb->s_magic = HUGETLBFS_MAGIC;
        sb->s_op = &hugetlbfs_ops;
        sb->s_time_gran = 1;
+
+       /*
+        * Due to the special and limited functionality of hugetlbfs, it does
+        * not work well as a stacking filesystem.
+        */
+       sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
        sb->s_root = d_make_root(hugetlbfs_get_root(sb, ctx));
        if (!sb->s_root)
                goto out_free;