[PATCH] afs: Don't cross .backup mountpoint from backup volume

[Marc Dionne]

Don't cross a mountpoint that explicitly specifies a backup volume
(target is <vol>.backup) when starting from a backup volume.

It it not uncommon to mount a volume's backup directly in the volume
itself.  This can cause tools that are not paying attention to get
into a loop mounting the volume onto itself as they attempt to
traverse the tree, leading to a variety of problems.

This doesn't prevent the general case of loops in a sequence of
mountpoints, but addresses a common special case in the same way
as other afs clients.

Signed-off-by: Marc Dionne <marc.dionne at auristor.com>
---
 fs/afs/mntpt.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index 97f50e9fd9eb..fb6eea31ff31 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -140,6 +140,11 @@ static int afs_mntpt_set_params(struct fs_context *fc, struct dentry *mntpt)
 		put_page(page);
 		if (ret < 0)
 			return ret;
+
+		/* Don't cross a backup volume mountpoint from a backup volume */
+		if (src_as->volume && src_as->volume->type == AFSVL_BACKVOL
+		    && ctx->type == AFSVL_BACKVOL)
+			return -ENODEV;
 	}
 
 	return 0;
-- 
2.45.0