[PATCH] NFS: allow name_to_handle_at() to work for Amazon EFS.

[NeilBrown]

Amazon EFS provides an NFSv4.1 filesystem which appears to use
(close to) full length (128 byte) file handles.
This causes the handle reported by name_to_handle_at() to exceed
MAX_HANDLE_SZ, resulting in
  EOVERFLOW if 128 bytes were requested, or
  EINVAL if the size reported by the previous call was requested.

To fix this, increase MAX_HANDLE_SIZE a little, and add a BUILD_BUG
so that this sort of inconsistent error reporting won't happen again.

Link: https://github.com/systemd/systemd/issues/7082#issuecomment-347380436
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

Sorry for the scatter-gun To: list.  This is really more of a VFS patch
than an NFS patch and sending patches in either direction has been a bit
hit-and-miss lately, so I'm hoping Andrew will take it unless someone
else objects or beats him to it...

Thanks,
NeilBrown

 fs/nfs/export.c          | 2 ++
 include/linux/exportfs.h | 7 +++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/export.c b/fs/nfs/export.c
index 83fd09fc8f77..23b2fc3ab2bb 100644
--- a/fs/nfs/export.c
+++ b/fs/nfs/export.c
@@ -39,6 +39,8 @@ nfs_encode_fh(struct inode *inode, __u32 *p, int *max_len, struct inode *parent)
 	size_t fh_size = offsetof(struct nfs_fh, data) + server_fh->size;
 	int len = EMBED_FH_OFF + XDR_QUADLEN(fh_size);
 
+	BUILD_BUG_ON_MSG(EMBED_FH_OFF + NFS_MAXFHSIZE > MAX_HANDLE_SZ,
+		"MAX_HANDLE_SZ is too small");
 	dprintk("%s: max fh len %d inode %p parent %p",
 		__func__, *max_len, inode, parent);
 
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 0d3037419bc7..e7ab1b071c5e 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -11,8 +11,11 @@ struct iomap;
 struct super_block;
 struct vfsmount;
 
-/* limit the handle size to NFSv4 handle size now */
-#define MAX_HANDLE_SZ 128
+/* Must be larger than NFSv4 file handle, but small
+ * enough for an on-stack allocation. overlayfs doesn't
+ * want this too close to 255.
+ */
+#define MAX_HANDLE_SZ 200
 
 /*
  * The fileid_type identifies how the file within the filesystem is encoded.
-- 
2.14.0.rc0.dirty

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