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

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@suse.com>
---

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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