[PATCH] fhandle: avoid -EINVAL if requested size is too large.

[NeilBrown]

Lennart Poettering observes that if the newly increased
MAX_HANDLE_SZ is exported to user space, and then used in an
application running on an old kernel, name_to_handle_at() will
report -EINVAL, which is unhelpful and inconsistent with
the documentation.

So:
 1/ add a comment making it clear that the new value must
    not be exposed to user-space.
 2/ remove the completely unnecessary restriction on the size
    of buffer provided by the application.

Reported-by: Lennart Poettering <lennart@xxxxxxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

This is a followup to the previous fhandle-for-Amazon-EFS
patch which adds further refinements.

Thanks,
NeilBrown


 fs/fhandle.c             | 2 +-
 include/linux/exportfs.h | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 0ace128f5d23..30bf1c20e143 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -35,7 +35,7 @@ static long do_sys_name_to_handle(struct path *path,
 		return -EFAULT;
 
 	if (f_handle.handle_bytes > MAX_HANDLE_SZ)
-		return -EINVAL;
+		f_handle.handle_bytes = MAX_HANDLE_SZ;
 
 	handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
 			 GFP_KERNEL);
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index e7ab1b071c5e..097704e80c89 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -14,6 +14,10 @@ struct vfsmount;
 /* Must be larger than NFSv4 file handle, but small
  * enough for an on-stack allocation. overlayfs doesn't
  * want this too close to 255.
+ * NOTE: This value MUST NOT be exported to user-space.
+ * Applications must only ever see MAX_HANDLE_SZ == 128.
+ * If they try a larger number on older kernels, they
+ * will get -EINVAL which will be confusing.
  */
 #define MAX_HANDLE_SZ 200
 
-- 
2.14.0.rc0.dirty

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