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

    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@poettering.net>
    Signed-off-by: NeilBrown <neilb@suse.com>
    ---

    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
    [unhandled content-type:application/pgp-signature]
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