The recent addition of NFS re-export and the possibility of using name_to_handle_at() on an NFS filesystem raises issues with name_to_handle_at() which have not been properly documented. Getting the filehandle for an untriggered automount point is arguably meaningless and in certainly not supported by NFS. name_to_handle_at() will return -EOVERFLOW even though the requested "handle_bytes" is large enough. This is an unfortunate overloading of the error code, but is manageable. So clarify this and also note that the mount_id is returned when EOVERFLOW is reported. Thought: it would be nice if mount_id were returned in the EOPNOTSUPP case too. I guess it is too late to fix that (?). Link: https://github.com/systemd/systemd/issues/7082 Signed-off-by: NeilBrown <neilb@xxxxxxxx> --- man2/open_by_handle_at.2 | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/man2/open_by_handle_at.2 b/man2/open_by_handle_at.2 index 91300e80fe24..9a30dd2ed476 100644 --- a/man2/open_by_handle_at.2 +++ b/man2/open_by_handle_at.2 @@ -109,6 +109,12 @@ and is set to indicate the required size; the caller can then use this information to allocate a structure of the correct size (see EXAMPLE below). +Some care is needed here as +.BR EOVERFLOW +can also indicate that no filehandle is available for this particular +name in a filesystem which does normally support filehandle lookup. +This case can be detected when EOVERFLOW is returned without +handle_bytes being increased. .PP Other than the use of the .IR handle_bytes @@ -193,6 +199,10 @@ Opening the pathname in the fifth field of that record yields a file descriptor for the mount point; that file descriptor can be used in a subsequent call to .BR open_by_handle_at (). +.I mount_id +is returned both for a successful call and for a call that results +in the error +.BR EOVERFLOW . .PP By default, .BR name_to_handle_at () @@ -206,6 +216,21 @@ is specified in .I pathname is dereferenced if it is a symbolic link (so that the call returns a handle for the file referred to by the link). +.PP +.BR name_to_handle_at ) +does not trigger a mount when the final component of the path is an +automount point. When a filesystem supports both filehandles and +automount points, a +.BR name_to_handle_at () +call on an automount point will return with error +.BR EOVERFLOW +without having increased +.IR handle_bytes . +This can happen since Linux 4.13 with NFS when accessing a directory +which is on a separate filesystem on the server. +.\" commit 20fa19027286983ab2734b5910c4a687436e0c31 +In this case the automount can be triggered by adding a "/" to the end +of the path. .SS open_by_handle_at() The .BR open_by_handle_at () -- 2.14.0.rc0.dirty
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