kcmp: Add KCMP_EPOLL_TFD description

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/kcmp.2

@@ -156,6 +156,44 @@ See the discussion of the
 .BR CLONE_VM
 flag in
 .BR clone (2).
+.TP
+.BR KCMP_EPOLL_TFD
+Check whether the file descriptor
+.I idx1
+of the process
+.I pid1
+present in
+.BR epoll (7)
+instance described by
+.I idx2
+of the process
+.IR pid2 .
+The argument
+.I idx2
+is an address of a structure where the target file is described.
+.in +4n
+.nf
+.sp
+struct kcmp_epoll_slot {
+	__u32 efd;
+	__u32 tfd;
+	__u64 toff;
+};
+.fi
+.in
+.sp
+where
+.I efd
+is epoll file descriptor from
+.BR epoll_create (2)
+call,
+.I tfd
+is a target file number,
+.I toff
+is a target file offset counted from zero.
+Several different targets may be registered with
+the same file descriptor number and setting specific
+offset helps to investigate each of them.
 .PP
 Note the
 .BR kcmp ()
@@ -246,6 +284,16 @@ Process
 or
 .I pid2
 does not exist.
+.TP
+.B EFAULT
+The epoll slot addressed by
+.I idx2
+is outside of the user's address space.
+.TP
+.B ENOENT
+The target file is not present in
+.BR epoll ()
+instance.
 .SH VERSIONS
 The
 .BR kcmp ()