pivot_root.2: Add some more detail to the remaining EBUSY error

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/pivot_root.2

@@ -115,7 +115,28 @@ may fail with any of the same errors as
 Additionally, it may fail with the following errors:
 .TP
 .B EBUSY
-\fInew_root\fP or \fIput_old\fP are on the current root filesystem.
+.\" Reconfirmed that the following error occurs on Linux 5.0 by
+.\" specifying 'new_root' as "/rootfs" and 'put_old' as
+.\" "/rootfs/oldrootfs", and *not* bind mounting "/rootfs" on top of
+.\" itself. Of course, this is an odd situation, since a later check
+.\" in the kernel code will in any case yield EINVAL if 'new_root' is
+.\" not a mount point. However, when the system call was first added,
+.\" 'new_root' was not required to be a mount point. So, this this
+.\" error is nowadays probably just the result of crufty accumulation.
+.\" This error can also occur if we bind mount "/" on top of itself
+.\" and try to specify "/" as the 'new' (again, an odd situation). So,
+.\" the EBUSY check in the kernel does still seem necessary to prevent
+.\" that case.  Furthermore, the "or put_old" piece is probably
+.\" redundant text (although the check is in the kernel), since,
+.\" in another check, 'put_old' is required to be under 'new_root'.
+.I new_root
+or
+.I put_old
+is on the current root filesystem.
+(This error covers the pathological case where
+.I new_root
+is
+.IR """/""" .)
 .TP
 .B EINVAL
 .I new_root