Implement proper Linux /dev/fd and /proc/self/fd behavior by adding

Linux specific things to the native fdescfs file system.

Unlike FreeBSD, the Linux fdescfs is a directory containing a symbolic
links to the actual files, which the process has open.
A readlink(2) call on this file returns a full path in case of regular file
or a string in a special format (type:[inode], anon_inode:<file-type>, etc..).
As well as in a FreeBSD, opening the file in the Linux fdescfs directory is
equivalent to duplicating the corresponding file descriptor.

Here we have mutually exclusive requirements:
- in case of readlink(2) call fdescfs lookup() method should return VLNK
vnode otherwise our kern_readlink() fail with EINVAL error;
- in the other calls fdescfs lookup() method should return non VLNK vnode.

For what new vnode v_flag VV_READLINK was added, which is set if fdescfs has beed
mounted with linrdlnk option an modified kern_readlinkat() to properly handle it.

For now For Linux ABI compatibility mount fdescfs volume with linrdlnk option:

    mount -t fdescfs -o linrdlnk null /compat/linux/dev/fd

Reviewed by:	kib@
MFC after:	1 week
Relnotes:	yes

1 files changed, 15 insertions, 1 deletions

diff --git a/share/man/man5/fdescfs.5 b/share/man/man5/fdescfs.5
index 30e72a5f3b1c..3f16104c0ac8 100644
--- a/share/man/man5/fdescfs.5
+++ b/share/man/man5/fdescfs.5
@@ -34,7 +34,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 18, 2010
+.Dd August 1, 2017
 .Dt FDESCFS 5
 .Os
 .Sh NAME
@@ -92,6 +92,14 @@ and
 files are created by default when devfs alone is mounted.
 .Nm
 creates entries for all file descriptors opened by the process.
+.Pp
+For
+.Xr linux 4
+ABI compatibility mount
+.Nm
+volume with
+.Cm linrdlnk
+option.
 .Sh FILES
 .Bl -tag -width /dev/stderr -compact
 .It Pa /dev/fd/#
@@ -103,6 +111,12 @@ volume located on
 .Pa /dev/fd :
 .Pp
 .Dl "mount -t fdescfs null /dev/fd"
+.Pp
+For
+.Xr linux 4
+ABI compatibility:
+.Pp
+.Dl "mount -t fdescfs -o linrdlnk null /compat/linux/dev/fd"
 .Sh SEE ALSO
 .Xr devfs 5 ,
 .Xr mount 8