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, 5 insertions, 2 deletions

diff --git a/share/man/man4/linux.4 b/share/man/man4/linux.4
index 4fd95d10894d..edd582e357be 100644
--- a/share/man/man4/linux.4
+++ b/share/man/man4/linux.4
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 8, 2010
+.Dd August 1, 2017
 .Dt LINUX 4
 .Os
 .Sh NAME
@@ -127,9 +127,11 @@ regardless of whether the
 module is statically linked into the kernel
 or loaded as a module.
 .Sh FILES
-.Bl -tag -width /compat/linux/proc -compact
+.Bl -tag -width /compat/linux/dev/fd -compact
 .It Pa /compat/linux
 minimal Linux run-time environment
+.It Pa /compat/linux/dev/fd
+limited Linux file-descriptor file system
 .It Pa /compat/linux/proc
 limited Linux process file system
 .It Pa /compat/linux/sys
@@ -138,6 +140,7 @@ limited Linux system file system
 .Sh SEE ALSO
 .Xr brandelf 1 ,
 .Xr elf 5 ,
+.Xr fdescfs 5 ,
 .Xr linprocfs 5 ,
 .Xr linsysfs 5
 .Sh HISTORY