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, 2 insertions, 0 deletions

diff --git a/sys/fs/fdescfs/fdesc_vfsops.c b/sys/fs/fdescfs/fdesc_vfsops.c
index ef5fd5e4afab..f63eea35581c 100644
--- a/sys/fs/fdescfs/fdesc_vfsops.c
+++ b/sys/fs/fdescfs/fdesc_vfsops.c
@@ -101,6 +101,8 @@ fdesc_mount(struct mount *mp)
 	 */
 	mp->mnt_data = fmp;
 	fmp->flags = 0;
+	if (vfs_getopt(mp->mnt_optnew, "linrdlnk", NULL, NULL) == 0)
+		fmp->flags |= FMNT_LINRDLNKF;
 	error = fdesc_allocvp(Froot, -1, FD_ROOT, mp, &rvp);
 	if (error) {
 		free(fmp, M_FDESCMNT);