chflags: Add a new UF_NOCACHE flag

This internet draft (which is close to being an RFC)
specifies a new NFSv4.2 attribute which tells the NFSv4.2
client to not cache file data. (Similar to O_DIRECT, but
triggered by this attribute set on the file on the NFSv4.2
server and not by the application's open(2).)

https://datatracker.ietf.org/doc/draft-ietf-nfsv4-uncacheable-files/

This patch adds a new chflags(1) flag called UF_NOCACHE to
implement this.

Patches for NFS and ZFS will be done separately.

This is a redo of the patch, with a requested name change
and a #ifdef in strtofflags.c so that it doesn't break some
Linux cross build.  The name change was requested by fuz@.

Reviewed by:	kib (earlier version)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D58181

lib/libc/gen/strtofflags.c

@@ -80,7 +80,13 @@ static struct {
 	{ "nosparse",		0, UF_SPARSE	},
 	{ "nousparse",		0, UF_SPARSE	},
 	{ "nosystem",		0, UF_SYSTEM	},
+	/* Needed for some Linux cross-build that uses a stale stat.h. */
+#ifdef UF_NOCACHE
+	{ "nousystem",		0, UF_SYSTEM	},
+	{ "noucache",		1, UF_NOCACHE	}
+#else
 	{ "nousystem",		0, UF_SYSTEM	}
+#endif
 };
 #define nmappings	(sizeof(mapping) / sizeof(mapping[0]))
 

sys/fs/tmpfs/tmpfs_subr.c

@@ -2042,7 +2042,7 @@ tmpfs_chflags(struct vnode *vp, u_long flags, struct ucred *cred,
 	if ((flags & ~(SF_APPEND | SF_ARCHIVED | SF_IMMUTABLE | SF_NOUNLINK |
 	    UF_APPEND | UF_ARCHIVE | UF_HIDDEN | UF_IMMUTABLE | UF_NODUMP |
 	    UF_NOUNLINK | UF_OFFLINE | UF_OPAQUE | UF_READONLY | UF_REPARSE |
-	    UF_SPARSE | UF_SYSTEM)) != 0)
+	    UF_SPARSE | UF_SYSTEM | UF_NOCACHE)) != 0)
 		return (EOPNOTSUPP);
 
 	/* Disallow this operation if the file system is mounted read-only. */

sys/sys/stat.h

@@ -327,6 +327,7 @@ struct nstat {
 #define	UF_REPARSE	0x00000400	/* Windows reparse point file bit */
 #define	UF_ARCHIVE	0x00000800	/* file needs to be archived */
 #define	UF_READONLY	0x00001000	/* Windows readonly file bit */
+#define	UF_NOCACHE	0x00002000	/* don't cache file data (NFSv4) */
 /* This is the same as the MacOS X definition of UF_HIDDEN. */
 #define	UF_HIDDEN	0x00008000	/* file is hidden */
 

sys/ufs/ufs/ufs_vnops.c

@@ -667,7 +667,7 @@ ufs_setattr(
 		    SF_NOUNLINK | SF_SNAPSHOT | UF_APPEND | UF_ARCHIVE |
 		    UF_HIDDEN | UF_IMMUTABLE | UF_NODUMP | UF_NOUNLINK |
 		    UF_OFFLINE | UF_OPAQUE | UF_READONLY | UF_REPARSE |
-		    UF_SPARSE | UF_SYSTEM)) != 0)
+		    UF_SPARSE | UF_SYSTEM | UF_NOCACHE)) != 0)
 			return (EOPNOTSUPP);
 		if (vp->v_mount->mnt_flag & MNT_RDONLY)
 			return (EROFS);