nfsd: Add support for the birth time attribute

For filesystems that supports "btime" timestamp (i.e. most modern
filesystems do) we share it via kernel nfsd. Btime support for NFS
client has already been added by Trond recently.
Suggested-by: Bruce Fields <bfields@fieldses.org>
Signed-off-by: Ondrej Valousek <ondrej.valousek.xm@renesas.com>
[ cel: addressed some whitespace/checkpatch nits ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -2854,6 +2854,9 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 	err = vfs_getattr(&path, &stat, STATX_BASIC_STATS, AT_STATX_SYNC_AS_STAT);
 	if (err)
 		goto out_nfserr;
+	if (!(stat.result_mask & STATX_BTIME))
+		/* underlying FS does not offer btime so we can't share it */
+		bmval1 &= ~FATTR4_WORD1_TIME_CREATE;
 	if ((bmval0 & (FATTR4_WORD0_FILES_AVAIL | FATTR4_WORD0_FILES_FREE |
 			FATTR4_WORD0_FILES_TOTAL | FATTR4_WORD0_MAXNAME)) ||
 	    (bmval1 & (FATTR4_WORD1_SPACE_AVAIL | FATTR4_WORD1_SPACE_FREE |
@@ -3254,6 +3257,13 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 		p = xdr_encode_hyper(p, (s64)stat.mtime.tv_sec);
 		*p++ = cpu_to_be32(stat.mtime.tv_nsec);
 	}
+	if (bmval1 & FATTR4_WORD1_TIME_CREATE) {
+		p = xdr_reserve_space(xdr, 12);
+		if (!p)
+			goto out_resource;
+		p = xdr_encode_hyper(p, (s64)stat.btime.tv_sec);
+		*p++ = cpu_to_be32(stat.btime.tv_nsec);
+	}
 	if (bmval1 & FATTR4_WORD1_MOUNTED_ON_FILEID) {
 		struct kstat parent_stat;
 		u64 ino = stat.ino;

fs/nfsd/nfsd.h

@@ -364,7 +364,7 @@ void		nfsd_lockd_shutdown(void);
  | FATTR4_WORD1_OWNER	        | FATTR4_WORD1_OWNER_GROUP  | FATTR4_WORD1_RAWDEV           \
  | FATTR4_WORD1_SPACE_AVAIL     | FATTR4_WORD1_SPACE_FREE   | FATTR4_WORD1_SPACE_TOTAL      \
  | FATTR4_WORD1_SPACE_USED      | FATTR4_WORD1_TIME_ACCESS  | FATTR4_WORD1_TIME_ACCESS_SET  \
- | FATTR4_WORD1_TIME_DELTA   | FATTR4_WORD1_TIME_METADATA    \
+ | FATTR4_WORD1_TIME_DELTA      | FATTR4_WORD1_TIME_METADATA   | FATTR4_WORD1_TIME_CREATE      \
  | FATTR4_WORD1_TIME_MODIFY     | FATTR4_WORD1_TIME_MODIFY_SET | FATTR4_WORD1_MOUNTED_ON_FILEID)
 
 #define NFSD4_SUPPORTED_ATTRS_WORD2 0