With NFSv4 ACLs, it is possible that applying a mode to an ACL which

is identical to the mode computed from that ACL will modify the ACL.
For example, mode computed from the following ACL is 0600:

   user:kamila:rwx--------C--:------:allow
        owner@:--x-----------:------:deny
        owner@:rw-p---A-W-Co-:------:allow
        group@:rwxp----------:------:deny
        group@:--------------:------:allow
     everyone@:rwxp---A-W-Co-:------:deny
     everyone@:------a-R-c--s:------:allow

However, applying that mode (chmod 0600) changes the ACL into this:

   user:kamila:rwx-----------:------:deny
   user:kamila:rwx--------C--:------:allow
        owner@:--x-----------:------:deny
        owner@:rw-p---A-W-Co-:------:allow
        group@:rwxp----------:------:deny
        group@:--------------:------:allow
     everyone@:rwxp---A-W-Co-:------:deny
     everyone@:------a-R-c--s:------:allow

In chmod(1) utility, there is an optimisation, which makes it not
call chmod(2) if the mode of the file is the same as the new mode.
Disable that optimisation for files which may have NFSv4 ACLs.

Reviewed by:	rwatson
Approved by:	re (kib)

bin/chmod/chmod.c

@@ -54,6 +54,7 @@ __FBSDID("$FreeBSD$");
 #include <unistd.h>
 
 static void usage(void);
+static int may_have_nfs4acl(const FTSENT *ent);
 
 int
 main(int argc, char *argv[])
@@ -180,8 +181,14 @@ done:	argv += optind;
 			break;
 		}
 		newmode = getmode(set, p->fts_statp->st_mode);
-		if ((newmode & ALLPERMS) == (p->fts_statp->st_mode & ALLPERMS))
-			continue;
+		/*
+		 * With NFSv4 ACLs, it is possible that applying a mode
+		 * identical to the one computed from an ACL will change
+		 * that ACL.
+		 */
+		if (may_have_nfs4acl(p) == 0 &&
+		    (newmode & ALLPERMS) == (p->fts_statp->st_mode & ALLPERMS))
+				continue;
 		if ((*change_mode)(p->fts_accpath, newmode) && !fflag) {
 			warn("%s", p->fts_path);
 			rval = 1;
@@ -219,3 +226,24 @@ usage(void)
 	    "usage: chmod [-fhv] [-R [-H | -L | -P]] mode file ...\n");
 	exit(1);
 }
+
+static int
+may_have_nfs4acl(const FTSENT *ent)
+{
+	int ret;
+	static dev_t previous_dev = (dev_t)-1;
+	static int supports_acls = -1;
+
+	if (previous_dev != ent->fts_statp->st_dev) {
+		previous_dev = ent->fts_statp->st_dev;
+		supports_acls = 0;
+
+		ret = pathconf(ent->fts_accpath, _PC_ACL_NFS4);
+		if (ret > 0)
+			supports_acls = 1;
+		else if (ret < 0 && errno != EINVAL)
+			warn("%s", ent->fts_path);
+	}
+
+	return (supports_acls);
+}