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libauth: re-implement procsetuser() to use /proc instead of #c/user

--- a/sys/src/libauth/procsetuser.c

+++ b/sys/src/libauth/procsetuser.c

@@ -5,16 +5,30 @@

 int

 procsetuser(char *user)

 {

-	int fd, n;

+	char name[32];

+	Dir dir;

-	fd = open("#c/user", OWRITE|OCEXEC);

-	if(fd < 0)

-		return -1;

-	n = strlen(user);

-	if(write(fd, user, n) != n){

+	nulldir(&dir);

+	dir.uid = user;

+	snprint(name, sizeof(name), "/proc/%lud/ctl", (ulong)getpid());

+	if(dirwstat(name, &dir) < 0){

+		/*

+		 * this is backwards compatibility code as

+		 * devproc initially didnt allow changing

+		 * the user to none.

+		 */

+		int fd;

+

+		if(strcmp(user, "none") != 0)

+			return -1;

+		fd = open("#c/user", OWRITE|OCEXEC);

+		if(fd < 0)

+			return -1;

+		if(write(fd, "none", 4) != 4){

+			close(fd);

+			return -1;

+		}

 		close(fd);

-		return -1;

 	}

-	close(fd);

 	return 0;

 }