dma -- sending mail from non-root to non-root

[Michael Krauss <hippodriver@xxxxxxx> (by way of Michael Krauss <hippodriver@xxxxxxx>)]

Hello DragonFly experts,

I am running into problems when sending mail from one unprivileged user
to another with the DragonFly Mail Agent. 

Actually I want to run dma on Arch Linux. Porting the program itself
was not a problem at all, it is running now, but suffering the same
mail folder access problems as on DragonFly BSD. On Arch Linux it is
getting even worse as no mail folder is automatically created for a
new user account. Here is a protocol from DragonFly 1.12.2:

After creation of user bob:
gimli# ls -l /var/mail/bob
total 0
-rw-------  1 bob       bob       0 Sep  8 23:10 bob

Try to send a mail from non-root:
> /usr/libexec/dma -D bob

Hallo bob
dma[878]: d0.28299080: mail from=<mickraus@gimli.gondor> to=<bob>
dma[878]: d0.28299080: trying delivery
dma[878]: d0.28299080: local delivery deferred: can not open
`/var/mail/bob': Permission denied ^C

Try to send a mail as root:
gimli# /usr/libexec/dma -D bob

Hallo Bob
dma[887]: d4.28299080: mail from=<mickraus@gimli.gondor> to=<bob>
dma[887]: d4.28299080: trying delivery
dma[887]: d4.28299080: delivery successful
gimli# ls -l /var/mail/bob
total 2
-rw-------  1 bob       bob       215 Sep  8 23:14 bob

changing gid of /var/mail/bob to mail and switch to 660.
gimli# ls -l /var/mail/bob
-rw-rw----  1 bob  mail  0 Sep  8 23:17 /var/mail/bob

sending mail from non-root:
> /usr/libexec/dma -D bob

Hallo bob
dma[915]: d5.28299080: mail from=<mickraus@gimli.gondor> to=<bob>
dma[915]: d5.28299080: trying delivery
dma[915]: d5.28299080: delivery successful

Removing bobs mail folder and send a mail from non-root:
> /usr/libexec/dma -D bob

Hallo bob
dma[924]: d5.28299080: mail from=<mickraus@gimli.gondor> to=<bob>
dma[924]: d5.28299080: trying delivery
dma[924]: d5.28299080: delivery successful
> ls -l /var/mail/bob
--w-------  1 mickraus  mail  218 Sep  8 23:30 /var/mail/bob


I tried to fix the creation of the new mail folder in dma.c in function
deliver_local, but I can't change the file owner to the mail recipient
with fchown, only root could do that. Excerpt from dma.c function
deliver_local (only added code into the __linux block to not pollute 
your code):

	error = snprintf(fn, sizeof(fn), "%s/%s", _PATH_MAILDIR, it->addr);
	if (error < 0 || (size_t)error >= sizeof(fn)) {
		syslog(LOG_ERR, "%s: local delivery deferred: %m",
		       it->queueid);
		return (1);
	}

	/* mailx removes users mailspool file if empty, so open with O_CREAT */
#ifdef __linux
	if ((mbox = open(fn, O_WRONLY | O_APPEND)) == -1) {
	  /* file doesn't exist, create file and set owner properly. */
	  mbox = open(fn, O_WRONLY | O_APPEND | O_CREAT);
	  
	  if (fchmod(mbox, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP) == -1) {
	    syslog(LOG_ERR, "%s: local delivery deferred: can not set file "
		   "permissions properly for `%s': %m",
		   it->queueid, fn);
	    return (1); 
	  }
	  /* Setting owner and group to $recipient and mail */
	  uid_t uid;
	  gid_t gid;
	  uid = getpwnam(it->addr)->pw_uid;
	  gid = getgrnam("mail")->gr_gid;
	  if (fchown(mbox, uid, gid) == -1) { /* <---- here it breaks */
	   syslog(LOG_ERR, "%s: local delivery deferred: can not set file "
		   "owners properly for `%s': %m",
		   it->queueid, fn);
	    return (1);
	  } 
	}
	flock(mbox, LOCK_EX);
#else
	mbox = open(fn, O_WRONLY | O_EXLOCK | O_APPEND | O_CREAT);
#endif /* __linux */
	if (mbox < 0) {
		syslog(LOG_ERR, "%s: local delivery deferred: can not open `%s': %m",
		       it->queueid, fn);
		return (1);
	}
	mboxlen = lseek(mbox, 0, SEEK_CUR);


Is there an other way to create an appropriate mail folder for the recipient of a mail?
I don't want to set uid root on dma.

Kind regards,
Michael Krauss