ntpshm as non-root

[Håkan Johansson-5]

Hi again,

I'd like to run gpsd under a non-root user and use the ntpshm interface to
set the system clock.  For that, I'd use ntpshm segments 2 and 3 which are
opened for everyone by ntpd.  However, gpsd.c tests for getuid() == 0, and
will not even try to setup with segments 2 and 3 for non-root users.  The
attached patch changes that, such that only segments 0 and 1 are avoided
for non-root users.  It also corrects the error handling for the
nice(NICEVAL) call.

Cheers,
Håkan
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[Eric S. Raymond-2]

Håkan Johansson <f96hajo@...>:
> I'd like to run gpsd under a non-root user and use the ntpshm interface
> to set the system clock.  For that, I'd use ntpshm segments 2 and 3 which
> are opened for everyone by ntpd.  However, gpsd.c tests for getuid() ==
> 0, and will not even try to setup with segments 2 and 3 for non-root
> users.  The attached patch changes that, such that only segments 0 and 1
> are avoided for non-root users.  It also corrects the error handling for
> the nice(NICEVAL) call.

This, unlike some of the other changes in ntpshm.c discussed recenely,
seems to me like a relatively safe it-will-work-or-it=won't sort of chanmge.
But I didn't see an actual patch attached.
--
                <a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
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[Håkan Johansson-5]

Sorry,
Håkan


On Wed, 4 Nov 2009, Eric S. Raymond wrote:

> Håkan Johansson <f96hajo@...>:
>> I'd like to run gpsd under a non-root user and use the ntpshm interface
>> to set the system clock.  For that, I'd use ntpshm segments 2 and 3 which
>> are opened for everyone by ntpd.  However, gpsd.c tests for getuid() ==
>> 0, and will not even try to setup with segments 2 and 3 for non-root
>> users.  The attached patch changes that, such that only segments 0 and 1
>> are avoided for non-root users.  It also corrects the error handling for
>> the nice(NICEVAL) call.
>
> This, unlike some of the other changes in ntpshm.c discussed recenely,
> seems to me like a relatively safe it-will-work-or-it=won't sort of chanmge.
> But I didn't see an actual patch attached.
> --
> <a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
> _______________________________________________
> Gpsd-dev mailing list
> Gpsd-dev@...
> https://lists.berlios.de/mailman/listinfo/gpsd-dev
>

[ntpshm_getuid.diff]

---

Index: gpsd.c
===================================================================
--- gpsd.c (revision 6410)
+++ gpsd.c (arbetskopia)
@@ -1907,14 +1907,10 @@
     gpsd_report(LOG_INF, "listening on port %s\n", gpsd_service);
 
 #ifdef NTPSHM_ENABLE
-    if (getuid() == 0) {
- errno = 0;
- if (nice(NICEVAL) != -1 || errno == 0)
-    gpsd_report (2, "Priority setting failed.\n");
- (void)ntpshm_init(&context, nowait);
-    } else {
- gpsd_report (LOG_INF, "Unable to start ntpshm.  gpsd must run as root.\n");
-    }
+    errno = 0;
+    if (nice(NICEVAL) == -1 && errno != 0)
+ gpsd_report (LOG_INF, "Priority setting failed.\n");
+    (void)ntpshm_init(&context, nowait);
 #endif /* NTPSHM_ENABLE */
 
 #ifdef DBUS_ENABLE
Index: ntpshm.c
===================================================================
--- ntpshm.c (revision 6410)
+++ ntpshm.c (arbetskopia)
@@ -6,6 +6,9 @@
  */
 
 #include <sys/types.h>
+#ifndef S_SPLINT_S
+#include <unistd.h>
+#endif /* S_SPLINT_S */
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
@@ -72,7 +75,7 @@
     int i;
 
     for (i = 0; i < NTPSHMSEGS; i++)
- context->shmTime[i] = getShmTime(i);
+ context->shmTime[i] = (i >= 2 || getuid() == 0) ? getShmTime(i) : NULL;
     memset(context->shmTimeInuse,0,sizeof(context->shmTimeInuse));
 # ifdef PPS_ENABLE
     context->shmTimePPS = enablepps;

---

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[Gary E. Miller]

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Hash: SHA1

Yo Håkan

If you start ntpd first then it will setup the required segments. gpsd
can make them just as a backup, but in the real world that is rarely
used.  So no need for most of your patch.

Check out the gpsd-dev archives for more discussion.

RGDS
GARY
- ---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        gem@...  Tel:[phone]

On Wed, 4 Nov 2009, Håkan Johansson wrote:

> I'd like to run gpsd under a non-root user and use the ntpshm interface to set
> the system clock.  For that, I'd use ntpshm segments 2 and 3 which are opened
> for everyone by ntpd.  However, gpsd.c tests for getuid() == 0, and will not
> even try to setup with segments 2 and 3 for non-root users.  The attached
> patch changes that, such that only segments 0 and 1 are avoided for non-root
> users.  It also corrects the error handling for the nice(NICEVAL) call.
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[Håkan Johansson-5]

Hi Gary,

the problem is gpcd.c, line 1910, which prevents it from even trying as
non-root:

#ifdef NTPSHM_ENABLE
     if (getuid() == 0) {
         errno = 0;
         if (nice(NICEVAL) != -1 || errno == 0)
             gpsd_report (2, "Priority setting failed.\n");
         (void)ntpshm_init(&context, nowait);
     } else {
         gpsd_report (LOG_INF, "Unable to start ntpshm.  gpsd must run as root.\n");
     }
#endif /* NTPSHM_ENABLE */

The only reason for patching in ntpshm.c was to avoid the spurios error
messages when gpsd cannot (and should not) create segments 0 and 1 as
non-root.  Their creation would be wrong in case gpsd happen to be started
before ntpd.  Is that the not needed part?

Regards,
Håkan


On Thu, 5 Nov 2009, Gary E. Miller wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Yo Håkan
>
> If you start ntpd first then it will setup the required segments. gpsd
> can make them just as a backup, but in the real world that is rarely
> used.  So no need for most of your patch.
>
> Check out the gpsd-dev archives for more discussion.
>
> RGDS
> GARY
> - ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
> gem@...  Tel:[phone]
>
> On Wed, 4 Nov 2009, Håkan Johansson wrote:
>
>> I'd like to run gpsd under a non-root user and use the ntpshm interface to set
>> the system clock.  For that, I'd use ntpshm segments 2 and 3 which are opened
>> for everyone by ntpd.  However, gpsd.c tests for getuid() == 0, and will not
>> even try to setup with segments 2 and 3 for non-root users.  The attached
>> patch changes that, such that only segments 0 and 1 are avoided for non-root
>> users.  It also corrects the error handling for the nice(NICEVAL) call.
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.3 (GNU/Linux)
>
> iD8DBQFK8i2dBmnRqz71OvMRAsPbAJ9xeHfEf4/6kvKD4WmjwhzjQzeQWgCeNQBQ
> gjrEjtABVexNpwsd3Bet5Bs=
> =pS8u
> -----END PGP SIGNATURE-----
>

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