[Savannah-hackers-public] Difference between sysconf(_SC_NGROUPS_MAX) and /proc/sys/kernel/ngroups_max

[Sylvain Beucler]

Package: glibc
Version: 2.3.2.ds1-22
Severity: important

There is a difference between sysconf(_SC_NGROUPS_MAX) and
/proc/sys/kernel/ngroups_max:

---
#include <stdio.h>
#include <unistd.h>

int main(void) {
  printf("%d\n", sysconf(_SC_NGROUPS_MAX));
  return 0;
}
---
=> 32

$ cat /proc/sys/kernel/ngroups_max
=> 65536


This applies under stable and testing, unstable works properly :)


More details: I sent this bug report to the shadow/passwd people, but
they think the problem comes from the difference above.

====
We would really need to get more groups per users in our stable
installation.

The stable kernel has a limit of 65536 groups per users:
$ cat /proc/sys/kernel/ngroups_max
65536

However usermod is still limited to 32 users:
$ usermod -G 
testgroup1,testgroup2,testgroup3,testgroup4,testgroup5,testgroup6,testgroup7,testgroup8,testgroup9,testgroup10,testgroup11,testgroup12,testgroup13,testgroup14,testgroup15,testgroup16,testgroup17,testgroup18,testgroup19,testgroup20,testgroup21,testgroup22,testgroup23,testgroup24,testgroup25,testgroup26,testgroup27,testgroup28,testgroup29,testgroup30,testgroup31,testgroup32,testgroup33,testgroup34,testgroup35,testgroup36,testgroup37,testgroup38,testgroup39,testgroup40,testgroup41,testgroup42,testgroup43,testgroup44,testgroup45,testgroup46,testgroup47,testgroup48,testgroup49,testgroup50,testgroup51,testgroup52,testgroup53,testgroup54,testgroup55,testgroup56,testgroup57,testgroup58,testgroup59,testgroup60,testgroup61,testgroup62,testgroup63,testgroup64,testgroup65,testgroup66,testgroup67,testgroup68,testgroup69,testgroup70,testgroup71,testgroup72,testgroup73,testgroup74,testgroup75,testgroup76,testgroup77,testgroup78,testgroup79,testgroup80,testgroup81,testgroup82,testgroup83,testgroup84,testgroup85,testgroup86,testgroup87,testgroup88,testgroup89,testgroup90,testgroup91,testgroup92,testgroup93,testgroup94,testgroup95,testgroup96,testgroup97,testgroup98,testgroup99,testgroup100
 test
usermod: too many groups specified (max 32).



Incidentally 'id' is also limited to 32
users. sysconf(_SC_NGROUPS_MAX) also returns 32.

stable's /usr/include/linux/limits.h has NGROUPS_MAX set to 32. In
testing and unstable it is set to 65536.


The usermod limitation happens in stable and testing (tested with
their respective default kernel), but not in unstable (tested with the
testing kernel - all tested kernels having ngroups_max=65536).


Basically we have a kernel without the limitation, but the executables
have the limitation, which was probably set at compile time.


Do you know what's the origin of the limitation, and how it could be
fixed to match the default kernel's?

-- 
Sylvain
GNU Savannah: http://savannah.gnu.org
====