Max groups one can be a member of linux/sched.h and NGROUPS_SMALL

Max groups one can be a member of linux/sched.h and NGROUPS_SMALL

[Joseph Pingenot]

Hello.

In my quest to try and figure out the max number of groups one can be a
  member of (and to learn more about the kernel internals), I stumbled
  across the following tidbit:

(excerpted from linux/sched.h)
#define NGROUPS_SMALL           32
#define NGROUPS_PER_BLOCK       ((int)(PAGE_SIZE / sizeof(gid_t)))
struct group_info {
        int ngroups;
        atomic_t usage;
        gid_t small_block[NGROUPS_SMALL];
        int nblocks;
        gid_t *blocks[0];
};

This seems to be the place where group information is stored (linked to from
  task_struct).

So, it appears to hold 32 gids, but what is this blocks bit?  Is 32 the max
  number of groups one can be a member of?

Thanks!

-Joseph

-- 
Joseph===============================================trelane@digitasaru.net
      Graduate Student in Physics, Freelance Free Software Developer

Re: Max groups one can be a member of linux/sched.h and NGROUPS_SMALL

[Tim Hockin]

On Thu, Oct 28, 2004 at 01:02:30PM -0500, Joseph Pingenot wrote:
> #define NGROUPS_SMALL           32
> #define NGROUPS_PER_BLOCK       ((int)(PAGE_SIZE / sizeof(gid_t)))
> struct group_info {
>         int ngroups;
>         atomic_t usage;
>         gid_t small_block[NGROUPS_SMALL];
>         int nblocks;
>         gid_t *blocks[0];
> };

> So, it appears to hold 32 gids, but what is this blocks bit?  Is 32 the max
>   number of groups one can be a member of?

By default, every task has enough room for 32 gids.  If you need more than
32, it uses a dynamically allocated 2-d array, stored in blocks.  Always
use the GROUP_AT() macro, and you can treat it like a 1-d array.  I think
there is a hardlimit of 64k groups, but that is simply a #define that can
be changed.  You can have as many groups as you need, dynamically
allocated per-task, with CoW between tasks.  Sorted and bsearch()ed.