Re: [RFC] Groups beyond 32

[H. Peter Anvin]

Followup to:  <1026936556.25347.48.camel@UberGeek>
By author:    Austin Gonyou <[e-mail]>
In newsgroup: linux.dev.kernel
> 
> The problem now is more one of maintenance. Most distributions do not
> support groups > 32 AFAIK. So, it's lead me to ask the following
> questions:
> 
> 1. Why, in general, is the limit so low? 
>    For specific application, mainly auditing and such, this would be    
>    advantageous I think.
> 

Mostly to cap the amount of storage to maintain in kernel space, and
for historical reasons.

> 2. What is required to limit the dependence on groups to just GLIBC or
> just the kernel? Is that even possible?

The main problem is programs who do things like:

gid_t mygroups[NGROUPS];

Other than that, it should all be in kernel space.  According to
POSIX, the NGROUPS above really should be sysconf(_SC_NGROUPS_MAX) --
NGROUPS_MAX is defined as a *guaranteed minimum* of
sysconf(_SC_NGROUPS_MAX).  Obviously there needs to be a kernel ->
libc interface for the sysconf.

FWIW, POSIX specifies:

      Application writers should note that {NGROUPS_MAX} is not
      necessarily a constant on all implementations.

(glibc has #define NGROUPS NGROUPS_MAX).

> 3. Is there any true advantage to supporting more than 32 groups, or
> creating "meta-groups" to get around the problem? 

There probably is.

      -hpa
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