proc: pid/status: show all supplementary groups

commit 8d238027b87e654be552eabdf492042a34c5c300 upstream.

We display a list of supplementary group for each process in
/proc/<pid>/status.  However, we show only the first 32 groups, not all of
them.

Although this is rare, but sometimes processes do have more than 32
supplementary groups, and this kernel limitation breaks user-space apps
that rely on the group list in /proc/<pid>/status.

Number 32 comes from the internal NGROUPS_SMALL macro which defines the
length for the internal kernel "small" groups buffer.  There is no
apparent reason to limit to this value.

This patch removes the 32 groups printing limit.

The Linux kernel limits the amount of supplementary groups by NGROUPS_MAX,
which is currently set to 65536.  And this is the maximum count of groups
we may possibly print.

Change-Id: Iee760113e5e6d79fba0952b56224d2668236c939
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Qiang Huang <h.huangqiang@huawei.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Jianguo Wu <wujianguo@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>