In preparation for raising NGROUPS and NGROUPS_MAX, change base

system callers of getgroups(), getgrouplist(), and setgroups() to
allocate buffers dynamically.  Specifically, allocate a buffer of size
sysconf(_SC_NGROUPS_MAX)+1 (+2 in a few cases to allow for overflow).

This (or similar gymnastics) is required for the code to actually follow
the POSIX.1-2008 specification where {NGROUPS_MAX} may differ at runtime
and where getgroups may return {NGROUPS_MAX}+1 results on systems like
FreeBSD which include the primary group.

In id(1), don't pointlessly add the primary group to the list of all
groups, it is always the first result from getgroups().  In principle
the old code was more portable, but this was only done in one of the two
places where getgroups() was called to the overall effect was pointless.

Document the actual POSIX requirements in the getgroups(2) and
setgroups(2) manpages.  We do not yet support a dynamic NGROUPS, but we
may in the future.

MFC after:	2 weeks

1 files changed, 6 insertions, 2 deletions

diff --git a/usr.sbin/chown/chown.c b/usr.sbin/chown/chown.c
index 0918265..b79deca 100644
--- a/usr.sbin/chown/chown.c
+++ b/usr.sbin/chown/chown.c
@@ -269,7 +269,8 @@ chownerr(const char *file)
 {
 	static uid_t euid = -1;
 	static int ngroups = -1;
-	gid_t groups[NGROUPS_MAX];
+	static long ngroups_max;
+	gid_t *groups;
 
 	/* Check for chown without being root. */
 	if (errno != EPERM || (uid != (uid_t)-1 &&
@@ -281,7 +282,10 @@ chownerr(const char *file)
 	/* Check group membership; kernel just returns EPERM. */
 	if (gid != (gid_t)-1 && ngroups == -1 &&
 	    euid == (uid_t)-1 && (euid = geteuid()) != 0) {
-		ngroups = getgroups(NGROUPS_MAX, groups);
+		ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1;
+		if ((groups = malloc(sizeof(gid_t) * ngroups_max)) == NULL)
+			err(1, "malloc");
+		ngroups = getgroups(ngroups_max, groups);
 		while (--ngroups >= 0 && gid != groups[ngroups]);
 		if (ngroups < 0) {
 			warnx("you are not a member of group %s", gname);