From d4e0d4d92e010b74404bddc87c1a1e8dbcaa25dd Mon Sep 17 00:00:00 2001
From: Olivier Certner <olce@FreeBSD.org>
Date: Wed, 2 Oct 2024 16:16:20 +0200
Subject: [PATCH] cred: New crsetgroups_fallback()

Similar to crsetgroups(), but allows an empty group array in input,
treating it like a one-element array containing the passed fallback
group.

Approved by:    markj (mentor)
MFC after:      3 days
Differential Revision:  https://reviews.freebsd.org/D46917
---
 sys/kern/kern_prot.c | 18 ++++++++++++++++++
 sys/sys/ucred.h      |  2 ++
 2 files changed, 20 insertions(+)

diff --git a/sys/kern/kern_prot.c b/sys/kern/kern_prot.c
index 0f49d000dfd..504c7931340 100644
--- a/sys/kern/kern_prot.c
+++ b/sys/kern/kern_prot.c
@@ -2487,6 +2487,24 @@ crsetgroups(struct ucred *cr, int ngrp, const gid_t *groups)
 	groups_normalize(&cr->cr_ngroups, cr->cr_groups);
 }
 
+/*
+ * Same as crsetgroups() but accepts an empty groups array.
+ *
+ * This function ensures that an effective GID is always present in credentials.
+ * An empty array is treated as a one-size one holding the passed effective GID
+ * fallback.
+ */
+void
+crsetgroups_fallback(struct ucred *cr, int ngrp, const gid_t *groups,
+    const gid_t fallback)
+{
+	if (ngrp == 0)
+		/* Shortcut. */
+		crsetgroups_internal(cr, 1, &fallback);
+	else
+		crsetgroups(cr, ngrp, groups);
+}
+
 /*
  * Get login name, if available.
  */
diff --git a/sys/sys/ucred.h b/sys/sys/ucred.h
index b8a3b609aba..75156cca09a 100644
--- a/sys/sys/ucred.h
+++ b/sys/sys/ucred.h
@@ -167,6 +167,8 @@ void	crcowfree(struct thread *td);
 void	cru2x(struct ucred *cr, struct xucred *xcr);
 void	cru2xt(struct thread *td, struct xucred *xcr);
 void	crsetgroups(struct ucred *cr, int ngrp, const gid_t *groups);
+void	crsetgroups_fallback(struct ucred *cr, int ngrp, const gid_t *groups,
+	    const gid_t fallback);
 
 /*
  * Returns whether gid designates a primary group in cred.