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+=== ucred / group changes in FreeBSD 15.0
+
+Links: +
+link:https://lists.freebsd.org/archives/freebsd-hackers/2025-August/004825.html[freebsd-arch@ discussion] URL: https://lists.freebsd.org/archives/freebsd-hackers/2025-August/004825.html[] +
+link:https://cgit.freebsd.org/src/commit/sys/sys/ucred.h?id=be1f7435ef218b1df35aebf3b90dd65ffd8bbe51[Primary kernel change] URL: https://cgit.freebsd.org/src/commit/sys/sys/ucred.h?id=be1f7435ef218b1df35aebf3b90dd65ffd8bbe51[] +
+link:https://cgit.freebsd.org/src/commit/sys/kern/kern_prot.c?id=9da2fe96ff2ea227e4d5f03ef92b55aabeabb7fc[Primary userspace change] URL: https://cgit.freebsd.org/src/commit/sys/kern/kern_prot.c?id=9da2fe96ff2ea227e4d5f03ef92b55aabeabb7fc
+
+Contact: Kyle Evans <kevans@FreeBSD.org> +
+Contact: Olivier Certner <olce@FreeBSD.org>
+
+FreeBSD 15.0 will change how supplementary groups are handled in both userspace and the kernel in FreeBSD 15.0 in a way that warrants additional attention and feedback.
+
+For some background: FreeBSD has historically tracked the effective group-ID of a process in the man:ucred[9] cr_groups array as the first element, with the rest of the array describing its supplementary groups.
+The natural consequence of this decision is that the arrays used in man:setgroups[2] and man:getgroups[2] follow the same format, and man:setgroups[2] has the documented side effect of setting the effective group-ID.
+The vast majority of other platforms do not exhibit this behavior anymore, including NetBSD and OpenBSD.
+macOS appears to be the only exception found in testing.
+
+The problem is that the vast majority of software in the FreeBSD Ports Collection comes from other platforms, where man:setgroups[2] and man:setgroups[2] operate purely on the supplementary groups.
+This kind of a behavior difference is very subtle and would need to be audited more carefully to be sure that we have not introduced a potential security issue in ported software.
+
+In FreeBSD 15.0, the primary user-facing change is that man:setgroups[2], man:getgroups[2], and man:initgroups[3] behavior will change to match other platforms, and users are requested to be extra vigilant in areas that may be affected as we proceed through the release cycle.
+In general, the expectation is that this change may:
+
+* Fix some small number of bugs where we would have lost either our expected effective group membership or one of the supplementary groups we should have been in
+* (Less likely) Introduce some even smaller number of bugs where something expected man:setgroups[2] to change our effective group membership but now it is just a supplementary group and our effective group-ID is unchanged
+
+Software included in the base system is largely unaffected or improved by this change, with OpenSSH being a notable example of a link:https://cgit.freebsd.org/src/commit/?id=239e8c98636a7578cc67a6f9d54d14c71b095e36[strange bug] caused by the historical implementation.