From: Luca Boccassi Date: Tue, 19 May 2026 15:28:39 +0000 (+0100) Subject: liveupdate: document systemd support X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=608d1642f20077abff382f04bcd74a4a6cec3018;p=thirdparty%2Flinux.git liveupdate: document systemd support systemd v261 will add native support for LUO via the file descriptor store mechanism. Add a brief paragraph in the LUO userspace documentation page to inform users. Signed-off-by: Luca Boccassi Reviewed-by: Pasha Tatashin Acked-by: Pratyush Yadav Link: https://patch.msgid.link/20260519152957.548119-1-luca.boccassi@gmail.com Signed-off-by: Pasha Tatashin Signed-off-by: Mike Rapoport (Microsoft) --- diff --git a/Documentation/userspace-api/liveupdate.rst b/Documentation/userspace-api/liveupdate.rst index 41c0473e4f160..f6efc32fb40b7 100644 --- a/Documentation/userspace-api/liveupdate.rst +++ b/Documentation/userspace-api/liveupdate.rst @@ -14,6 +14,29 @@ ioctl uAPI =========== .. kernel-doc:: include/uapi/linux/liveupdate.h +Userspace Integration +===================== + +systemd (since version v261) uses LUO to preserve its per-service file +descriptor store across a kexec-based live update. Services opt in by setting +``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit, +and push file descriptors with a name into the store via +``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``. + +Services may also create their own LUO sessions (via ``/dev/liveupdate``) and +push the resulting session fds into their file descriptor store like any other +fd. systemd detects such session fds and handles them accordingly, and +hands the re-retrieved session fd back to the service after kexec, using the +existing file descriptor store service interface. + +For details, see: + +- `File Descriptor Store `_ +- `systemd.service(5) FileDescriptorStorePreserve= + `_ +- `sd_pid_notify_with_fds(3) + `_ + See Also ========