There is a limit on the number of lines in the /proc/<pid>/{g,u}id_map files. In Linux 4.14 and earlier, this limit was (arbitrarily) set at 5 lines. Since Linux 4.15, which was released on 28 Jan 2018, the limit is 340 lines. This change is documented in user_namespaces(7). https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit... Signed-off-by: Radostin Stoyanov <rstoyanov1(a)gmail.com&gt; --- src/lxc/lxc_controller.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/src/lxc/lxc_controller.c b/src/lxc/lxc_controller.c index 99360397a7..2bec8846aa 100644 --- a/src/lxc/lxc_controller.c +++ b/src/lxc/lxc_controller.c @@ -1390,6 +1390,13 @@ virLXCControllerSetupUsernsMap(virDomainIdMapEntryPtr map, size_t i; int ret = -1; + /* The kernel supports up to 340 lines in /proc/<pid>/{g,u}id_map */ + if (num > 340) { + virReportError(VIR_ERR_INVALID_ARG, "%s", + _("Too many id mappings defined.")); + goto cleanup; + } + for (i = 0; i < num; i++) virBufferAsprintf(&map_value, "%u %u %u\n", map[i].start, map[i].target, map[i].count); -- 2.19.2