* Jürg Billeter: > On Fri, 2018-11-30 at 14:40 +0100, Florian Weimer wrote: >> * Jürg Billeter: >> >> > This introduces a new thread group flag that can be set by calling >> > >> > prctl(PR_SET_KILL_DESCENDANTS_ON_EXIT, 1, 0, 0, 0) >> > >> > When a thread group exits with this flag set, it will send SIGKILL to >> > all descendant processes. This can be used to prevent stray child >> > processes. >> > >> > This flag is cleared on privilege gaining execve(2) to ensure an >> > unprivileged process cannot get a privileged process to send SIGKILL. >> >> So this is inherited across regular execve? I'm not sure that's a good >> idea. > > Yes, this matches PR_SET_CHILD_SUBREAPER (and other process > attributes). Besides consistency and allowing a parent to configure the > flag for a spawned process, this is also needed to prevent a process > from clearing the flag (in combination with a seccomp filter). I think the semantics of PR_SET_CHILD_SUBREAPER are different, and the behavior makes more sense there. >> > Descendants that are orphaned and reparented to an ancestor of the >> > current process before the current process exits, will not be killed. >> > PR_SET_CHILD_SUBREAPER can be used to contain orphaned processes. >> >> For double- or triple-forking daemons, the reparenting will be racy, if >> I understand things correctly. > > Can you please elaborate, if you're concerned about a particular race? > As the commit message mentions, for containment this flag can be > combined with PR_SET_CHILD_SUBREAPER (and PR_SET_NO_NEW_PRIVS). Without PR_SET_CHILD_SUBREAPER, if a newly execve'ed daemon performs double/triple forking to disentangle itself from the parent process session, and the parent process which set PR_SET_KILL_DESCENDANTS_ON_EXIT terminates, behavior depends on when exactly the parent process terminates. The daemon process will leak if it has completed its reparenting. I think this could be sufficiently common that solution is needed here. Thanks, Florian
