Messages in this thread | | | Subject | Re: [PATCH] prctl: add PR_[GS]ET_PDEATHSIG_PROC | From | Jürg Billeter <> | Date | Tue, 12 Sep 2017 20:54:55 +0200 |
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Hi Oleg,
Thanks for the review.
On Tue, 2017-09-12 at 19:05 +0200, Oleg Nesterov wrote: > On 09/09, Jürg Billeter wrote: > > Unlike > > PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole > > subtree without race conditions. > > but I am still not sure this is right... at least I can't understand the > "without race conditions" above. > > IOW, the child can do prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) right after fork(), > why this is not enough to kill a whole subtree without race conditions?
What if the parent dies between fork() and prctl()? Besides avoiding this race condition, it also makes it relatively easy to enforce PDEATHSIG_PROC for all descendants of a process. You simply set PDEATHSIG_PROC and then block further changes using seccomp (and set no_new_privs) to avoid runaway children.
> OTOH. If you want to kill a whole sub-tree then perhaps the exiting process > should simply send the ->pdeath_signal_proc to the whole sub-tree? Not that > I really think this makes more sense, but if we add the new API we should > discuss everything we can.
While this would likely work for my use case of avoiding runaway processes, I don't think it would make sense for non-SIGKILL use cases of cooperating processes. Inheritance across fork still allows resetting PDEATHSIG_PROC in the child after fork and I don't expect the parent death race to be a significant issue in the case of cooperating processes.
> Say, CLONE_PARENT. Should it succeed if ->pdeath_signal_proc != 0 ?
Yes, I don't see an issue with that. The new process will be a sibling and inheriting pdeath_signal_proc seems sensible to me for this.
Jürg
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