On 04/29, Matthew Dempsky wrote: > > On Tue, Apr 29, 2014 at 3:11 PM, Andrew Morton <[email protected]> > wrote: > > More Oleg review would be nice, please ;) > > FWIW, Oleg "acked" v4 earlier in the thread. Are you asking for > further review from him beyond that?
Yes, still/again Acked-by: Oleg Nesterov <[email protected]> > > Well that's a scary comment. If we're going to leave the code in this > > state then please carefully describe (within this comment) the > > *consequences* of the race. Does the kernel crash? Give away your ssh > > keys? If not then what. > > Sorry, I can see how that comment could be scary without proper > context. I added another sentence explaining the consequences are > limited to the ptracer receiving a bogus pid_t value from > PTRACE_GETEVENTMSG. > > > And how would userspace recognize and/or recover from the race? > > If the ptracer attaches via PTRACE_ATTACH, then there shouldn't be a > race: the ptracer can't use PTRACE_SETOPTIONS to request fork events > until after the child has already stopped. So any SIGTRAP fork events > that it receives before using PTRACE_SETOPTIONS it should disregard, > because it hasn't asked the kernel to send them yet. > > If the ptracer attaches via PTRACE_SEIZE and also requests fork events > at the same time, then it would need to discard the first SIGTRAP it > receives for the child if: > > 1. it's a fork event; > 2. the ptracer can't otherwise prove the fork happened after the > PTRACE_SEIZE rather than concurrently; and > 3. the ptracer is concerned a ptracer from a different pid namespace > may have just detached. And I think we should just ignore this very unlikely and harmless race. We do not see a simple way to close it and in fact this ptrace_event() is inherently racy anyway. Even without namespaces, if we race with DETACH + ATTACH, the new tracer gets the correct child's pid, but the child can be already untraced. _Perhaps_ we can do something better later (to remind, we can setup ->ptrace_message beforehand and change ATTACH to clear it), but this is more subtle and needs more changes. This patch is straightforward, and it fixes the old/known problem: currently this pid_t is always wrong unless the tracer is from the root namespace. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
