#systemtap on 2021-01-26 — irc logs at freenode.irclog.whitequark.org

2015-11-12 23:18 fche changed the topic of #systemtap to: http://sourceware.org/systemtap; email systemtap@sourceware.org if answers here not timely, conversations may be logged

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16:12 <irker316> systemtap: fche systemtap.git:master * release-4.4-74-ga790dc64a / util.cxx: PR27251: support buildid probing on --x--x--x perm binaries

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23:21 <kerneltoast> fche, yo

23:26 <fche> yo ma

23:26 <kerneltoast> n

23:26 <kerneltoast> fixed it for you

23:27 <kerneltoast> so we found some interesting new crashes on -rt kernels

23:27 <kerneltoast> but i'm not entirely certain if it's caused by -rt

23:27 <kerneltoast> remember this patch? https://sourceware.org/git/?p=systemtap.git;a=commit;h=5c6c7a994cf485a20f423440aa8dbc65ccf5e0e8

23:29 <kerneltoast> that patch makes utrace->attached.next become NULL when kfree_rcu() is called

23:29 <kerneltoast> this should be fine since the utrace struct isn't supposed to be in use anymore

23:29 <kerneltoast> except on some -rt kernels, a stale utrace struct was pulled from the hashlist

23:30 <kerneltoast> and then a list_for_each_entry on utrace->attached occurred and there was a NULL pointer panic

23:31 <kerneltoast> you might be thinking my kfree_rcu() patch is responsible, but it just happened to expose this bug

23:31 <kerneltoast> SO have you ever seen utrace struct use-after-free panics on -rt?

23:32 <fche> dunno

23:32 <fche> we know multiple problems exist on -rt

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23:32 <kerneltoast> i could fix this but i'm not sure if it's just -rt insanity or a real bug

23:33 <kerneltoast> have you ever seen any bizarre panics that could've alluded to utrace use-after-free?

23:33 <kerneltoast> in the past 10 year

23:33 <kerneltoast> s

23:34 <fche> i am HUMBLED that you would think that I have a greater than zero probability of even being able to address such a question

23:34 <kerneltoast> rack those floppy disks in your brain man

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