Messages in this thread |  | | From | Song Liu <> | Subject | Re: [PATCH v2 bpf-next 5/5] bpf: trampoline: support FTRACE_OPS_FL_SHARE_IPMODIFY | Date | Wed, 6 Jul 2022 22:15:47 +0000 |
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> On Jul 6, 2022, at 2:40 PM, Steven Rostedt <rostedt@goodmis.org> wrote: > > On Wed, 6 Jul 2022 21:37:52 +0000 > Song Liu <songliubraving@fb.com> wrote: > >>> Can you comment here that returning -EAGAIN will not cause this to repeat. >>> That it will change things where the next try will not return -EGAIN? >> >> Hmm.. this is not the guarantee here. This conflict is a real race condition >> that an IPMODIFY function (i.e. livepatch) is being registered at the same time >> when something else, for example bpftrace, is updating the BPF trampoline. >> >> This EAGAIN will propagate to the user of the IPMODIFY function (i.e. livepatch), >> and we need to retry there. In the case of livepatch, the retry is initiated >> from user space. > > We need to be careful here then. If there's a userspace application that > runs at real-time and does a: > > do { > errno = 0; > regsiter_bpf(); > } while (errno != -EAGAIN);
Actually, do you mean:
do { errno = 0; regsiter_bpf(); } while (errno == -EAGAIN);
(== -EAGAIN) here?
In this specific race condition, register_bpf() will succeed, as it already got tr->mutex. But the IPMODIFY (livepatch) side will fail and retry.
Since both livepatch and bpf trampoline changes are rare operations, I think the chance of the race condition is low enough.
Does this make sense?
Thanks, Song
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