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SubjectRe: [PATCH v2 bpf-next 5/5] bpf: trampoline: support FTRACE_OPS_FL_SHARE_IPMODIFY

> On Jul 6, 2022, at 2:40 PM, Steven Rostedt <> wrote:
> On Wed, 6 Jul 2022 21:37:52 +0000
> Song Liu <> 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;
} 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?


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