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

[Song Liu <songliubraving@fb.com>]

> 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