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

[Steven Rostedt]

On Wed, 6 Jul 2022 21:37:52 +0000
Song Liu <songliubraving@xxxxxx> 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);

it could in theory preempt the owner of the lock and never make any
progress.

-- Steve