linux-kernel - Re: perf_event_detach_bpf

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Message-ID: <Zxey1hxhnp9_BebL@gofer.mess.org>
Date: Tue, 22 Oct 2024 15:12:38 +0100
From: Sean Young <sean@...s.org>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, andrii@...nel.org, yhs@...com,
	linux-kernel@...r.kernel.org, daniel@...earbox.net
Subject: Re: perf_event_detach_bpf_prog() broken?

On Tue, Oct 22, 2024 at 04:03:40PM +0200, Jiri Olsa wrote:
> On Tue, Oct 22, 2024 at 01:16:38PM +0200, Peter Zijlstra wrote:
> > Hi guys,
> > 
> > Per commit 170a7e3ea070 ("bpf: bpf_prog_array_copy() should return
> > -ENOENT if exclude_prog not found") perf_event_detach_bpf_prog() can now
> > return without doing bpf_prog_put() and leaving event->prog set.
> > 
> > This is very 'unexpected' behaviour.
> > 
> > I'm not sure what's sane from the BPF side of things here, but leaving
> > event->prog set is really rather unexpected.
> > 
> > Help?
> 
> IIUC the ENOENT should never happen in perf event context, so not
> sure why we have that check.. also does not seem to be used from
> lirc code, Sean?

You can deattach a lirc program using the bpf syscall with command 
BPF_PROG_DETACH, and if you pass an incorrect (as in, not attached) program,
then this commit ensures you get ENOENT rather than success.


Sean

> perf_event_detach_bpf_prog is called when the event is being freed
> so I think we should always put and clear the event->prog
> 
> jirka