Messages in this thread |  | | From | Dmitry Vyukov <> | Date | Thu, 4 Feb 2021 10:54:42 +0100 | Subject | Re: Process-wide watchpoints |
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On Thu, Feb 4, 2021 at 10:39 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, Feb 04, 2021 at 09:10:11AM +0100, Dmitry Vyukov wrote: > > On Wed, Feb 3, 2021 at 2:37 PM Peter Zijlstra <peterz@infradead.org> wrote: > > > > Letting perf send a signal to the monitored task is intrusive.. let me > > > think on that. > > > > I was thinking of something very similar to that bpf_send_signal that > > delays sending to exit from irq: > > https://elixir.bootlin.com/linux/latest/source/kernel/trace/bpf_trace.c#L1091 > > Oh, making code to do it isn't the problem. The problem stems from the > fact that perf is supposed to be observant only. The exception is when > you monitor yourself, in that case you can send signals to yourself, > because you know what you're doing (supposedly ;-). > > But if you go send signals to the task you're monitoring, you're > actually changing their code-flow, you're an active participant instead > of an observer. > > Also, they might not be able to handle the signal, in which case you're > not changing the program but terminating it entirely. > > That's a big conceptual shift. > > OTOH, we're using ptrace permission checks, and ptrace() can inject > signals just fine. But it's a fairly big departure from what perf set > out to be.
Oh, I see, I did not think about this.
FWIW it's doable today by attaching a BPF program.
Will it help if this mode is restricted to monitoring the current process? Sending signals indeed usually requires cooperation, so doing it for the current process looks like a reasonable restriction. This may be not a fundamental restriction, but rather "we don't have any use cases and are not sure about implications, so this is a precaution measure, may be relaxed in future".
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