netdev - Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers for generated code

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Date:   Fri, 14 Jun 2019 09:41:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Song Liu <songliubraving@...com>,
        Kairui Song <kasong@...hat.com>
Subject: Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers
 for generated code

On Thu, Jun 13, 2019 at 11:00:09PM -0700, Alexei Starovoitov wrote:

> There is something wrong with
> commit d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")

It assumes we can always unwind stack, which is, imo, not a weird thing.

> If I simply revert it and have CONFIG_UNWINDER_FRAME_POINTER=y
> JITed stacks work just fine, because
> bpf_get_stackid()->get_perf_callchain()
> need to start unwinding before any bpf stuff.

How does stack unwinding work if we try and unwind from an interrupt
that hits inside a BPF program? That too needs to work properly.

> After that commit it needs to go through which is a bug on its own.
> imo patch 1 doesn't really fix that issue.

This we agree on, patch 1 doesn't solve that at all. But we also should
not loose the initial regs->ip value.

> As far as mangled rbp can we partially undo old
> commit 177366bf7ceb ("bpf: change x86 JITed program stack layout")
> that introduced that rbp adjustment.

> Going through bpf code is only interesting in case of panics somewhere
> in bpf helpers. Back then we didn't even have ksym of jited code.

I disagree here, interrupts/NMIs hitting inside BPF should be able to
reliably unwind the entire stack. Back then is irrelevant, these days we
expect a reliable unwind.

> Anyhow I agree that we need to make the jited frame proper,
> but unwinding need to start before any bpf stuff.
> That's a bigger issue.

I strongly disagree, we should be able to unwind through bpf.