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Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with
 nmi_enter()")
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Masami Hiramatsu <masami.hiramatsu@gmail.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>, bpf@vger.kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>
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Date:   Fri, 29 Jan 2021 08:36:30 +0200
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On 29.01.21 г. 3:34 ч., Alexei Starovoitov wrote:
> On Thu, Jan 28, 2021 at 07:24:14PM +0100, Peter Zijlstra wrote:
>> On Thu, Jan 28, 2021 at 06:45:56PM +0200, Nikolay Borisov wrote:
>>> it would be placed on the __fentry__ (and not endbr64) hence it works.
>>> So perhaps a workaround outside of bpf could essentially detect this
>>> scenario and adjust the probe to be on the __fentry__ and not preceding
>>> instruction if it's detected to be endbr64 ?
>>
>> Arguably the fentry handler should also set the nmi context, it can,
>> after all, interrupt pretty much any other context by construction.
> 
> But that doesn't make it a real nmi.
> nmi can actually interrupt anything. Whereas kprobe via int3 cannot
> do nokprobe and noinstr sections. The exposure is a lot different.
> ftrace is even more contained. It's only at the start of the functions.
> It's even smaller subset of places than kprobes.
> So ftrace < kprobe < nmi.
> Grouping them all into nmi doesn't make sense to me.
> That bpf breaking change came unnoticed mostly because people use
> kprobes in the beginning of the functions only, but there are cases
> where kprobes are in the middle too. People just didn't upgrade kernels
> fast enough to notice.

nit: slight correction - I observed while I was putting kprobes at the
beginning of the function but __fentry__ wasn't the first thing in the
function's code. The reason why people haven't observed is because
everyone is running with retpolines enabled which disables CFI/CET.

> imo appropriate solution would be to have some distinction between
> ftrace < kprobe_via_int3 < nmi, so that bpf side can react accordingly.
> That code in trace_call_bpf:
>   if (in_nmi()) /* not supported yet */
> was necessary in the past. Now we have all sorts of protections built in.
> So it's probably ok to just drop it, but I think adding
> called_via_ftrace vs called_via_kprobe_int3 vs called_via_nmi
> is more appropriate solution long term.
>