linux-kernel - Re: [PATCH perf/core 02/11] uprobes: Skip emulate/sstep on unique uprobe when ip is changed

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Message-ID: <CAADnVQ+DHGc8R0Tdxf7eUj1R0TDGHXLwk5D4i_0==2_rfXGbfw@mail.gmail.com>
Date: Thu, 4 Sep 2025 08:01:53 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Jiri Olsa <olsajiri@...il.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>, 
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, 
	John Fastabend <john.fastabend@...il.com>, Hao Luo <haoluo@...gle.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH perf/core 02/11] uprobes: Skip emulate/sstep on unique
 uprobe when ip is changed

On Thu, Sep 4, 2025 at 4:26 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 09/04, Jiri Olsa wrote:
> >
> > On Thu, Sep 04, 2025 at 10:49:50AM +0200, Oleg Nesterov wrote:
> > > On 09/03, Jiri Olsa wrote:
> > > >
> > > > On Wed, Sep 03, 2025 at 01:26:48PM +0200, Oleg Nesterov wrote:
> > > > > On 09/02, Jiri Olsa wrote:
> > > > > >
> > > > > > If user decided to take execution elsewhere, it makes little sense
> > > > > > to execute the original instruction, so let's skip it.
> > > > >
> > > > > Exactly.
> > > > >
> > > > > So why do we need all these "is_unique" complications? Only a single
> > > > > is_unique/exclusive consumer can change regs->ip, so I guess handle_swbp()
> > > > > can just do
> > > > >
> > > > >         handler_chain(uprobe, regs);
> > > > >         if (instruction_pointer(regs) != bp_vaddr)
> > > > >                 goto out;
> > > >
> > > > hum, that's what I did in rfc [1] but I thought you did not like that [2]
> > > >
> > > > [1] https://lore.kernel.org/bpf/20250801210238.2207429-2-jolsa@kernel.org/
> > > > [2] https://lore.kernel.org/bpf/20250802103426.GC31711@redhat.com/
> > > >
> > > > I guess I misunderstood your reply [2], I'd be happy to drop the
> > > > unique/exclusive flag
> > >
> > > Well, but that rfc didn't introduce the exclusive consumers, and I think
> > > we agree that even with these changes the non-exclusive consumers must
> > > never change regs->ip?
> >
> > ok, got excited too soon.. so you meant getting rid of is_unique
> > check only for this patch and have just change below..  but keep
> > the unique/exclusive flag from patch#1
>
> Yes, this is what I meant,
>
> > IIUC Andrii would remove the unique flag completely?
>
> Lets wait for Andrii...

Not Andrii, but I see only negatives in this extra flag.
It doesn't add any safety or guardrails.
No need to pollute uapi with pointless flags.