linux-kernel - Re: [RFC 1/4] uprobe: Do not emulate/sstep original instruction when ip is changed

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Message-ID: <20250802103426.GC31711@redhat.com>
Date: Sat, 2 Aug 2025 12:34:27 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	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: [RFC 1/4] uprobe: Do not emulate/sstep original instruction when
 ip is changed

On 08/01, Jiri Olsa wrote:
>
> If uprobe handler changes instruction pointer we still execute single
> step) or emulate the original instruction and increment the (new) ip
> with its length.

Yes... but what if we there are multiple consumers? The 1st one changes
instruction_pointer, the next is unaware. Or it may change regs->ip too...

Oleg.

> This makes the new instruction pointer bogus and application will
> likely crash on illegal instruction execution.
> 
> If user decided to take execution elsewhere, it makes little sense
> to execute the original instruction, so let's skip it.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/events/uprobes.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 4c965ba77f9f..dff5509cde67 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2742,6 +2742,9 @@ static void handle_swbp(struct pt_regs *regs)
>  
>  	handler_chain(uprobe, regs);
>  
> +	if (instruction_pointer(regs) != bp_vaddr)
> +		goto out;
> +
>  	if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
>  		goto out;
>  
> -- 
> 2.50.1
>