Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Andy Lutomirski]

> On Jun 7, 2019, at 10:34 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Sat, Jun 08, 2019 at 12:47:08AM +0900, Masami Hiramatsu wrote:
> 
>>> This fits almost all text_poke_bp() users, except
>>> arch_unoptimize_kprobe() which restores random text, and for that site
>>> we have to build an explicit emulate instruction.
>> 
>> Hm, actually it doesn't restores randome text, since the first byte
>> must always be int3. As the function name means, it just unoptimizes
>> (jump based optprobe -> int3 based kprobe).
>> Anyway, that is not an issue. With this patch, optprobe must still work.
> 
> I thought it basically restored 5 bytes of original text (with no
> guarantee it is a single instruction, or even a complete instruction),
> with the first byte replaced with INT3.
> 

I am surely missing some kprobe context, but is it really safe to use this mechanism to replace more than one instruction?