linux-kernel - Re: [PATCH -tip] x86/kprobes: Drop removed INT3 handling code

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Message-Id: <20221130012817.638dea0fbf4b8abcd4a70019@kernel.org>
Date:   Wed, 30 Nov 2022 01:28:17 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Huacai Chen <chenhuacai@...ngson.cn>,
        Jinyang He <hejinyang@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>
Subject: Re: [PATCH -tip] x86/kprobes: Drop removed INT3 handling code

On Mon, 28 Nov 2022 15:49:05 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 25 Nov 2022 23:44:47 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > Drop removed INT3 handling code from kprobe_int3_handler() because this
> > case (get_kprobe() doesn't return corresponding kprobe AND the INT3 is
> > removed) must not happen with the kprobe managed INT3, but can happen
> > with the non-kprobe INT3, which should be handled by other callbacks.
> > 
> > For the kprobe managed INT3, the arch_disarm_kprobe() removes the INT3
> > and then calls text_poke_sync(). Since this text_poke_sync() uses IPI
> > to call sync_core() on all online cpus, that ensures that all running
> > INT3 exception handlers have done.
> > And, the unregister_kprobe() will remove the kprobe from the hash table
> > after arch_disarm_kprobe().
> > 
> > Thus, when the kprobe managed INT3 hits, kprobe_int3_handler() should
> > be able to find corresponding kprobe always by get_kprobe(). If it can
> > not find any kprobe, this means that is NOT a kprobe managed INT3.
> > 
> 
> I believe this was fixed by:
> 
>   5c02ece81848d ("x86/kprobes: Fix ordering while text-patching")
> 
> That should be mentioned in the commit log.

Thanks for pointing! Yes, it ensures since it add text_poke_sync()
in arch_arm/disarm_kprobe(). So this is the last part of the
commit 5c02ece81848d.

> 
> Anyway, looks good.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Thank you!

> 
> -- Steve
> 
> 
> 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> >  arch/x86/kernel/kprobes/core.c |   14 --------------
> >  1 file changed, 14 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index 66299682b6b7..33390ed4dcf3 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -986,20 +986,6 @@ int kprobe_int3_handler(struct pt_regs *regs)
> >  			kprobe_post_process(p, regs, kcb);
> >  			return 1;
> >  		}
> > -	}
> > -
> > -	if (*addr != INT3_INSN_OPCODE) {
> > -		/*
> > -		 * The breakpoint instruction was removed right
> > -		 * after we hit it.  Another cpu has removed
> > -		 * either a probepoint or a debugger breakpoint
> > -		 * at this address.  In either case, no further
> > -		 * handling of this interrupt is appropriate.
> > -		 * Back up over the (now missing) int3 and run
> > -		 * the original instruction.
> > -		 */
> > -		regs->ip = (unsigned long)addr;
> > -		return 1;
> >  	} /* else: not a kprobe fault; let the kernel handle it */
> >  
> >  	return 0;
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>