linux-kernel - Re: [PATCH] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

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Date:   Fri, 9 Sep 2022 09:48:09 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...e.de>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

On Fri, Sep 09, 2022 at 10:16:13AM +0200, Peter Zijlstra wrote:
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -419,7 +419,9 @@ static void emit_indirect_jump(u8 **ppro
>  		OPTIMIZER_HIDE_VAR(reg);
>  		emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
>  	} else {
> -		EMIT2(0xFF, 0xE0 + reg);
> +		EMIT2(0xFF, 0xE0 + reg);	/* jmp *%\reg */
> +		if (IS_ENABLED(CONFIG_RETPOLINE) || IS_ENABLED(CONFIG_SLS))
> +			EMIT1(0xCC);		/* int3 */
>  	}

Hm, if you have retpolines disabled at runtime, why would you want this.

-- 
Josh