linux-kernel - Re: [PATCH -v5 04/17] x86/alternatives: Add and use text_gen

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Message-ID: <20191112121028.3ef25207@gandalf.local.home>
Date:   Tue, 12 Nov 2019 12:10:28 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, jeyu@...nel.org,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH -v5 04/17] x86/alternatives: Add and use text_gen_insn()
 helper

On Mon, 11 Nov 2019 14:12:56 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> +void *text_gen_insn(u8 opcode, const void *addr, const void *dest)
> +{
> +	static union text_poke_insn insn; /* text_mutex */
> +	int size = 0;
> +
> +	lockdep_assert_held(&text_mutex);
> +
> +	insn.opcode = opcode;
> +
> +#define __CASE(insn)	\
> +	case insn##_INSN_OPCODE: size = insn##_INSN_SIZE; break
> +
> +	switch(opcode) {
> +	__CASE(INT3);
> +	__CASE(CALL);
> +	__CASE(JMP32);
> +	__CASE(JMP8);
> +	}
> +
> +	if (size > 1) {
> +		insn.disp = (long)dest - (long)(addr + size);
> +		if (size == 2)

Could we add a comment here. It took me a little bit to figure out why
you have this BUG_ON().

-- Steve


> +			BUG_ON((insn.disp >> 31) != (insn.disp >> 7));
> +	}
> +
> +	return &insn.text;
> +}