linux-kernel - Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG

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Date:   Wed, 7 Sep 2022 09:06:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check
 with CONFIG_RETHUNK

On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:

> +/* Return the jump target address or 0 */
> +static inline unsigned long insn_get_branch_addr(struct insn *insn)
> +{
> +	switch (insn->opcode.bytes[0]) {
> +	case 0xe0:	/* loopne */
> +	case 0xe1:	/* loope */
> +	case 0xe2:	/* loop */

Oh cute, objtool doesn't know about those, let me go add them.

> +	case 0xe3:	/* jcxz */
> +	case 0xe9:	/* near relative jump */

 /* JMP.d32 */

> +	case 0xeb:	/* short relative jump */

 /* JMP.d8 */

> +		break;
> +	case 0x0f:
> +		if ((insn->opcode.bytes[1] & 0xf0) == 0x80) /* jcc near */

 /* Jcc.d32 */

Are the GNU AS names for these things.

> +			break;
> +		return 0;

> +	default:
> +		if ((insn->opcode.bytes[0] & 0xf0) == 0x70) /* jcc short */
> +			break;
> +		return 0;

You could write that as:

	case 0x70 ... 0x7f: /* Jcc.d8 */
		break;

	default:
		return 0;

> +	}
> +	return (unsigned long)insn->next_byte + insn->immediate.value;
> +}