linux-kernel - Re: [PATCH RFC 34/43] objtool: Adapt indirect call of __fentry_

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Date:   Fri, 28 Apr 2023 17:18:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hou Wenlong <houwenlong.hwl@...group.com>
Cc:     linux-kernel@...r.kernel.org,
        Thomas Garnier <thgarnie@...omium.org>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [PATCH RFC 34/43] objtool: Adapt indirect call of __fentry__()
 for PIE support

On Fri, Apr 28, 2023 at 05:51:14PM +0800, Hou Wenlong wrote:

> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -747,15 +747,21 @@ void arch_initial_func_cfi_state(struct cfi_init_state *state)
>  
>  const char *arch_nop_insn(int len)
>  {
> -	static const char nops[5][5] = {
> +	static const char nops[6][6] = {
>  		{ BYTES_NOP1 },
>  		{ BYTES_NOP2 },
>  		{ BYTES_NOP3 },
>  		{ BYTES_NOP4 },
>  		{ BYTES_NOP5 },
> +		/*
> +		 * For PIE kernel, use a 5-byte nop
> +		 * and 1-byte nop to keep the frace
> +		 * hooking algorithm working correct.
> +		 */
> +		{ BYTES_NOP5, BYTES_NOP1 },
>  	};
> -	if (len < 1 || len > 5) {
> +	if (len < 1 || len > 6) {
>  		WARN("invalid NOP size: %d\n", len);
>  		return NULL;
>  	}

Like Steve already said, this is broken, we hard rely on these things
being single instructions, this must absolutely be BYTES_NOP6.

And yes, then you get to fix a whole lot more.