[PATCH v2 5/9] x86/cfi: Add "debug" option to "cfi=" bootparam

[Peter Zijlstra]

On Wed, Sep 03, 2025 at 08:46:44PM -0700, Kees Cook wrote:

> @@ -1734,6 +1737,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  	 * rewrite them. This disables all CFI. If this succeeds but any of the
>  	 * later stages fails, we're without CFI.
>  	 */
> +	if (cfi_debug)
> +		pr_info("CFI: disabling all indirect call checking\n");
>  	ret = cfi_disable_callers(start_retpoline, end_retpoline);
>  	if (ret)
>  		goto err;
> @@ -1744,14 +1749,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  			cfi_bpf_hash = cfi_rehash(cfi_bpf_hash);
>  			cfi_bpf_subprog_hash = cfi_rehash(cfi_bpf_subprog_hash);
>  		}
> +		if (cfi_debug)
> +			pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
>  
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing all preambles\n");

So this repeated if() bugs the heck out of me ... :-)

What would you prefer, this:

		if (cfi_debug) {
			pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
			pr_info("CFI: rehashing all preambles\n");
		}

or something like:

#define pr_cfi_debug(X...) if (cfi_debug) pr_info(X)

		pr_cfi_debug("CFI: cfi_seed: 0x%08x\n", cfi_seed);
		pr_cfi_debug("CFI: rehashing all preambles\n");

?

>  		ret = cfi_rand_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing all indirect calls\n");
>  		ret = cfi_rand_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
> +	} else {
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing disabled\n");
>  	}
>  
>  	switch (cfi_mode) {
> @@ -1761,6 +1775,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  		return;
>  
>  	case CFI_KCFI:
> +		if (cfi_debug)
> +			pr_info("CFI: enabling all indirect call checking\n");

This should be "CFI: re-enabling all..." I suppose, to better match the
earlier "CFI: disabling all ..." message.

>  		ret = cfi_enable_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
> @@ -1771,17 +1787,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  		return;
>  
>  	case CFI_FINEIBT:
> +		if (cfi_debug)
> +			pr_info("CFI: adding FineIBT to all preambles\n");
>  		/* place the FineIBT preamble at func()-16 */
>  		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
>  		/* rewrite the callers to target func()-16 */
> +		if (cfi_debug)
> +			pr_info("CFI: rewriting indirect call sites to use FineIBT\n");
>  		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
>  
>  		/* now that nobody targets func()+0, remove ENDBR there */
> +		if (cfi_debug)
> +			pr_info("CFI: removing old endbr insns\n");
>  		cfi_rewrite_endbr(start_cfi, end_cfi);
>  
>  		if (builtin) {
> @@ -2324,6 +2346,7 @@ void __init alternative_instructions(void)
>  
>  	__apply_fineibt(__retpoline_sites, __retpoline_sites_end,
>  			__cfi_sites, __cfi_sites_end, true);
> +	cfi_debug = false;
>  
>  	/*
>  	 * Rewrite the retpolines, must be done before alternatives since