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

Peter Zijlstra
2025-09-04T07:00:51Z

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