linux-kernel - Re: [PATCH] Compiler Attributes: Add __kcfi

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Message-ID: <CABCJKucfFyVqarstWRCRMf1DbvM7-SajkxAphsH-X8MgSPPN8A@mail.gmail.com>
Date: Thu, 17 Jul 2025 09:45:19 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Bill Wendling <morbo@...gle.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Justin Stitt <justinstitt@...gle.com>, 
	llvm@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Compiler Attributes: Add __kcfi_salt

Hi Kees,

On Thu, Jul 17, 2025 at 1:54 AM Kees Cook <kees@...nel.org> wrote:
>
> +/*
> + * This tries to call an indirect function with a mismatched hash salt.
> + */
> +static void lkdtm_CFI_FORWARD_SALT(void)
> +{
> +       /*
> +        * Matches lkdtm_increment_void()'s and lkdtm_increment_void_again()'s
> +        * prototypes, but they have different hash salts.
> +        */
> +       pr_info("Calling matched prototype ...\n");
> +       lkdtm_indirect_call(lkdtm_increment_void);
> +
> +       pr_info("Calling mismatched hash salt ...\n");
> +       lkdtm_indirect_call(lkdtm_increment_void_again);
> +
> +       pr_err("FAIL: survived mismatched salt function call!\n");
> +       pr_expected_config(CONFIG_CFI_CLANG);

Should this test also have a "This is probably expected" message if
the compiler doesn't support the __kcfi_salt__ attribute?

Sami