linux-kernel - Re: [PATCH v4 1/4] arm64: Handle KCOV _

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Message-ID: <aIiCD5V1MaI3ORqA@willie-the-truck>
Date: Tue, 29 Jul 2025 09:10:55 +0100
From: Will Deacon <will@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>, Ard Biesheuvel <ardb@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
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Subject: Re: [PATCH v4 1/4] arm64: Handle KCOV __init vs inline mismatches

On Wed, Jul 23, 2025 at 10:50:25PM -0700, Kees Cook wrote:
> GCC appears to have kind of fragile inlining heuristics, in the
> sense that it can change whether or not it inlines something based on
> optimizations. It looks like the kcov instrumentation being added (or in
> this case, removed) from a function changes the optimization results,
> and some functions marked "inline" are _not_ inlined. In that case,
> we end up with __init code calling a function not marked __init, and we
> get the build warnings I'm trying to eliminate in the coming patch that
> adds __no_sanitize_coverage to __init functions:
> 
> WARNING: modpost: vmlinux: section mismatch in reference: acpi_get_enable_method+0x1c (section: .text.unlikely) -> acpi_psci_present (section: .init.text)
> 
> This problem is somewhat fragile (though using either __always_inline
> or __init will deterministically solve it), but we've tripped over
> this before with GCC and the solution has usually been to just use
> __always_inline and move on.
> 
> For arm64 this requires forcing one ACPI function to be inlined with
> __always_inline.
> 
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Will Deacon <will@...nel.org>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Cc: Gavin Shan <gshan@...hat.com>
> Cc: "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>
> Cc: James Morse <james.morse@....com>
> Cc: Oza Pawandeep <quic_poza@...cinc.com>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: <linux-arm-kernel@...ts.infradead.org>
> ---
>  arch/arm64/include/asm/acpi.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/acpi.h b/arch/arm64/include/asm/acpi.h
> index a407f9cd549e..c07a58b96329 100644
> --- a/arch/arm64/include/asm/acpi.h
> +++ b/arch/arm64/include/asm/acpi.h
> @@ -150,7 +150,7 @@ acpi_set_mailbox_entry(int cpu, struct acpi_madt_generic_interrupt *processor)
>  {}
>  #endif
>  
> -static inline const char *acpi_get_enable_method(int cpu)
> +static __always_inline const char *acpi_get_enable_method(int cpu)
>  {
>  	if (acpi_psci_present())
>  		return "psci";

Thanks for improving the commit message:

Acked-by: Will Deacon <will@...nel.org>

Will