linux-kernel - Re: [RFC PATCH 02/12] riscv: add landing pad for asm routines.

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Message-ID: <CABCJKuee-6GGDDjvByCkikR02gka2BNhwRVBw6UAwEcmSQposQ@mail.gmail.com>
Date: Thu, 11 Apr 2024 17:15:17 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Deepak Gupta <debug@...osinc.com>
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Subject: Re: [RFC PATCH 02/12] riscv: add landing pad for asm routines.

On Tue, Apr 9, 2024 at 6:12 AM Deepak Gupta <debug@...osinc.com> wrote:
>
> SYM_* macros are used to define assembly routines. In this patch series,
> re-define those macros in risc-v arch specific include file to include
> a landing pad instruction at the beginning. This is done only when the
> compiler flag for landing pad is enabled (i.e. __riscv_zicfilp).
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>  arch/riscv/include/asm/linkage.h | 42 ++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
>
> diff --git a/arch/riscv/include/asm/linkage.h b/arch/riscv/include/asm/linkage.h
> index 9e88ba23cd2b..bb43ae7dadeb 100644
> --- a/arch/riscv/include/asm/linkage.h
> +++ b/arch/riscv/include/asm/linkage.h
> @@ -6,7 +6,49 @@
>  #ifndef _ASM_RISCV_LINKAGE_H
>  #define _ASM_RISCV_LINKAGE_H
>
> +#ifdef __ASSEMBLY__
> +#include <asm/assembler.h>
> +#endif
> +
>  #define __ALIGN                .balign 4
>  #define __ALIGN_STR    ".balign 4"
>
> +#ifdef __riscv_zicfilp
> +/*
> + * A landing pad instruction is needed at start of asm routines
> + * re-define macros for asm routines to have a landing pad at
> + * the beginning of function. Currently use label value of 0x1.
> + * Eventually, label should be calculated as a hash over function
> + * signature.
> + */

I haven't seen the compiler implementation for fine-grained Zicfilp
yet, but in the kernel at least, this would ideally reuse as much of
the KCFI plumbing as possible. For example, since only C code has type
information, we left the type hash computation for the compiler, which
allows assembly functions to just reference the appropriate
__kcfi_typeid_* symbol.

Sami