Re: [PATCH v9 03/26] riscv: zicfiss / zicfilp enumeration

[Clément Léger]

On 05/02/2025 02:21, Deepak Gupta wrote:
> This patch adds support for detecting zicfiss and zicfilp. zicfiss and
> zicfilp stands for unprivleged integer spec extension for shadow stack
> and branch tracking on indirect branches, respectively.
> 
> This patch looks for zicfiss and zicfilp in device tree and accordinlgy
> lights up bit in cpu feature bitmap. Furthermore this patch adds detection
> utility functions to return whether shadow stack or landing pads are
> supported by cpu.
> 
> Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cpufeature.h | 13 +++++++++++++
>  arch/riscv/include/asm/hwcap.h      |  2 ++
>  arch/riscv/include/asm/processor.h  |  1 +
>  arch/riscv/kernel/cpufeature.c      |  2 ++
>  4 files changed, 18 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index 569140d6e639..69007b8100ca 100644
> --- a/arch/riscv/include/asm/cpufeature.h
> +++ b/arch/riscv/include/asm/cpufeature.h
> @@ -12,6 +12,7 @@
>  #include <linux/kconfig.h>
>  #include <linux/percpu-defs.h>
>  #include <linux/threads.h>
> +#include <linux/smp.h>
>  #include <asm/hwcap.h>
>  #include <asm/cpufeature-macros.h>
>  
> @@ -137,4 +138,16 @@ static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsi
>  	return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
>  }
>  
> +static inline bool cpu_supports_shadow_stack(void)
> +{
> +	return (IS_ENABLED(CONFIG_RISCV_USER_CFI) &&
> +		riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_ZICFISS));
> +}
> +
> +static inline bool cpu_supports_indirect_br_lp_instr(void)
> +{
> +	return (IS_ENABLED(CONFIG_RISCV_USER_CFI) &&
> +		riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_ZICFILP));
> +}
> +
>  #endif
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 869da082252a..2dc4232bdb3e 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -100,6 +100,8 @@
>  #define RISCV_ISA_EXT_ZICCRSE		91
>  #define RISCV_ISA_EXT_SVADE		92
>  #define RISCV_ISA_EXT_SVADU		93
> +#define RISCV_ISA_EXT_ZICFILP		94
> +#define RISCV_ISA_EXT_ZICFISS		95
>  
>  #define RISCV_ISA_EXT_XLINUXENVCFG	127
>  
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index 5f56eb9d114a..e3aba3336e63 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -13,6 +13,7 @@
>  #include <vdso/processor.h>
>  
>  #include <asm/ptrace.h>
> +#include <asm/hwcap.h>
>  
>  #define arch_get_mmap_end(addr, len, flags)			\
>  ({								\
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index c6ba750536c3..e72de12e5b99 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -333,6 +333,8 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
>  	__RISCV_ISA_EXT_SUPERSET_VALIDATE(zicboz, RISCV_ISA_EXT_ZICBOZ, riscv_xlinuxenvcfg_exts,
>  					  riscv_ext_zicboz_validate),
>  	__RISCV_ISA_EXT_DATA(ziccrse, RISCV_ISA_EXT_ZICCRSE),
> +	__RISCV_ISA_EXT_SUPERSET(zicfilp, RISCV_ISA_EXT_ZICFILP, riscv_xlinuxenvcfg_exts),
> +	__RISCV_ISA_EXT_SUPERSET(zicfiss, RISCV_ISA_EXT_ZICFISS, riscv_xlinuxenvcfg_exts),

Hey Deepak,

I think these definitions can benefit from using a validation callback:

static int riscv_cfi_validate(const struct riscv_isa_ext_data *data,
				  const unsigned long *isa_bitmap)
{
	if (!IS_ENABLED(CONFIG_RISCV_USER_CFI)
		return -EINVAL;
		
	return 0;
}

__RISCV_ISA_EXT_SUPERSET_VALIDATE(zicfilp, RISCV_ISA_EXT_ZICFILP,
riscv_xlinuxenvcfg_exts, riscv_cfi_validate),
__RISCV_ISA_EXT_SUPERSET_VALIDATE(zicfiss, RISCV_ISA_EXT_ZICFISS,
riscv_xlinuxenvcfg_exts, riscv_cfi_validate),

That way, ZICFISS/ZICFILP wont be enable if the kernel does not have
builtin support for them. Additionally, this solve a bug you have with
your hwprobe patch (19/26) that exposes ZICFILP/ZICFISS unconditionally
(ie, even if the kernel does not have CONFIG_RISCV_USER_CFI).

BTW, patch 23/26 introduce CONFIG_RISCV_USER_CFI but it is used in that
patch.

Thanks,

Clément

>  	__RISCV_ISA_EXT_DATA(zicntr, RISCV_ISA_EXT_ZICNTR),
>  	__RISCV_ISA_EXT_DATA(zicond, RISCV_ISA_EXT_ZICOND),
>  	__RISCV_ISA_EXT_DATA(zicsr, RISCV_ISA_EXT_ZICSR),
>