Re: [PATCH 2/3] riscv: Deprecate vector state prctl/sysctl

[Andy Chiu <tchiu@tenstorrent.com>]

On Wed, Oct 07, 2026 at 01:51:17AM -0700, Samuel Holland wrote:
> The ability to prevent userspace from executing vector instructions was
> provided as a escape hatch for software that was broken by adding vector
> state to the signal context. This appears to be unused in practice, and
> creates confusion in the extension support returned by hwprobe(), as in
> this one special case an extension reported as present by hwprobe() is
> unusable.
> 
> Since we believe it is safe to always allow vector usage, hide the prctl
> and sysctl behind a Kconfig option and mark them as deprecated, so they
> can be removed in the future. Note that significant userspace software
> like glibc already considers these controls explicitly unsupported.
> 
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

Reviewed-by: Andy Chiu <tchiu@tenstorrent.com>

> ---
> 
>  arch/riscv/Kconfig                 | 10 ++++++++++
>  arch/riscv/include/asm/processor.h |  4 +++-
>  arch/riscv/include/asm/vector.h    | 16 ++++++++++++----
>  arch/riscv/kernel/vector.c         | 10 ++++++++++
>  4 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index e26450f0d38fb..83e52a0a85ba7 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -650,6 +650,16 @@ config RISCV_ISA_V
>  
>  	  If you don't know what to do here, say Y.
>  
> +config RISCV_ISA_V_VSTATE_CTRL
> +	bool "Deprecated prctl/sysctl for disallowing userspace vector access"
> +	depends on RISCV_ISA_V
> +	help
> +	  Add prctl and sysctl interfaces to disallow userspace to execute
> +	  vector instructions. These controls are deprecated because they create
> +	  ambiguity in the ISA extension availability reported by hwprobe.
> +
> +	  If you don't know what to do here, say N.
> +
>  config RISCV_ISA_V_UCOPY_THRESHOLD
>  	int "Threshold size for vectorized user copies"
>  	depends on RISCV_ISA_V
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index 815715c67f940..ad64df01417a2 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -121,7 +121,9 @@ struct thread_struct {
>  	unsigned long envcfg;
>  	unsigned long sum;
>  	u32 riscv_v_flags;
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
>  	u32 vstate_ctrl;
> +#endif
>  	struct __riscv_v_ext_state vstate;
>  	unsigned long align_ctl;
>  	struct __riscv_v_ext_state kernel_vstate;
> @@ -202,7 +204,7 @@ extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src
>  
>  extern unsigned long signal_minsigstksz __ro_after_init;
>  
> -#ifdef CONFIG_RISCV_ISA_V
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
>  /* Userspace interface for PR_RISCV_V_{SET,GET}_VS prctl()s: */
>  #define RISCV_V_SET_CONTROL(arg)	riscv_v_vstate_ctrl_set_current(arg)
>  #define RISCV_V_GET_CONTROL()		riscv_v_vstate_ctrl_get_current()

nit: consider changing the enclosing CONFIG_RISCV_ISA_V at the comment to
CONFIG_RISCV_ISA_V_VSTATE_CTRL if this needs respin.

> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> index fffe72a772080..5425a25f8a196 100644
> --- a/arch/riscv/include/asm/vector.h
> +++ b/arch/riscv/include/asm/vector.h
> @@ -404,9 +404,6 @@ static inline void __switch_to_vector(struct task_struct *prev,
>  	}
>  }
>  
> -void riscv_v_vstate_ctrl_init(struct task_struct *tsk);
> -bool riscv_v_vstate_ctrl_user_allowed(void);
> -
>  #else /* ! CONFIG_RISCV_ISA_V  */
>  
>  struct pt_regs;
> @@ -418,7 +415,6 @@ static __always_inline bool has_xtheadvector_no_alternatives(void) { return fals
>  static __always_inline bool has_xtheadvector(void) { return false; }
>  static inline bool riscv_v_first_use_handler(struct pt_regs *regs) { return false; }
>  static inline bool riscv_v_vstate_query(struct pt_regs *regs) { return false; }
> -static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
>  #define riscv_v_vsize (0)
>  #define riscv_v_vstate_discard(regs)		do {} while (0)
>  #define riscv_v_vstate_save(vstate, regs)	do {} while (0)
> @@ -435,6 +431,18 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
>  
>  #endif /* CONFIG_RISCV_ISA_V */
>  
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
> +
> +void riscv_v_vstate_ctrl_init(struct task_struct *tsk);
> +bool riscv_v_vstate_ctrl_user_allowed(void);
> +
> +#else
> +
> +static inline void riscv_v_vstate_ctrl_init(struct task_struct *tsk) {}
> +static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return true; }
> +
> +#endif /* CONFIG_RISCV_ISA_V_VSTATE_CTRL */
> +
>  /*
>   * Return the implementation's vlen value.
>   *
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 9641343732782..b35d11ff3cc89 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -20,7 +20,9 @@
>  #include <asm/ptrace.h>
>  #include <asm/bug.h>
>  
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
>  static bool riscv_v_implicit_uacc = true;
> +#endif
>  static struct kmem_cache *riscv_v_user_cachep;
>  #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
>  static struct kmem_cache *riscv_v_kernel_cachep;
> @@ -144,6 +146,8 @@ void riscv_v_thread_free(struct task_struct *tsk)
>  #endif
>  }
>  
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
> +
>  #define VSTATE_CTRL_GET_CUR(x) ((x) & PR_RISCV_V_VSTATE_CTRL_CUR_MASK)
>  #define VSTATE_CTRL_GET_NEXT(x) (((x) & PR_RISCV_V_VSTATE_CTRL_NEXT_MASK) >> 2)
>  #define VSTATE_CTRL_MAKE_NEXT(x) (((x) << 2) & PR_RISCV_V_VSTATE_CTRL_NEXT_MASK)
> @@ -182,6 +186,8 @@ bool riscv_v_vstate_ctrl_user_allowed(void)
>  }
>  EXPORT_SYMBOL_GPL(riscv_v_vstate_ctrl_user_allowed);
>  
> +#endif
> +
>  bool riscv_v_first_use_handler(struct pt_regs *regs)
>  {
>  	u32 __user *epc = (u32 __user *)regs->epc;
> @@ -227,6 +233,8 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
>  	return true;
>  }
>  
> +#ifdef CONFIG_RISCV_ISA_V_VSTATE_CTRL
> +
>  void riscv_v_vstate_ctrl_init(struct task_struct *tsk)
>  {
>  	bool inherit;
> @@ -330,3 +338,5 @@ static int __init riscv_v_init(void)
>  	return riscv_v_sysctl_init();
>  }
>  core_initcall(riscv_v_init);
> +
> +#endif
> -- 
> 2.52.0
>