linux-kernel - Re: [PATCH v5 2/9] riscv: vector: init vector context with proper vlenb

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Message-ID: <CAFTtA3MdoL5XjcjTrNhZ27UXZe4Mxot-rttc8cQ-K=E-Mh164A@mail.gmail.com>
Date: Wed, 7 Jan 2026 00:49:31 -0600
From: Andy Chiu <andybnac@...il.com>
To: Sergey Matyukevich <geomatsi@...il.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>, Oleg Nesterov <oleg@...hat.com>, 
	Shuah Khan <shuah@...nel.org>, Thomas Huth <thuth@...hat.com>, 
	Charlie Jenkins <charlie@...osinc.com>, Samuel Holland <samuel.holland@...ive.com>, 
	Joel Granados <joel.granados@...nel.org>, Conor Dooley <conor.dooley@...rochip.com>, 
	Yong-Xuan Wang <yongxuan.wang@...ive.com>, Heiko Stuebner <heiko@...ech.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v5 2/9] riscv: vector: init vector context with proper vlenb

On Sun, Dec 14, 2025 at 10:35 AM Sergey Matyukevich <geomatsi@...il.com> wrote:
>
> The vstate in thread_struct is zeroed when the vector context is
> initialized. That includes read-only register vlenb, which holds
> the vector register length in bytes. Zeroed state persists until
> mstatus.VS becomes 'dirty' and a context switch saves the actual
> hardware values.
>
> This can expose the zero vlenb value to the user-space in early
> debug scenarios, e.g. when ptrace attaches to a traced process
> early, before any vector instruction except the first one was
> executed.
>
> Fix this by specifying proper vlenb on vector context init.
>
> Signed-off-by: Sergey Matyukevich <geomatsi@...il.com>

Reviewed-by: Andy Chiu <andybnac@...il.com>

> ---
>  arch/riscv/kernel/vector.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 3ed071dab9d8..b112166d51e9 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -111,8 +111,8 @@ bool insn_is_vector(u32 insn_buf)
>         return false;
>  }
>
> -static int riscv_v_thread_zalloc(struct kmem_cache *cache,
> -                                struct __riscv_v_ext_state *ctx)
> +static int riscv_v_thread_ctx_alloc(struct kmem_cache *cache,
> +                                   struct __riscv_v_ext_state *ctx)
>  {
>         void *datap;
>
> @@ -122,13 +122,15 @@ static int riscv_v_thread_zalloc(struct kmem_cache *cache,
>
>         ctx->datap = datap;
>         memset(ctx, 0, offsetof(struct __riscv_v_ext_state, datap));
> +       ctx->vlenb = riscv_v_vsize / 32;
> +
>         return 0;
>  }
>
>  void riscv_v_thread_alloc(struct task_struct *tsk)
>  {
>  #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
> -       riscv_v_thread_zalloc(riscv_v_kernel_cachep, &tsk->thread.kernel_vstate);
> +       riscv_v_thread_ctx_alloc(riscv_v_kernel_cachep, &tsk->thread.kernel_vstate);
>  #endif
>  }
>
> @@ -214,12 +216,14 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
>          * context where VS has been off. So, try to allocate the user's V
>          * context and resume execution.
>          */
> -       if (riscv_v_thread_zalloc(riscv_v_user_cachep, &current->thread.vstate)) {
> +       if (riscv_v_thread_ctx_alloc(riscv_v_user_cachep, &current->thread.vstate)) {
>                 force_sig(SIGBUS);
>                 return true;
>         }
> +
>         riscv_v_vstate_on(regs);
>         riscv_v_vstate_set_restore(current, regs);
> +
>         return true;
>  }
>
> --
> 2.52.0
>