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

Andy Chiu
2026-01-07T06:49:45Z

On Sun, Dec 14, 2025 at 10:35 AM Sergey Matyukevich <geomatsi@gmail.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@gmail.com>

Reviewed-by: Andy Chiu <andybnac@gmail.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
>