linux-kernel - [PATCH v2 6/6] riscv: vector: initialize vlenb on the first context switch

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Message-ID: <20251007115840.2320557-7-geomatsi@gmail.com>
Date: Tue,  7 Oct 2025 14:58:22 +0300
From: Sergey Matyukevich <geomatsi@...il.com>
To: linux-riscv@...ts.infradead.org,
	linux-kselftest@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Paul Walmsley <pjw@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>,
	Oleg Nesterov <oleg@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	Jisheng Zhang <jszhang@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Huth <thuth@...hat.com>,
	Charlie Jenkins <charlie@...osinc.com>,
	Andy Chiu <andybnac@...il.com>,
	Han Gao <rabenda.cn@...il.com>,
	Samuel Holland <samuel.holland@...ive.com>,
	Nam Cao <namcao@...utronix.de>,
	Joel Granados <joel.granados@...nel.org>,
	Clément Léger <cleger@...osinc.com>,
	Conor Dooley <conor.dooley@...rochip.com>,
	Sergey Matyukevich <geomatsi@...il.com>
Subject: [PATCH v2 6/6] riscv: vector: initialize vlenb on the first context switch

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. This 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 forcing the vector context save on the first context switch.

Signed-off-by: Sergey Matyukevich <geomatsi@...il.com>
---
 arch/riscv/kernel/vector.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index 901e67adf576..3dd22a71aa18 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -120,6 +120,7 @@ static int riscv_v_thread_zalloc(struct kmem_cache *cache,
 
 	ctx->datap = datap;
 	memset(ctx, 0, offsetof(struct __riscv_v_ext_state, datap));
+
 	return 0;
 }
 
@@ -216,8 +217,11 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
 		force_sig(SIGBUS);
 		return true;
 	}
+
 	riscv_v_vstate_on(regs);
 	riscv_v_vstate_set_restore(current, regs);
+	set_tsk_thread_flag(current, TIF_RISCV_V_FORCE_SAVE);
+
 	return true;
 }
 
-- 
2.51.0