[PATCH -next v21 22/27] riscv: Add sysctl to set the default vector rule for new processes

[Andy Chiu]

To support Vector extension, the series exports variable-length vector
registers on the signal frame. However, this potentially breaks abi if
processing vector registers is required in the signal handler for old
binaries. For example, there is such need if user-level context switch
is triggerred via signals[1].

For this reason, it is best to leave a decision to distro maintainers,
where the enablement of userspace Vector for new launching programs can
be controlled. Developers may also need the switch to experiment with.
The parameter is configurable through sysctl interface so a distro may
turn off Vector early at init script if the break really happens in the
wild.

The switch will only take effects on new execve() calls once set. This
will not effect existing processes that do not call execve(), nor
processes which has been set with a non-default vstate_ctrl by making
explicit PR_RISCV_V_SET_CONTROL prctl() calls.

Link: https://lore.kernel.org/all/87cz4048rp.fsf@all.your.base.are.belong.to.us/
Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
Reviewed-by: Greentime Hu <greentime.hu at sifive.com>
Reviewed-by: Vincent Chen <vincent.chen at sifive.com>
Reviewed-by: Björn Töpel <bjorn at rivosinc.com>
---
Changelog v20:
 - Use READ_ONCE to access riscv_v_implicit_uacc (Björn)
---
 arch/riscv/kernel/vector.c | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index a7dec9230164..f9c8e19ab301 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -180,7 +180,7 @@ void riscv_v_vstate_ctrl_init(struct task_struct *tsk)
 
 	next = riscv_v_ctrl_get_next(tsk);
 	if (!next) {
-		if (riscv_v_implicit_uacc)
+		if (READ_ONCE(riscv_v_implicit_uacc))
 			cur = PR_RISCV_V_VSTATE_CTRL_ON;
 		else
 			cur = PR_RISCV_V_VSTATE_CTRL_OFF;
@@ -243,3 +243,34 @@ long riscv_v_vstate_ctrl_set_current(unsigned long arg)
 
 	return -EINVAL;
 }
+
+#ifdef CONFIG_SYSCTL
+
+static struct ctl_table riscv_v_default_vstate_table[] = {
+	{
+		.procname	= "riscv_v_default_allow",
+		.data		= &riscv_v_implicit_uacc,
+		.maxlen		= sizeof(riscv_v_implicit_uacc),
+		.mode		= 0644,
+		.proc_handler	= proc_dobool,
+	},
+	{ }
+};
+
+static int __init riscv_v_sysctl_init(void)
+{
+	if (has_vector())
+		if (!register_sysctl("abi", riscv_v_default_vstate_table))
+			return -EINVAL;
+	return 0;
+}
+
+#else /* ! CONFIG_SYSCTL */
+static int __init riscv_v_sysctl_init(void) { return 0; }
+#endif /* ! CONFIG_SYSCTL */
+
+static int riscv_v_init(void)
+{
+	return riscv_v_sysctl_init();
+}
+core_initcall(riscv_v_init);
-- 
2.17.1