On Mon, Nov 10, 2025 at 10:04:17AM +0100, Thomas Weißschuh wrote:

On Sat, Nov 08, 2025 at 12:46:05AM +0100, Jason A. Donenfeld wrote:

quoted

I'm not a huge fan of this change:

On Thu, Nov 06, 2025 at 11:02:12AM +0100, Thomas Weißschuh wrote:

quoted

+static DEFINE_STATIC_KEY_FALSE(random_vdso_is_ready);
 
 /* Control how we warn userspace. */
 static struct ratelimit_state urandom_warning =

@@ -252,6 +253,9 @@ static void random_vdso_update_generation(unsigned long next_gen)
 	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
 		return;
 
+	if (!static_branch_likely(&random_vdso_is_ready))
+		return;
+
 	/* base_crng.generation's invalid value is ULONG_MAX, while
 	 * vdso_k_rng_data->generation's invalid value is 0, so add one to the
 	 * former to arrive at the latter. Use smp_store_release so that this

@@ -274,6 +278,9 @@ static void random_vdso_set_ready(void)
 	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
 		return;
 
+	if (!static_branch_likely(&random_vdso_is_ready))
+		return;
+
 	WRITE_ONCE(vdso_k_rng_data->is_ready, true);
 }
 

@@ -925,6 +932,9 @@ void __init random_init(void)
 	_mix_pool_bytes(&entropy, sizeof(entropy));
 	add_latent_entropy();
 
+	if (IS_ENABLED(CONFIG_VDSO_GETRANDOM))
+		static_branch_enable(&random_vdso_is_ready);
+
 	/*
 	 * If we were initialized by the cpu or bootloader before jump labels
 	 * or workqueues are initialized, then we should enable the static

@@ -934,8 +944,10 @@ void __init random_init(void)
 		crng_set_ready(NULL);
 
 	/* Reseed if already seeded by earlier phases. */
-	if (crng_ready())
+	if (crng_ready()) {
 		crng_reseed(NULL);
+		random_vdso_set_ready();
+	}

The fact that the vdso datapage is set up by the time random_init() is
called seems incredibly contingent on init details. Why not, instead,
make this a necessary part of the structure of vdso setup code, which
can actually know about what happens when?

The whole early init is "carefully" ordered in any case. I would have been
happy to allocate the data pages before the random initialization, but the
allocator is not yet usable by then.
We could also make the ordering more visible by having the vDSO datastore call
into a dedicated function to allow the random core to touch the data pages:
random_vdso_enable_datapages().

quoted

For example, one clean way of
doing that would be to make vdso_k_rng_data always valid by having it
initially point to __initdata memory, and then when it's time to
initialize the real datapage, memcpy() the __initdata memory to the new
specially allocated memory. Then we don't need the complex state
tracking that this commit and the prior one introduce.

Wouldn't that require synchronization between the update path and the memcpy()
path? Also if the pointer is going to change at some point we'll probably need
to use READ_ONCE()/WRITE_ONCE(). In general I would be happy about a cleaner
solution for this but didn't find a great one.

This is still before userspace has started, and interrupts are disabled,
so I don't think so? Also, you only care about being after
mm_core_init(), right? So move your thing before sched_init() and then
you'll really have nothing to worry about.

But I think globally I agree with Andy/Arnd -- this is kind of ugly and
not worth it. Disable vDSO for these old CPUs with cache aliasing
issues.

Jason