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SubjectRe: [PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr_key
On Tue, 15 May 2018 09:55:13 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, May 15, 2018 at 7:06 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> > - smp_mb();
> > + smp_wmb();
> > WRITE_ONCE(have_filled_random_ptr_key, true);
>
>
> > + /* Read ptr_key after reading have_filled_random_ptr_key */
> > + smp_rmb();
> > +
> > #ifdef CONFIG_64BIT
> > hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
>
> Hmm. smp_wmb/rmb are basically free on x86, but on some architectures
> smp_rmb() in particular can be pretty expensive.
>
> So when you have a "handoff" situation like this, it's _probably_ better to
> use use "smp_store_release()" and "smp_load_acquire()". To some degree that
> might also be better for documentation purposes, because that's exactly the
> "release-acquire" pattern.
>
> That said, I'm not convinced this really matters all that much for a
> boot-time flag like this. The race is pretty theoretical.
>

I was thinking the same. But since the smp_mb() is there, then it
should be correct, which it currently isn't.

We could change this to a static key, and enable it after we set up
the ptr_key. That would be a one time change at boot up, wont have
races, and have no overhead.

-- Steve

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 30c0cb8cc9bc..da4ea056a309 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1669,19 +1669,21 @@ char *pointer_string(char *buf, char *end, const void *ptr,
return number(buf, end, (unsigned long int)ptr, spec);
}

-static bool have_filled_random_ptr_key __read_mostly;
+static DEFINE_STATIC_KEY_TRUE(not_filled_random_ptr_key);
static siphash_key_t ptr_key __read_mostly;

+static void enable_ptr_key_workfn(struct work_struct *work)
+{
+ /* Needs to run from preemptable context */
+ static_branch_disable(&not_filled_random_ptr_key);
+}
+
+static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);
+
static void fill_random_ptr_key(struct random_ready_callback *unused)
{
get_random_bytes(&ptr_key, sizeof(ptr_key));
- /*
- * have_filled_random_ptr_key==true is dependent on get_random_bytes().
- * ptr_to_id() needs to see have_filled_random_ptr_key==true
- * after get_random_bytes() returns.
- */
- smp_mb();
- WRITE_ONCE(have_filled_random_ptr_key, true);
+ queue_work(system_unbound_wq, &enable_ptr_key_work);
}

static struct random_ready_callback random_ready = {
@@ -1709,7 +1711,7 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
unsigned long hashval;
const int default_width = 2 * sizeof(ptr);

- if (unlikely(!have_filled_random_ptr_key)) {
+ if (static_branch_unlikely(&not_filled_random_ptr_key)) {
spec.field_width = default_width;
/* string length must be less than default_width */
return string(buf, end, "(ptrval)", spec);
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