random: Add comment to random_initialize()

commit cbc96b7594b5691d61eba2db8b2ea723645be9ca upstream.

Many platforms have per-machine instance data (serial numbers,
asset tags, etc.) squirreled away in areas that are accessed
during early system bringup. Mixing this data into the random
pools has a very high value in providing better random data,
so we should allow (and even encourage) architecture code to
call add_device_randomness() from the setup_arch() paths.

However, this limits our options for internal structure of
the random driver since random_initialize() is not called
until long after setup_arch().

Add a big fat comment to rand_initialize() spelling out
this requirement.

Suggested-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 918979fb4131f6e2d7c414b7865689822ebcf9a1..8ae92357378160fba36ced80412a835be204a263 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1114,6 +1114,16 @@ static void init_std_data(struct entropy_store *r)
        mix_pool_bytes(r, utsname(), sizeof(*(utsname())), NULL);
 }
 
+/*
+ * Note that setup_arch() may call add_device_randomness()
+ * long before we get here. This allows seeding of the pools
+ * with some platform dependent data very early in the boot
+ * process. But it limits our options here. We must use
+ * statically allocated structures that already have all
+ * initializations complete at compile time. We should also
+ * take care not to overwrite the precious per platform data
+ * we were given.
+ */
 static int rand_initialize(void)
 {
        init_std_data(&input_pool);