Merge "refactor(trng): discarding the used entropy bits" into integration
diff --git a/services/std_svc/trng/trng_entropy_pool.c b/services/std_svc/trng/trng_entropy_pool.c
index 30105b3..dd08c5e 100644
--- a/services/std_svc/trng/trng_entropy_pool.c
+++ b/services/std_svc/trng/trng_entropy_pool.c
@@ -66,7 +66,7 @@
bool trng_pack_entropy(uint32_t nbits, uint64_t *out)
{
bool ret = true;
-
+ uint32_t bits_to_discard = nbits;
spin_lock(&trng_pool_lock);
if (!trng_fill_entropy(nbits)) {
@@ -111,9 +111,66 @@
* 5 4 3 2 1 0 7 6
* [e,e,e,e,e,e,e,e]
*/
- out[word_i] = 0;
out[word_i] |= entropy[ENTROPY_WORD_INDEX(word_i)] >> rshift;
+ /**
+ * Discarding the used/packed entropy bits from the respective
+ * words, (word_i) and (word_i+1) as applicable.
+ * In each iteration of the loop, we pack 64bits of entropy to
+ * the output buffer. The bits are picked linearly starting from
+ * 1st word (entropy[0]) till 4th word (entropy[3]) and then
+ * rolls back (entropy[0]). Discarding of bits is managed
+ * similarly.
+ *
+ * The following diagram illustrates the logic:
+ *
+ * |---------entropy pool----------|
+ * C var |--(word_i + 1)-|----word_i-----|
+ * bit idx |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+ * [e,e,e,e,e,e,e,e|e,e,0,0,0,0,0,0]
+ * | [e,e,e,e,e,e,e,e] |
+ * | |--out[word_i]--| |
+ * lshift|---| |--rshift---|
+ * |e,e|0,0,0,0,0,0,0,0|0,0,0,0,0,0|
+ * |<== || ==>|
+ * bits_to_discard (from these bytes)
+ *
+ * variable(bits_to_discard): Tracks the amount of bits to be
+ * discarded and is updated accordingly in each iteration.
+ *
+ * It monitors these packed bits from respective word_i and
+ * word_i+1 and overwrites them with zeros accordingly.
+ * It discards linearly from the lowest index and moves upwards
+ * until bits_to_discard variable becomes zero.
+ *
+ * In the above diagram,for example, we pack 2bytes(7th and 6th
+ * from word_i) and 6bytes(0th till 5th from word_i+1), combine
+ * and pack them as 64bit to output buffer out[i].
+ * Depending on the number of bits requested, we discard the
+ * bits from these packed bytes by overwriting them with zeros.
+ */
+
+ /*
+ * If the bits to be discarded is lesser than the amount of bits
+ * copied to the output buffer from word_i, we discard that much
+ * amount of bits only.
+ */
+ if (bits_to_discard < (BITS_PER_WORD - rshift)) {
+ entropy[ENTROPY_WORD_INDEX(word_i)] &=
+ (~0ULL << ((bits_to_discard+rshift) % BITS_PER_WORD));
+ bits_to_discard = 0;
+ } else {
+ /*
+ * If the bits to be discarded is more than the amount of valid
+ * upper bits from word_i, which has been copied to the output
+ * buffer, we just set the entire word_i to 0, as the lower bits
+ * will be already zeros from previous operations, and the
+ * bits_to_discard is updated precisely.
+ */
+ entropy[ENTROPY_WORD_INDEX(word_i)] = 0;
+ bits_to_discard -= (BITS_PER_WORD - rshift);
+ }
+
/*
* Note that a shift of 64 bits is treated as a shift of 0 bits.
* When the shift amount is the same as the BITS_PER_WORD, we
@@ -123,6 +180,35 @@
if (lshift != BITS_PER_WORD) {
out[word_i] |= entropy[ENTROPY_WORD_INDEX(word_i + 1)]
<< lshift;
+ /**
+ * Discarding the remaining packed bits from upperword
+ * (word[i+1]) which was copied to output buffer by
+ * overwriting with zeros.
+ *
+ * If the remaining bits to be discarded is lesser than
+ * the amount of bits from [word_i+1], which has been
+ * copied to the output buffer, we overwrite that much
+ * amount of bits only.
+ */
+ if (bits_to_discard < (BITS_PER_WORD - lshift)) {
+ entropy[ENTROPY_WORD_INDEX(word_i+1)] &=
+ (~0ULL << ((bits_to_discard) % BITS_PER_WORD));
+ bits_to_discard = 0;
+ } else {
+ /*
+ * If bits to discard is more than the bits from word_i+1
+ * which got packed into the output, then we discard all
+ * those copied bits.
+ *
+ * Note: we cannot set the entire word_i+1 to 0, as
+ * there are still some unused valid entropy bits at the
+ * upper end for future use.
+ */
+ entropy[ENTROPY_WORD_INDEX(word_i+1)] &=
+ (~0ULL << ((BITS_PER_WORD - lshift) % BITS_PER_WORD));
+ bits_to_discard -= (BITS_PER_WORD - lshift);
+ }
+
}
}
const uint64_t mask = ~0ULL >> (BITS_PER_WORD - (nbits % BITS_PER_WORD));