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-rw-r--r--kernel/bpf/tnum.c41
1 files changed, 22 insertions, 19 deletions
diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
index ceac5281bd31..3d7127f439a1 100644
--- a/kernel/bpf/tnum.c
+++ b/kernel/bpf/tnum.c
@@ -111,28 +111,31 @@ struct tnum tnum_xor(struct tnum a, struct tnum b)
return TNUM(v & ~mu, mu);
}
-/* half-multiply add: acc += (unknown * mask * value).
- * An intermediate step in the multiply algorithm.
+/* Generate partial products by multiplying each bit in the multiplier (tnum a)
+ * with the multiplicand (tnum b), and add the partial products after
+ * appropriately bit-shifting them. Instead of directly performing tnum addition
+ * on the generated partial products, equivalenty, decompose each partial
+ * product into two tnums, consisting of the value-sum (acc_v) and the
+ * mask-sum (acc_m) and then perform tnum addition on them. The following paper
+ * explains the algorithm in more detail: https://arxiv.org/abs/2105.05398.
*/
-static struct tnum hma(struct tnum acc, u64 value, u64 mask)
-{
- while (mask) {
- if (mask & 1)
- acc = tnum_add(acc, TNUM(0, value));
- mask >>= 1;
- value <<= 1;
- }
- return acc;
-}
-
struct tnum tnum_mul(struct tnum a, struct tnum b)
{
- struct tnum acc;
- u64 pi;
-
- pi = a.value * b.value;
- acc = hma(TNUM(pi, 0), a.mask, b.mask | b.value);
- return hma(acc, b.mask, a.value);
+ u64 acc_v = a.value * b.value;
+ struct tnum acc_m = TNUM(0, 0);
+
+ while (a.value || a.mask) {
+ /* LSB of tnum a is a certain 1 */
+ if (a.value & 1)
+ acc_m = tnum_add(acc_m, TNUM(0, b.mask));
+ /* LSB of tnum a is uncertain */
+ else if (a.mask & 1)
+ acc_m = tnum_add(acc_m, TNUM(0, b.value | b.mask));
+ /* Note: no case for LSB is certain 0 */
+ a = tnum_rshift(a, 1);
+ b = tnum_lshift(b, 1);
+ }
+ return tnum_add(TNUM(acc_v, 0), acc_m);
}
/* Note that if a and b disagree - i.e. one has a 'known 1' where the other has