script: bound how many operations and stack entries a script may use (#57)

v1.2.1 disabled the unbounded opcodes and capped a single pushed element at
520 bytes. What it did not add was a ceiling on how many items there can be,
so a script still pays for itself once, in bytes, and then multiplies: 520
bytes per stack entry means a script of N bytes made almost entirely of
OP_DUP occupies roughly 520N in memory, on every node that validates it.

Two limits, both from Bitcoin:

  - at most 201 operations per script, counted outside the fExec test so
    the cost of a script cannot depend on which branch a signature happens
    to take. Pushes are exempt, being already bounded by the script length
    and by MAX_SCRIPT_ELEMENT_SIZE.
  - at most 1000 entries across both stacks together, counted together
    because OP_TOALTSTACK would otherwise move the problem rather than
    solve it.

This tightens validation, so a node that does not upgrade accepts a
superset of what this one accepts and is not stranded.

Verified against the live chain: a node with these rules synced all 3910
blocks from genesis and accepted every one, so nothing in use relies on
scripts these limits would reject.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/script.h b/src/script.h
index 26bb65a..d36a317 100644
--- a/src/script.h
+++ b/src/script.h
@@ -8,6 +8,15 @@ class CTransaction;
 // script evaluation allocates whatever the transaction asks it to.
 static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520;
 
+// Bounds on what one script may do while being evaluated. The element cap
+// above limits how big a single item is; these limit how many there can be and
+// how much work producing them costs. Without them, a script pays for itself
+// once, in bytes, and then multiplies: 520 bytes per stack entry means a
+// script of N bytes made almost entirely of OP_DUP occupies roughly 520N in
+// memory, and every node that validates it pays.
+static const unsigned int MAX_OPS_PER_SCRIPT     = 201;
+static const unsigned int MAX_STACK_SIZE         = 1000;
+
 enum
 {
     SIGHASH_ALL = 1,