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[SMTChecker] Docs

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12
docs/layout-of-source-files.rst
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@ -89,12 +89,12 @@ It is activated for the Ubuntu PPA releases in most versions,
but not for solc-js, the Docker images, Windows binaries or the
statically-built Linux binaries.
If you use
``pragma experimental SMTChecker;``, then you get additional
safety warnings which are obtained by querying an SMT solver.
The component does not yet support all features of the Solidity language
and likely outputs many warnings. In case it reports unsupported
features, the analysis may not be fully sound.
If you use ``pragma experimental SMTChecker;``, then you get additional
:ref:`safety warnings<formal_verification>` which are obtained by querying an
SMT solver.
The component does not yet support all features of the Solidity language and
likely outputs many warnings. In case it reports unsupported features, the
analysis may not be fully sound.
.. index:: source file, ! import, module

184
docs/security-considerations.rst
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@ -331,6 +331,8 @@ The more people examine a piece of code, the more issues are found.
Asking people to review your code also helps as a cross-check to find out whether your code
is easy to understand - a very important criterion for good smart contracts.
.. _formal_verification:
*******************
Formal Verification
*******************
@ -344,3 +346,185 @@ Note that formal verification itself can only help you understand the
difference between what you did (the specification) and how you did it
(the actual implementation). You still need to check whether the specification
is what you wanted and that you did not miss any unintended effects of it.
Solidity implements a formal verification approach based on SMT solving. The
SMTChecker module automatically tries to prove that the code satisfies the
specification given by ``require/assert`` statements. That is, it considers
``require`` statements as assumptions and tries to prove that the conditions
inside ``assert`` statements are always true. If an assertion failure is
found, a counterexample is given to the user, showing how the assertion can be
violated.
The SMTChecker also checks automatically for arithmetic underflow/overflow,
trivial conditions and unreachable code.
It is currently an experimental feature, therefore in order to use it you need
to enable it via :ref:`a pragma directive<smt_checker>`.
The SMTChecker traverses the Solidity AST creating and collecting program constraints.
When it encounters a verification target, an SMT solver is invoked to determine the outcome.
If a check fails, the SMTChecker provides specific input values that lead to the failure.
For more details on how the SMT encoding works internally, see the paper
`SMT-based Verification of Solidity Smart Contracts <https://github.com/leonardoalt/text/blob/master/solidity_isola_2018/main.pdf>`_.
Abstraction and False Positives
===============================
The SMTChecker implements abstractions in an incomplete and sound way: If a bug
is reported, it might be a false positive introduced by abstractions (due to
erasing knowledge or using a non-precise type). If it determines that a
verification target is safe, it is indeed safe, that is, there are no false
negatives (unless there is a bug in the SMTChecker).
The SMT encoding tries to be as precise as possible, mapping Solidity types
and expressions to their closest `SMT-LIB <http://smtlib.cs.uiowa.edu/>`_
representation, as shown in the table below.
+-----------------------+--------------+-----------------------------+
|Solidity type |SMT sort |Theories (quantifier-free) |
+=======================+==============+=============================+
|Boolean |Bool |Bool |
+-----------------------+--------------+-----------------------------+
|intN, uintN, address, |Integer |LIA, NIA |
|bytesN, enum | | |
+-----------------------+--------------+-----------------------------+
|array, mapping |Array |Arrays |
+-----------------------+--------------+-----------------------------+
|other types |Integer |LIA |
+-----------------------+--------------+-----------------------------+
Types that are not yet supported are abstracted by a single 256-bit unsigned integer,
where their unsupported operations are ignored.
Function calls to the same contract (or base contracts) are inlined when
possible, that is, when their implementation is available.
Calls to functions in other contracts are not inlined even if their code is
available, since we cannot guarantee that the actual deployed code is the same.
Complex pure functions are abstracted by an uninterpreted function (UF) over
the arguments.
+-----------------------------------+--------------------------------------+
|Functions |SMT behavior |
+===================================+======================================+
|``assert`` |Verification target |
+-----------------------------------+--------------------------------------+
|``require`` |Assumption |
+-----------------------------------+--------------------------------------+
|internal |Inline function call |
+-----------------------------------+--------------------------------------+
|external |Inline function call |
| |Erase knowledge about state variables |
| |and local storage references |
+-----------------------------------+--------------------------------------+
|``gasleft``, ``blockhash``, |Abstracted with UF |
|``keccak256``, ``ecrecover`` | |
|``ripemd160``, ``addmod``, | |
|``mulmod`` | |
+-----------------------------------+--------------------------------------+
|pure functions without |Abstracted with UF |
|implementation (external or | |
|complex) | |
+-----------------------------------+--------------------------------------+
|external functions without |Unsupported |
|implementation | |
+-----------------------------------+--------------------------------------+
|others |Currently unsupported |
+-----------------------------------+--------------------------------------+
Using abstraction means loss of precise knowledge, but in many cases it does
not mean loss of proving power.
::
pragma solidity >=0.5.0;
pragma experimental SMTChecker;
contract Recover
{
function f(
bytes32 hash,
uint8 _v1, uint8 _v2,
bytes32 _r1, bytes32 _r2,
bytes32 _s1, bytes32 _s2
) public pure returns (address) {
address a1 = ecrecover(hash, _v1, _r1, _s1);
require(_v1 == _v2);
require(_r1 == _r2);
require(_s1 == _s2);
address a2 = ecrecover(hash, _v2, _r2, _s2);
assert(a1 == a2);
return a1;
}
}
In the example above, the SMTChecker is not expressive enough to actually
compute ``ecrecover``, but by modelling the function calls as uninterpreted
functions we know that the return value is the same when called on equivalent
parameters. This is enough to prove that the assertion above is always true.
Abstracting a function call with an UF can be done for functions known to be
deterministic, and can be easily done for pure functions. It is however
difficult to do this with general external functions, since they might depend
on state variables.
External function calls also imply that any current knowledge that the
SMTChecker might have regarding mutable state variables needs to be erased to
guarantee no false negatives, since the called external function might direct
or indirectly call a function in the analyzed contract that changes state
variables.
Reference Types and Aliasing
=============================
Solidity implements aliasing for reference types with the same :ref:`data
location<data-location>`.
That means one variable may be modified through a reference to the same data
area.
The SMTChecker does not keep track of which references refer to the same data.
This implies that whenever a local reference or state variable of reference
type is assigned, all knowledge regarding variables of the same type and data
location is erased.
If the type is nested, the knowledge removal also includes all the prefix base
types.
::
pragma solidity >=0.5.0;
pragma experimental SMTChecker;
// This will not compile
contract Aliasing
{
uint[] array;
function f(
uint[] memory a,
uint[] memory b,
uint[][] memory c,
uint[] storage d
) internal view {
require(array[0] == 42);
require(a[0] == 2);
require(c[0][0] == 2);
require(d[0] == 2);
b[0] = 1;
// Erasing knowledge about memory references should not
// erase knowledge about state variables.
assert(array[0] == 42);
// Fails because `a == b` is possible.
assert(a[0] == 2);
// Fails because `c[i] == b` is possible.
assert(c[0][0] == 2);
assert(d[0] == 2);
assert(b[0] == 1);
}
}
After the assignment to ``b[0]``, we need to clear knowledge about ``a`` since
it has the same type (``uint[]``) and data location (memory). We also need to
clear knowledge about ``c``, since its base type is also a ``uint[]`` located
in memory. This implies that some ``c[i]`` could refer to the same data as
``b`` or ``a``.
Notice that we do not clear knowledge about ``array`` and ``d`` because they
are located in storage, even though they also have type ``uint[]``. However,
if ``d`` was assigned, we would need to clear knowledge about ``array`` and
vice-versa.