Updated readme

Author: dvvrd

README.md

@@ -1,51 +1,108 @@
-# Universal Symbolic Virtual Machine
+# What is USVM?
 
-**Universal Symbolic Virtual Machine**, or **USVM**, is an ultimately powerful _language-agnostic_ core for implementing
-custom symbolic execution based products.
+USVM is a powerful symbolic execution core designed for analysis of programs in multiple programming languages. Symbolic execution is known to be a very precise but resource demanding technique. USVM makes it faster, more stable, and versatile.
 
-## How we got here
+USVM main features include
 
-USVM is the result of years of experience designing symbolic execution engines for various programming languages. Many
-engines have a lot in common — so why don't we extract and polish it? Many of them still feature different advantages
-— so why don't we unify them to get the most out of symbolic execution?
+* extensible and highly optimized symbolic memory model
+* optimized constraint management to reduce SMT solver workload
+* improved symbolic models for containers (`Map`, `Set`, `List`, etc.)
+* targeted symbolic execution
+* solving type constraints with no SMT solver involved
+* bit-precise reasoning
+* forward and backward symbolic exploration
 
-USVM abstracts language primitives into the generic ones, so, with a simple DSL provided, you are free to implement an
-interpreter for your language. USVM integrates the particular symbolic execution enhancements into one compact form
-— an
-efficient and configurable
-core with a unified API.
+# How can I use it?
 
-## Key features
+With USVM, you can achieve completely automated
+* [static code analysis](#taint-analysis-with-usvm),
+* [unit test cases generation](#usvm-for-unit-test-generation),
+* [targeted fuzzing and more symbolic execution based solutions](#using-usvm-to-confirm-sarif-reports).
 
-What does it mean to be an _efficient_ core?
+Right now, we have ready-to-be-used implementation for [Java](https://github.com/UnitTestBot/usvm/tree/main/usvm-jvm) and experimental implementation for [Python](https://github.com/UnitTestBot/usvm/tree/tochilinak/python/usvm-python).
 
-The USVM core redefines language primitives, so they become symbolic and language-independent, and provides us
-with a range of benefits:
-* optimized constraint management to reduce SMT solver workload;
-* extensible symbolic memory model;
-* forward and backward symbolic exploration;
-* improved symbolic models for containers (`map`, `set`, `list`, etc.);
-* solving type constraints with no SMT solver involved;
-* bit-precise reasoning.
+# Taint analysis with USVM
 
-## Language-specific implementations
+USVM supports interprocedural condition-sensitive taint analysis of JVM bytecode. For instance, it is able to automatically find the following SQL injection:
+![True positive sample](./docs/assets/images/injection.png)
 
-We are now developing a user-friendly USVM API, so that you can easily adapt the core to analyzing programs
-in the required language.
+By default, USVM is able to find other problems:
+* null reference exceptions,
+* out-of-bounds access for collections,
+* integer overflows,
+* division by zero.
 
-For now, we have already implemented symbolic interpreters for _Java_ and _Python_ (the latter is an experimental but
-working example). And the interpreters for _Go_ and _JavaScript_ are under development, so stay tuned.
+You can also extend its analysis rules by [writing custom checkers](#writing-custom-checkers).
 
-These ready-to-use symbolic interpreters are for the managed languages mostly, but you have everything you need to
-analyze programs in whatever language — whether it is _C++_ or an exotic (or even custom) one.
 
-## Applicable scope
+You can run USVM in your repo CI by configuring the [ByteFlow](https://github.com/UnitTestBot/byteflow) runner. 
 
-Symbolic execution is known to be a powerful but slow and demanding technique. USVM makes it faster, more stable, and
-versatile.
+## About condition-sensitive analysis
 
-You can build custom symbolic execution engines with the USVM core inside to show better performance in
-* test generation,
-* static analysis,
-* verification
-* targeted fuzzing,</br>and more symbolic execution based solutions.
+If we modify the above program a little, things change drastically:
+![False positive sample](./docs/assets/images/injection_fp.png)
+
+All interprodecural dataflow analysers we've tried report the similar warning for this program. However, this is false alarm: untrusted data is read only in development mode (that is, when `production` field is false), but the real database query happens only in production mode.
+
+The reason why the existing analysers are wrong is the lack of condition-sensitive analysis: they simply do not understand that untrusted data is emitted only under conditions that prevent program from getting into `checkUserIsAdminProd` method. 
+
+The major reason for this is that condition-sensitive analysis is complex and expensive. USVM makes condition-sensitive analysis robust and scalable. In particular, USVM does not report warning in this program.
+
+You can run the this example online in our [demo repository](https://github.com/unitTestBot/byteflow/security/code-scanning).
+
+# Writing custom checkers
+
+USVM allows to customize its behaviour by writing custom analysis rules. To achieve this, USVM can share its internal analysis states into the attached *interpreter observers*. So the first step to write a custom checker is to implement [`JcInterpreterObserver`](https://github.com/UnitTestBot/usvm/blob/b6ed4682063f1ff6008b3f3c8aa15be663706c74/usvm-jvm/src/main/kotlin/org/usvm/machine/JcInterpreterObserver.kt) interface. This observer can be attached to symbolic machine [in its constructor](https://github.com/UnitTestBot/usvm/blob/b6ed4682063f1ff6008b3f3c8aa15be663706c74/usvm-jvm/src/main/kotlin/org/usvm/machine/JcMachine.kt#L35C17-L35C36).
+
+Now, before every instruction gets symbolically executed, the symbolic engine will notify the observer about the next instruction. For instance, if the engine has reached the `throw` instruction, the attached observer will recieve the corresponding event:
+```kotlin
+fun onThrowStatement(simpleValueResolver: JcSimpleValueResolver, stmt: JcThrowInst, stepScope: JcStepScope)
+```
+
+Here, `stmt` represents the instruction which was proven to be reachable. If, for example, your analysis looks for reachability of `HorrificException`, you can look into type of `stmt.throwable`.
+
+The internal state of the analysis is stored into `stepScope`. In fact, your checker gets the representation of the whole program state in the branch that reaches `stmt`. You can query the arbitrary data stored into state or even modify it (allocate new information or modify the existing) using `stepScope.calcOnState`. For example, to allocate new memory fragment in program state, you can write
+```kotlin
+stepScope.calcOnState { memory.allocateConcreteRef() }
+```
+
+You can compute the validity of arbitrary logical predicates on state. Warning: this can cause queries to SMT solver, which can be time and memory demanding. To query the validity, you can write
+```kotlin
+stepScope.calcOnState { 
+	clone().assert(your condition)?.let {
+		... handling case when condition is satisfiable ...
+	}
+}
+```
+
+To form and report warnings in SARIF format, use [built-in reporters](https://github.com/UnitTestBot/jacodb/blob/e61c2fa41533a2f0a39fad0beb220c3350987345/jacodb-analysis/src/main/kotlin/org/jacodb/analysis/sarif/DataClasses.kt#L137).
+
+You can browse the [existing checkers](https://github.com/UnitTestBot/usvm/blob/b6ed4682063f1ff6008b3f3c8aa15be663706c74/usvm-jvm/src/main/kotlin/org/usvm/api/targets/TaintAnalysis.kt) for more examples and details.
+
+# Using USVM to confirm SARIF reports
+
+In lots of cases, the exising static code analysers [report false alarms](#about-condition-sensitive-analysis). USVM has ability to confirm or reject the reported warnings. 
+
+To run USVM in trace reproduction mode, configure one of the [analyses](https://github.com/UnitTestBot/usvm/blob/saloed/usvm-demo/usvm-jvm/src/main/kotlin/org/usvm/api/targets/TaintAnalysis.kt) and [pass a set of traces](https://github.com/UnitTestBot/usvm/blob/a1e931e5e51f463ed4a33009cee1ffa01cd375bd/usvm-jvm/src/main/kotlin/org/usvm/api/targets/TaintAnalysis.kt#L29) into [JcMachine](https://github.com/UnitTestBot/usvm/blob/main/usvm-jvm/src/main/kotlin/org/usvm/machine/JcMachine.kt).
+
+Also, this process can be customized by a [rich set of options](https://github.com/UnitTestBot/usvm/blob/main/usvm-util/src/main/kotlin/org/usvm/UMachineOptions.kt).
+
+# USVM for unit test generation
+
+USVM has ability to discover all possible behaviours of a program. This is a key feature used in white-box test generation engines. In future, USVM will be the default code analysis engine in [UnitTestBot for Java](https://github.com/UnitTestBot/utbotjava).
+
+
+# Other languages support in USVM
+
+[USVM.Core](https://github.com/UnitTestBot/usvm/tree/main/usvm-core) is a framework which provides highly optimized primitives for symbolic execution:
+* construction and manipulation with symbolic expressions (based on [KSMT](https://github.com/UnitTestBot/ksmt) platform);
+* advanced modeling of [memory operations](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/memory);
+* efficient [constraint managenent](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/constraints) and [constraint solving](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/solver);
+* rich set of [search strategies](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/ps) in giant branching spaces;
+* special support of [standard collections](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/collection);
+* special support of [type system](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/types) constraints;
+* collection and reporting of [statistics](https://github.com/UnitTestBot/usvm/tree/main/usvm-core/src/main/kotlin/org/usvm/statistics).
+
+Thus, USVM.Core implements common primitives used in programming languages. This makes much easier instantiating USVM for new programming language: in fact, to support a programming language, you only need to write its interpreter in terms of operations provided by USVM.Core.
+
+If you want to support a new language, please take a look at [sample language support](https://github.com/UnitTestBot/usvm/tree/main/usvm-sample-language) in USVM.