# Communication model of fuzzilli with V8

## Source code

On a low level, Fuzzilli communicates with v8 through the REPRL protocol, implemented on the fuzzer side by the libreprl C library in `Sources/libreprl/`. The main way of using the library is through the following three functions:

`reprl_create_context()` this creates a new, empty REPRL context to be used by the following APIs.

`reprl_initialize_context(ctx, argv, envp)` this initializes the given context and sets the argv and envp vectors to use for the child processes.

`reprl_execute(ctx, code)` this executes the given code and returns the exit status. If necessary, a new child process is created for this. This involves creating pipes, forking itself, then setting filedescriptors, and using `execve` to execute the d8 binary. A child process can be reused for multiple executions, thus increasing fuzzing performance as the overhead of fork and execve are removed.

## Coverage

Coverage information are being monitored through shared memory. On the side of v8 it is monitored through SanitizerCoverage module of Clang compiler ( https://clang.llvm.org/docs/SanitizerCoverage.html ) Through shared memory information about edges are shared with fuzzilli which implements counter for error and covered branches of the V8 code in Sources/libcoverage/coverage.c