Issue #20448 has been updated by ms-tob (Matt S). **TL;DR: I've been further researching how fuzzers gather coverage information, and what types of coverage are most useful for fuzzing. I think the best course of action regarding this request would be to implement a fairly general coverage instrumentation approach that can be used to implement different types of coverage.** Based on my research, I've found that there are four commonly used types of coverage: 1. Function 2. Basic block 3. Edge 4. Path libFuzzer's SanitizerCoverage tool offers [three coverage instrumentation points](https://clang.llvm.org/docs/SanitizerCoverage.html#instrumentation-points): `func`, `bb`, and `edge`. So these would correspond to (1), (2), and (3) above. AFL's coverage instrumentation implements [edge coverage](https://github.com/mirrorer/afl/blob/master/docs/technical_details.txt#L23-L...). This corresponds to (3) above. I have not seen any practical fuzzers that implement (4) above. I believe this is likely due to the state explosion of trying to track all unique paths through a program. So, if we can implement functionality in the `TracePoint` module to gather coverage information for instrumentation points (1), (2), and (3), then I think we will cover the vast majority of use-cases. libFuzzer and AFL are far and away the two most popular fuzzers. Instead of focusing solely on my specific use-case, I think it's a good idea to enable a generalized solution that others can test out different fuzzing strategies with. The good news is I think supporting (1) above is already possible with the `call`, `a_call`, `b_call`, and `c_call` `TracePoint` events. However, function based instrumentation is the least useful in my opinion. I see function -> basic block -> edge tracking as least to most granular. The tradeoff here is higher granularity requires potentially more memory and compute time, although I think it's negligible in this case. So, if Ruby already supports events for (1), and we're discarding (4), then that leaves us with (2) and (3). Fortunately, both (2) and (3) can be implemented based on a [control-flow graph](https://en.wikipedia.org/wiki/Control-flow_graph) (CFG) generated from branch events. The main difference here is that basic block coverage is simply branch destinations, whereas edge coverage is branch sources + branch destinations. In other words, basic blocks are simply `(branch_dst)` and edges are `(branch_src, branch_dst)`. This is described well in AFL's [coverage measurement](https://github.com/mirrorer/afl/blob/master/docs/technical_details.txt#L23-L...) technical documentation. I've been reading up on this quite a bit, so I've picked out a few quotations to back up my claims:
... tracking accurate path coverage is infeasible in practice, but tracking edge and block coverage are possible. The edge coverage provided by AFL provides more information than block coverage solutions.
- [CollAFL: Path Sensitive Fuzzing](https://wcventure.github.io/FuzzingPaper/Paper/SP18_ColLAFL.pdf)
AFL [60] leverages edge coverage (a.k.a. branch coverage or transition coverage), and libFuzzer [47] supports both edge and block coverage.
- [Not All Coverage Measurements Are Equal: Fuzzing by Coverage Accounting for Input Prioritization](https://www.ndss-symposium.org/wp-content/uploads/2020/02/24422.pdf)
The number of unique paths (#paths) continues to be a common performance measure for greybox fuzzers [17, 18, 68] despite its obvious flaws [36, 38]. The number of unique edges (#edges), reported as map size by AFL-based fuzzers, is often used as a proxy for branch coverage. AFL maintains a fixed-size hashmap containing an entry for every tuple of conditional jumps that are sequentially exercised in the program.
- [On the Reliability of Coverage-Based Fuzzer Benchmarking](https://mboehme.github.io/paper/ICSE22.pdf)
Branch coverage is a straightforward yet effective enhancement over block coverage, which is the most basic one that can only tell which code block is visited. By involving the code block preceding the currently visited one, branch coverage can differentiate the visits of the same code block from different predecessors. Branch here means an edge from one code block to another one. Ideally, branch coverage should be measured as a tuple (prev_block, cur_block), where prev_block and cur_block stand for the previous block ID and the current block ID, respectively. In practice, branch coverage is usually measured by hashing this tuple (as key) into a hash table (e.g., a hit_count map).
- [Be Sensitive and Collaborative: Analyzing Impact of Coverage Metrics in Greybox Fuzzing](https://www.usenix.org/system/files/raid2019-wang-jinghan.pdf) I believe we can achieve both basic block and edge coverage instrumentation with the following API: ```ruby TracePoint.enable_branch_tracepoints TracePoint.new(:branch) do |tp| p tp.branch_src #=> [src_id, src_first_lineno, src_first_column, src_last_lineno, src_last_column] p tp.branch_dst #=> [dst_id, dst_first_lineno, dst_first_column, dst_last_lineno, dst_last_column] end.enable load "target.rb" TracePoint.disable_branch_tracepoints ``` In this case, the `src` is the location in code of the branch statement (e.g. the `if` statement), and the `dst` is the location in code where execution will continue after the branch statement (i.e. which branch is taken). If a fuzzer would like to track basic blocks, they can use only `branch_dst`. If they'd like to track edges, they can use both `branch_src` and `branch_dst`. This would allow fuzzers flexibility in their implementation and allow them to experiment with different instrumentation points to optimize the fuzzing process. It would also allow Ruby fuzzers to be built on both AFL and libFuzzer, which enables further experimentation and options. One small edge case: we will likely want to consider [critical edges](https://en.wikipedia.org/wiki/Control-flow_graph#Special_edges) as well. This is an implementation detail, and shouldn't impact the overall API of the functionality. I think this would be a good, generalized solution that many parties interested in fuzzing could experiment with. Let me know what you think of this proposal. I have a much better understanding of how fuzzers implement and use coverage information now, so I hope this additional information helps. ---------------------------------------- Feature #20448: Make coverage event hooking C API public https://bugs.ruby-lang.org/issues/20448#change-108322 * Author: ms-tob (Matt S) * Status: Open ---------------------------------------- # Abstract Gathering code coverage information is a well-known goal within software engineering. It is most commonly used to assess code coverage during automated testing. A lesser known use-case is coverage-guided fuzz testing, which will be the primary use-case presented in this issue. This issue exists to request that Ruby coverage event hooking be made part of its official, public C API. # Background Ruby currently provides a number of avenues for hooking events *or* gathering coverage information: 1. The [Coverage](https://ruby-doc.org/3.3.0/exts/coverage/Coverage.html) module 2. The [TracePoint](https://ruby-doc.org/3.3.0/TracePoint.html) module 3. The [rb_add_event_hook](https://ruby-doc.org/3.3.0/extension_rdoc.html#label-Hooks+for+the+interpret...) extension function Unfortunately, none of these pieces of functionality solve this issue's specific use-case. The `Coverage` module is not a great fit for real-time coverage analysis with an unknown start and stop point. Coverage-guided fuzz testing requires this. The `TracePoint` module and `rb_add_event_hook` are not able to hook branch and line coverage events. Coverage-guided fuzz testing typically tracks branch events. # Proposal The ultimate goal is to enable Ruby C extensions to process coverage events in real-time. I did some cursory investigation into the Ruby C internals to determine what it would take to achieve this, but I'm by no means an expert, so my list may be incomplete. The good news is that much of this functionality already exists, but it's part of the private, internal-only C API. 1. Make `RUBY_EVENT_COVERAGE_LINE` and `RUBY_EVENT_COVERAGE_BRANCH` public: https://github.com/ruby/ruby/blob/v3_3_0/vm_core.h#L2182-L2184 a. This would be an addition to the current public event types: https://github.com/ruby/ruby/blob/v3_3_0/include/ruby/internal/event.h#L32-L... 2. Allow initializing global coverage state so that coverage tracking can be fully enabled a. Currently, if `Coverage.setup` or `Coverage.start` is not called, then coverage events cannot be hooked. I do not fully understand why this is, but I believe it has something to do with `rb_get_coverages` and `rb_set_coverages`. If calls to `rb_get_coverages` return `NULL` (https://github.com/ruby/ruby/blob/v3_3_0/iseq.c#L641-L647, https://github.com/ruby/ruby/blob/v3_3_0/iseq.c#L864-L868), then coverage hooking will not be enabled. I believe the `Coverage` module initializes that state via a `rb_set_coverages` call here: https://github.com/ruby/ruby/blob/v3_3_0/ext/coverage/coverage.c#L112-L120. b. So, to achieve this goal, a C extension would need to be able to call `rb_set_coverages` or somehow initialize the global coverage state. I've actually been able to achieve this functionality by calling undocumented features and defining `RUBY_EVENT_COVERAGE_BRANCH`: ```c #include <ruby.h> #include <ruby/debug.h> #define RUBY_EVENT_COVERAGE_BRANCH 0x020000 // ... rb_event_flag_t events = RUBY_EVENT_COVERAGE_BRANCH; rb_event_hook_flag_t flags = ( RUBY_EVENT_HOOK_FLAG_SAFE | RUBY_EVENT_HOOK_FLAG_RAW_ARG ); rb_add_event_hook2( (rb_event_hook_func_t) event_hook_branch, events, counter_hash, flags ); ``` If I call `Coverage.setup(branches: true)`, and add this event hook, then branch hooking works as expected. `rb_add_event_hook2` will still respect the `RUBY_EVENT_COVERAGE_BRANCH` value if its passed. But it would be better if I could rely on official functionality rather than undocumented features. The above two points would be requirements for this functionality, but there's an additional nice-to-have: 3. Extend the public `tracearg` functionality to include additional coverage information a. Currently, `tracearg` offers information like `rb_tracearg_lineno` and `rb_tracearg_path`. It would be helpful if it also provided additional coverage information like `coverage.c`'s column information and a unique identifier for each branch. Currently, I can only use `(path, lineno)` as a unique identifier for a branch because that's what's offered by the public API, but more information like column number would be helpful for uniquely identify branches. Since there can be multiple `if` statements on a single line, this can provide ambiguous identification for a branch event. # Use cases This use-case was born out of a new coverage-guided Ruby fuzzer: https://github.com/trailofbits/ruzzy. You can read more about its implementation details here: https://blog.trailofbits.com/2024/03/29/introducing-ruzzy-a-coverage-guided-.... You can also find the Ruby C extension code behind its implementation here: https://github.com/trailofbits/ruzzy/blob/v0.7.0/ext/cruzzy/cruzzy.c#L220-L2.... So, the primary use-case here is enabling real-time, coverage-guided fuzz testing of Ruby code. However, as mentioned in the abstract, gathering code coverage information is useful in many domains. For example, it could enable new workflows in standard unit/integration test coverage. It could also enable gathering coverage information in real-time as an application is running. I see this as the most generalized form of gathering code coverage information, and something like the `Coverage` module as a specialized implementation. Another example, https://bugs.ruby-lang.org/issues/20282 may be solved by this more generalized solution. We are tracking this request downstream here: https://github.com/trailofbits/ruzzy/issues/9 # Discussion Fuzz testing is another tool in a testers toolbelt. It is an increasingly common way to improve software's robustness. Go has it built in to the standard library, Python has Atheris, Java has Jazzer, JavaScript has Jazzer.js, etc. OSS-Fuzz has helped identify and fix over 10,000 vulnerabilities and 36,000 bugs [using fuzzing](https://google.github.io/oss-fuzz/#trophies). Ruby deserves a good fuzzer, and improving coverage gathering would help achieve that goal. The `Coverage` module, `TracePoint` module, and `rb_add_event_hook` function seem like they could fulfill this goal. However, after deeper investigation, none of them fit the exact requirements for this use-case. # See also - https://bugs.ruby-lang.org/issues/20282 - https://github.com/google/atheris - https://security.googleblog.com/2020/12/how-atheris-python-fuzzer-works.html - https://github.com/CodeIntelligenceTesting/jazzer/ - https://www.code-intelligence.com/blog/java-fuzzing-with-jazzer - https://go.dev/doc/security/fuzz/ -- https://bugs.ruby-lang.org/