On 201104 1546, Peter Maydell wrote: > On Wed, 4 Nov 2020 at 15:26, Alexander Bulekov <alx...@bu.edu> wrote: > > If I understand correctly, this is analogous to what happens with > > Coverity reports. Access to Coverity is closed (not sure if there is a > > process to apply for access). It also seems that there is a push to fix > > CID issues prior to new releases. Maybe a similar process can be used for > > fuzzing? > > Coverity is only closed in the sense that you have to request > an account on the website. Anybody who's a QEMU developer > can look at the reports. >
OK thats good to know. Why should fuzzing reports be treated differently? Is it because they come with a ready-to-use reproducer? > The attempt to fix CID issues works because: > * Coverity reports a fairly small number of issues, so > "fix them all" is relatively tractable, and then once you > get down to "no outstanding issues" the only new ones > that get found are for new changes to QEMU (not entirely > true, but close enough) I think fuzzing is quite similar. After an initial wave of reports, they should slow to a trickle and eventually should just catch problems in new changes. > * Mostly issues are reported soon after the offending code > goes into the tree, so it's often possible to quickly > identify the patch that introduced the issue and ask > the person who wrote that patch to fix the bug While fuzzing can take some time to find issues after new code is committed, I don't think the problem of reports on stale code is unique to fuzzing. That should only be an issue for the initial wave of reports (the same as I'm guessing it was was when Coverity started analyzing QEMU in 2013). Additionally, we can filter crashes based on where they occured, which should give a similar level of control as the Coverity components/patterns. If problems in e.g. ati or vmxnet are unlikely to be looked at, we can 1.) Not fuzz them (easy to do by removing an entry from tests/qtest/fuzz/generic_fuzz_configs.h) 2.) Apply filters to ignore crash reports with ati.c or vmxnet.c in the callstack. That said, IIUC main purpose of the Coverity componsnets/patterns is to prevent false positives. This shouldn't be nearly as much of a problem for oss-fuzz reports for which we can provide qtest reproducers. > * Coverity reports are categorized by kind-of-failure, > so it's easy to prioritize important stuff (buffer overflows) > and leave less significant stuff (dead code) for later Fuzzer reports are the same. OSS-fuzz tells us if something is an overflow, UAF, overlapping-memcpy, double-free, assertion-failure, null-ptr derefs etc. If noise from assertion-failure/null-ptr derefs reports is a concern, maybe we can just ignore those for now? > * Coverity's reports include the automated analysis of > why Coverity thinks there's an issue -- this is not > always right but it's a solid head start on "what's the > bug here" compared to just having a repro case and an > assertion-failure message Thats true - the types of bugs found by fuzzing rather than static analysis usually are tough to automatically suggest (pretty) fixes for. I have been thinking about ways to make this situation better, beyond just enabling the relevant -trace events and hoping they provide context and make life easier. > * There's a set of people who care enough about Coverity > reports to put the time in to fixing them... I hope this set of people grows for fuzzing reports as well. Though fuzzing reports are nothing compared to the ~1250 fixed coverity defects, fuzzing has already helped highlight some serious issues that have been hiding in the code for a long time. Unfortunately, I think there is a gap between the types of problems reported by fuzzing and Coverity, where I don't see someone picking up a dozen random fuzzing reports and having a dozen patches ready by the end of the day. Multiple people working on fuzzer-discovered issues have mentioned that they are often quite time consuming to properly fix (for little preceived reward). Maybe the solution is to limit the scope of fuzzer reports so that they might be rarer, but are more likely to feature problems that people will care about? This might mean having a narrower selection of fuzzed devices than just "anything that works with KVM" -Alex > > thanks > -- PMM
