On Wed, Oct 18, 2023 at 09:24:05PM -0700, Andrii Nakryiko wrote: > Add tests that validate correctness and completeness of BPF verifier's > register range bounds. Nitpick: in abstract-interpretation-speak, completeness seems to mean something different. I believe what we're trying to check here is soundness[1], again, in abstraction-interpretation-speak), so using completeness here may be misleading to some. (I'll leave explanation to other that understand this concept better than I do, rather than making an ill attempt that would probably just make things worst) > The main bulk is a lot of auto-generated tests based on a small set of > seed values for lower and upper 32 bits of full 64-bit values. > Currently we validate only range vs const comparisons, but the idea is > to start validating range over range comparisons in subsequent patch set. CC Langston Barrett who had previously send kunit-based tnum checks[2] a while back. If this patch is merged, perhaps we can consider adding validation for tnum as well in the future using similar framework. More comments below > When setting up initial register ranges we treat registers as one of > u64/s64/u32/s32 numeric types, and then independently perform conditional > comparisons based on a potentially different u64/s64/u32/s32 types. This > tests lots of tricky cases of deriving bounds information across > different numeric domains. > > Given there are lots of auto-generated cases, we guard them behind > SLOW_TESTS=1 envvar requirement, and skip them altogether otherwise. > With current full set of upper/lower seed value, all supported > comparison operators and all the combinations of u64/s64/u32/s32 number > domains, we get about 7.7 million tests, which run in about 35 minutes > on my local qemu instance. So it's something that can be run manually > for exhaustive check in a reasonable time, and perhaps as a nightly CI > test, but certainly is too slow to run as part of a default test_progs run. FWIW an alternative approach that speeds things up is to use model checkers like Z3 or CBMC. On my laptop, using Z3 to validate tnum_add() against *all* possible inputs takes less than 1.3 seconds[3] (based on code from [1] paper, but I somehow lost the link to their GitHub repository). One of the potential issue with [3] is that Z3Py is written in Python. So there's the large over head of translating the C-implementation into Python using Z3Py APIs each time we changed relevant code. This overhead could potentially be removed with CBMC, which understand C, and we had a precedence of using CBMC[4] within the kernel source code, though it was later removed[5] due because SRCU changes are still happening too fast for the format tests to keep up, so it looks like CBMC is not a silver-bullet. I really meant to look into the CMBC approach for verification of ranges and tnum, but fails to allocate time for it, so far. Shung-Hsi > ... 1: https://people.cs.rutgers.edu/~sn349/papers/cgo-2022.pdf 2: https://lore.kernel.org/bpf/20220430215727.113472-1-langston.barrett@xxxxxxxxx/ 3: https://gist.github.com/shunghsiyu/a63e08e6231553d1abdece4aef29f70e 4: https://lore.kernel.org/all/1485295229-14081-3-git-send-email-paulmck@xxxxxxxxxxxxxxxxxx/
