On Fri, Apr 16, 2021 at 3:26 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > On Fri, Apr 16, 2021 at 3:13 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > On Fri, 16 Apr 2021 09:51:45 +0200 > > Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > > > > > If you look at substantial base of bisection logs, you will find lots > > > of cases where bug types, functions don't match. Kernel crashes > > > differently even on the same revision. And obviously things change if > > > you change revisions. Also if you see presumably a different bug, what > > > does it say regarding the original bug. > > > > Yes, but there are also several types of cases where the issue will be the > > same. Namely lockdep. I agree that use after free warnings can have a side > > effect, and may be more difficult. > > But how do we know it's lockdep, rather than a use-after-free > manifested as lockdep? > A Significant portion of kernel bugs are caused by concurrency and can > manifest in different ways, e.g. these are not lockdep, or WARN, or > use-after-free, but rather a race in nature. > > > But there's many other bugs that remain > > consistent across kernels. And if you stumble on one of them, look for it > > only. > > For example? Does not look to be true for WARN, BUG, KASAN, > "inconsistent lock state". > > > > And if you hit another bug, and if it doesn't crash, then ignore it (of > > course this could be an issue if you have panic on warning set). But > > otherwise, just skip it. > > It's not possible to skip, say, BUG. > And if we skip, say, a use-after-free, how do we know we are not > making things worse? Because now we are running on corrupted memory, > so anything can happen. Definitely a stray lockdep report can happen, > or other way around not happen when it should... > > > > I would very much like to improve automatic bisection quality, but it > > > does not look trivial at all. > > > > > > Some random examples where, say, your hypothesis of WARN-to-WARN, > > > BUG-to-BUG does not hold even on the same kernel revision (add to this > > > > At least lockdep to lockdep, as when I do manual bisects, that's exactly > > what I look for, and ignore all other warnings. And that has found the > > problem commit pretty much every time. > > What lockdep bug types do you mean? All? > In the examples above you can see at least "inconsistent lock state" > mixed with 2 other completely different bug types. I've looked for some examples. Here a reliable deadlock in io_uring changes to reliable task hung in io_uring: run #6: crashed: possible deadlock in io_poll_double_wake all runs: crashed: INFO: task hung in io_ring_exit_work https://syzkaller.appspot.com/text?tag=Log&x=135e4d31d00000 Here deadlock is mixed with "lock held when returning to user space": run #6: crashed: possible deadlock in inet_stream_connect run #7: crashed: WARNING: lock held when returning to user space in inet_stream_connect https://syzkaller.appspot.com/text?tag=Log&x=109b88f2d00000 Here a reliable deadlock in n_tty_receive_buf_common changes to UAF in n_tty_receive_buf_common: all runs: crashed: possible deadlock in n_tty_receive_buf_common all runs: crashed: KASAN: use-after-free Read in n_tty_receive_buf_common https://syzkaller.appspot.com/text?tag=Log&x=106596b5e00000 Here deadlock changes to UAF and corrupted list: run #4: crashed: possible deadlock in neigh_change_state run #1: crashed: KASAN: use-after-free Read in neigh_mark_dead run #2: crashed: BUG: corrupted list in ___neigh_create run #3: crashed: BUG: corrupted list in neigh_mark_dead https://syzkaller.appspot.com/text?tag=Log&x=1328a1e2e00000 But it also shows another interesting aspect, kernel output is sometimes corrupted and kernel does not manage to print proper complete oops message. In these cases we can't reliably say that it's also a deadlock or not, we can't say anything about the crash, it's unparsable. This problem also shows up here: https://syzkaller.appspot.com/text?tag=Log&x=145c3a6d200000 and here https://syzkaller.appspot.com/text?tag=Log&x=1006f18c600000 and in the second one also these 2 are mixed: run #1: crashed: WARNING: locking bug in corrupted run #2: crashed: WARNING: possible circular locking dependency detected This one shows a related issue. Kernel crash output periodically changes, and a testing system fails to parse it. This can happen if we go back to older releases, or when we do fix bisection and test a very new kernel with changed output: https://syzkaller.appspot.com/text?tag=Log&x=108f4416e00000 It's also easy to imagine that a program that triggers 2 deadlocks on different revisions, both are deadlocks, but completely different and unrelated. And we can't look at function names as they change over time. All these things are almost trivial when you do bisection manually. But for an automated system, they are quite challenging to handle. > > > different revisions and the fact that a different bug does not give > > > info regarding the original bug): > > > > > > > Can you tell me that all these examples bisected to the commit that caused > > the bug? Because if it did not, then you may have just proved my point ;-) > > I don't know now what was the result, but for a single run these were > manifestations of the same root bug. > E.g. see below, that's UAF in fuse_dev_do_read vs WARNING in > request_end. request_end is also fuse. And you can see that a memory > corruption causing a random bug type, in this case WARNING, but can as > well be LOCKDEP. > > > > > run #0: crashed: KASAN: use-after-free Read in fuse_dev_do_read > > > run #1: crashed: WARNING in request_end > > > run #2: crashed: KASAN: use-after-free Read in fuse_dev_do_read > > > run #3: OK > > > run #4: OK
