On Wed, Dec 01, 2021 at 07:16:25PM +0100, Marco Elver wrote: > On Wed, 1 Dec 2021 at 18:46, Mark Rutland <mark.rutland@xxxxxxx> wrote: > [...] > > > > Currently we mostly get away with disabling KCOV for while compilation units, > > > > so maybe it's worth waiting for the GCC 12.0 release, and restricting things > > > > once that's out? > > > > > > An alternative would be to express 'select ARCH_WANTS_NO_INSTR' more > > > precisely, say with an override or something. Because as-is, > > > ARCH_WANTS_NO_INSTR then doesn't quite reflect reality on arm64 > > > (yet?). > > > > It's more of a pragmatic thing -- ARCH_WANTS_NO_INSTR does reflect reality, and > > we do *want* to enforce that strictly, it's just that we're just struck between > > a rock and a hard place where until GCC 12 is released we either: > > > > a) Strictly enforce noinstr, and be sure there aren't any bugs from unexpected > > instrumentation, but we can't test GCC-built kernels under Syzkaller due to > > the lack of KCOV. > > > > b) Don't strictly enforce noinstr, and have the same latent bugs as today (of > > unknown severity), but we can test GCC-built kernels under Syzkaller. > > > > ... and since this (currently only affects KCOV, which people only practically > > enable for Syzkaller, I think it's ok to wait until GCC 12 is out, so that we > > can have the benefit of Sykaller in the mean time, and subsequrntly got for > > option (a) and say those people need to use GCC 12+ (and clang 13+). > > > > > But it does look simpler to wait, so I'm fine with that. I leave it to you. > > > > FWIW, for my purposes I'm happy to take this immediately and to have to apply a > > local patch to my fuzzing branches until GCC 12 is out, but I assume we'd want > > the upstream testing to work in the mean time without requiring additional > > patches. > > Agree, it's not an ideal situation. :-/ > > syzkaller would still work, just not as efficiently. Not sure what's > worse, less efficient fuzzing, or chance of random crashes. In fact, > on syzbot we already had to disable it: > https://github.com/google/syzkaller/blob/61f862782082c777ba335aa4b4b08d4f74d7d86e/dashboard/config/linux/bits/base.yml#L110 > https://lore.kernel.org/linux-arm-kernel/20210119130010.GA2338@C02TD0UTHF1T.local/T/#m78fdfcc41ae831f91c93ad5dabe63f7ccfb482f0 > > So if we ran into issues with KCOV on syzbot for arm64, I'm sure it's > not just us. I can't quite see what the reasons for the crashes are, > but ruling out noinstr vs. KCOV would be a first step. > > So I'm inclined to suggest we take this patch now and not wait for GCC > 12, given we're already crashing with KCOV and therefore have KCOV > disabled on arm64 syzbot. > > I'm still fine waiting, but just wanted to point out you can fuzz > without KCOV. Preferences? If it's not used by Syzbot, that's good enough for me -- I can apply local hacks to run with KCOV if I want to in the mean time, and I can debug my own mess if I have to. So FWIW, for taking that now: Acked-by: Mark Rutland <mark.rutland@xxxxxxx> Thanks, Mark.
