On Tue, 5 May 2020 at 16:59, Arnd Bergmann <[email protected]> wrote: > > On Tue, May 5, 2020 at 4:50 PM 'Dmitry Vyukov' via Clang Built Linux > <[email protected]> wrote: > > On Tue, May 5, 2020 at 4:36 PM Marco Elver <[email protected]> wrote: > > > > Clang does not allow -fsanitize-coverage=trace-{pc,cmp} together > > > > with -fsanitize=bounds or with ubsan: > > > > > > > > clang: error: argument unused during compilation: > > > > '-fsanitize-coverage=trace-pc' [-Werror,-Wunused-command-line-argument] > > > > clang: error: argument unused during compilation: > > > > '-fsanitize-coverage=trace-cmp' [-Werror,-Wunused-command-line-argument] > > > > > > > > menuconfig KCSAN > > > > bool "KCSAN: dynamic data race detector" > > > > - depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN > > > > + depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !KCOV > > > > > > This also disables KCOV with GCC. Why does this not work with KCSAN? > > My mistake, this should be kept enabled for gcc. If we can get the combination > to work in clang, that's something that should also get enabled.
See my suggestion below how we might dynamically determine if the combination is supported. > > > This is a huge problem for us, since syzbot requires KCOV. In fact > > > I've always been building KCSAN kernels with CONFIG_KCOV=y (with GCC > > > or Clang) and cannot reproduce the problem. > > I have some local patches that change the way we pick the warning options > for each compiler, and enable more of the warnings that are normally disabled. > > Maybe -Wunused-command-line-argument is disabled by default? > I only started seeing this problem recently. It's also possible that there > are some other options that interact with it so only Kcov+FOO leads to > KCSAN being ignored. I see. It certainly seems quite bad if one or the other option is effectively ignored. > > > Ditto, we really need KCOV for all sanitizers. I also just tried to > > > reproduce the problem but can't. > > > > > > Which version of clang is causing this? I'm currently using Clang 9. > > > My guess is that we should not fix this by disallowing KCOV, but > > > rather make Clang work with these configs. > > > > > > Dmitry, can you comment? > > > > FWIW I can reproduce both with clang: > > > > $ clang /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=bounds > > clang-11: warning: argument unused during compilation: > > '-fsanitize-coverage=trace-pc' [-Wunused-command-line-argument] > > > > $ clang /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=thread > > clang-11: warning: argument unused during compilation: > > '-fsanitize-coverage=trace-pc' [-Wunused-command-line-argument] > > > > with both my disto's 9.0.1 and fresher 11.0.0 > > (7b80cb7cf45faf462d6193cc41c2cb7ad556600d. > > > > But both work with gcc > > > > $ gcc /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=thread > > $ gcc /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=bounds > > > > Is it a known issue in clang? > > > > Can we somehow disable it only for clang and not gcc? > > > > This will immediately break KCSAN on syzbot as it enables KCSAN and KCOV: > > https://syzkaller.appspot.com/upstream?manager=ci2-upstream-kcsan-gce > > I can respin the patch with this fixup if you like: > > --- a/lib/Kconfig.kcsan > +++ b/lib/Kconfig.kcsan > @@ -5,7 +5,7 @@ config HAVE_ARCH_KCSAN > > menuconfig KCSAN > bool "KCSAN: dynamic data race detector" > - depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !KCOV > + depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !(KCOV > && CC_IS_CLANG) I wonder if we can just add this: depends on !(KCOV && !$(cc-option,-Werror -fsanitize=thread -fsanitize-coverage=trace-pc)) Similarly for UBSAN. That way, once Clang supports this combination, we don't need another patch to fix it. Thanks, -- Marco > select STACKTRACE > help > The Kernel Concurrency Sanitizer (KCSAN) is a dynamic > > As you both say, the combination seems to be quite important, so maybe there > is something else that can be to also enable it with clang. > > Arnd
