On Wed, 13 May 2020 at 14:40, Will Deacon <w...@kernel.org> wrote: > > On Wed, May 13, 2020 at 02:32:43PM +0200, Peter Zijlstra wrote: > > On Wed, May 13, 2020 at 01:48:41PM +0200, Marco Elver wrote: > > > > > Disabling most instrumentation for arch/x86 is reasonable. Also fine > > > with the __READ_ONCE/__WRITE_ONCE changes (your improved > > > compiler-friendlier version). > > > > > > We likely can't have both: still instrument __READ_ONCE/__WRITE_ONCE > > > (as Will suggested) *and* avoid double-instrumentation in arch_atomic. > > > If most use-cases of __READ_ONCE/__WRITE_ONCE are likely to use > > > data_race() or KCSAN_SANITIZE := n anyway, I'd say it's reasonable for > > > now. > > I agree that Peter's patch is the right thing to do for now. I was hoping we > could instrument __{READ,WRITE}_ONCE(), but that we before I realised that > __no_sanitize_or_inline doesn't seem to do anything. > > > Right, if/when people want sanitize crud enabled for x86 I need > > something that: > > > > - can mark a function 'no_sanitize' and all code that gets inlined into > > that function must automagically also not get sanitized. ie. make > > inline work like macros (again). > > > > And optionally: > > > > - can mark a function explicitly 'sanitize', and only when an explicit > > sanitize and no_sanitize mix in inlining give the current > > incompatible attribute splat. > > > > That way we can have the noinstr function attribute imply no_sanitize > > and frob the DEFINE_IDTENTRY*() macros to use (a new) sanitize_or_inline > > helper instead of __always_inline for __##func(). > > Sounds like a good plan to me, assuming the compiler folks are onboard. > In the meantime, can we kill __no_sanitize_or_inline and put it back to > the old __no_kasan_or_inline, which I think simplifies compiler.h and > doesn't mislead people into using the function annotation to avoid KCSAN? > > READ_ONCE_NOCHECK should also probably be READ_ONCE_NOKASAN, but I > appreciate that's a noisier change.
So far so good, except: both __no_sanitize_or_inline and __no_kcsan_or_inline *do* avoid KCSAN instrumenting plain accesses, it just doesn't avoid explicit kcsan_check calls, like those in READ/WRITE_ONCE if KCSAN is enabled for the compilation unit. That's just because macros won't be redefined just for __no_sanitize functions. Similarly, READ_ONCE_NOCHECK does work as expected, and its access is unchecked. This will have the expected result: __no_sanitize_or_inline void foo(void) { x++; } // no data races reported This will not work as expected: __no_sanitize_or_inline void foo(void) { READ_ONCE(x); } // data races are reported All this could be fixed if GCC devs would finally take my patch to make -fsanitize=thread distinguish volatile [1], but then we have to wait ~years for the new compilers to reach us. So please don't hold your breath for this one any time soon. [1] https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544452.html Thanks, -- Marco