On Thu, Oct 29, 2020 at 10:45 AM Song Liu <songliubraving@xxxxxx> wrote: > > > On Oct 29, 2020, at 9:09 AM, Ian Rogers <irogers@xxxxxxxxxx> wrote: > > > > If bits is 0, the case when the map is empty, then the >> is the size of > > the register which is undefined behavior - on x86 it is the same as a > > shift by 0. Fix by handling the 0 case explicitly when running with > > address sanitizer. > > > > A variant of this patch was posted previously as: > > https://lore.kernel.org/lkml/20200508063954.256593-1-irogers@xxxxxxxxxx/ > > > > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx> > > --- > > tools/lib/bpf/hashmap.h | 14 ++++++++++++++ > > 1 file changed, 14 insertions(+) > > > > diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h > > index d9b385fe808c..27d0556527d3 100644 > > --- a/tools/lib/bpf/hashmap.h > > +++ b/tools/lib/bpf/hashmap.h > > @@ -12,9 +12,23 @@ > > #include <stddef.h> > > #include <limits.h> > > > > +#ifdef __has_feature > > +#define HAVE_FEATURE(f) __has_feature(f) > > +#else > > +#define HAVE_FEATURE(f) 0 > > +#endif > > + > > static inline size_t hash_bits(size_t h, int bits) > > { > > /* shuffle bits and return requested number of upper bits */ > > +#if defined(ADDRESS_SANITIZER) || HAVE_FEATURE(address_sanitizer) > > I am not very familiar with these features. Is address sanitizer same > as undefined behavior sanitizer (mentioned in previous version)? My preference would be to special case bits == 0 without the feature guards as per the original change, this is the most correct. There is some feature support for detecting ubsan: https://github.com/google/sanitizers/issues/765 In my case I see this with address sanitizer and older versions of clang don't expose ubsan as a feature. > > + /* > > + * If the requested bits == 0 avoid undefined behavior from a > > + * greater-than bit width shift right (aka invalid-shift-exponent). > > + */ > > + if (bits == 0) > > + return -1; > > Shall we return 0 or -1 (0xffffffff) here? The value isn't used and so doesn't matter. -1 seemed less likely to silently succeed. > Also, we have HASHMAP_MIN_CAP_BITS == 2. Shall we just make sure we > never feed bits == 0 into hash_bits()? I think that'd be a different change. I'd be happy to see it. Thanks, Ian > Thanks, > Song > > > > +#endif > > #if (__SIZEOF_SIZE_T__ == __SIZEOF_LONG_LONG__) > > /* LP64 case */ > > return (h * 11400714819323198485llu) >> (__SIZEOF_LONG_LONG__ * 8 - bits); > > -- > > 2.29.1.341.ge80a0c044ae-goog > > >
