Generally, I think ubsan is discussed on the main clang list, cfe-dev. I don't think there's a separate group for it, since it's fairly low maintenance, and has more to do with checking fidly C++ language rules like this.
In any case, this seems pretty straightforward. If you request a certain field alignment, that will raise the minimum required alignment for the aggregate, and clang emits a ubsan check for that. It seems like the character arrays are enough to prevent false sharing. If you put another padding array between 'r' and 'w' you shouldn't need to worry about false sharing between them. On Wed, Oct 10, 2018 at 11:17 AM <[email protected]> wrote: > > (I'm posting this question about UBSAN on the ASAN list because I couldn't > find a UBSAN list; let me know if there is one.) > > Hello, I have a pretty large C++11 program that I just ran UBSAN on and > I'm getting errors related to misaligned addresses and alignments. It's > non-trivial to show a sample program so I'm hoping I can get pointed in the > right direction without that for now. > > The class C in question has the following characteristics: > > > - Class C itself doesn't do anything special regarding memory > alignment. No posix_memalign, no alignas, etc. > - Class C, however, has a member that is of class M > - Class M does some alignment for some members using alignas. It looks > like this: > > #define FALSE_SHARING_RANGE 128 > > class M { > char pad0_[FALSE_SHARING_RANGE]; > alignas(FALSE_SHARING_RANGE) std::atomic<unsigned int> r; > alignas(FALSE_SHARING_RANGE) std::atomic<unsigned int> w; > char pad1_[FALSE_SHARING_RANGE - sizeof(w)]; > > void* v1; > const uint32_t s; > const uint32_t b; > const uint32_t pb; > } > > > Here are the UBSAN errors: > > runtime error: constructor call on misaligned address 0x000003330ca0 for > type 'C', which requires 128 byte alignment > 0x000003330ca0: note: pointer points here > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 > 00 00 00 00 00 00 00 00 00 > ^ > runtime error: constructor call on misaligned address 0x000003330ca0 for > type 'C *', which requires 128 byte alignment > 0x000003330ca0: note: pointer points here > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 > 00 00 00 00 00 00 00 00 00 > ^ > runtime error: member call on misaligned address 0x000003330ca0 for type 'C > *', which requires 128 byte alignment > 0x000003330ca0: note: pointer points here > > > 00 00 00 00 85 eb 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 > 00 00 00 00 00 00 00 00 00 > ^ > ... similar errors continue > > Note that when the alignas specifiers on r and w are removed, the UBSAN > errors go away. > > I'd greatly appreciate if someone can explain 1) why this is occuring; 2) > and how to properly write the code to make the error go away. (Note: my > motivation for the alignas attributes is to prevent false sharing between r > and w as this class is used by concurrent threads.) > > -- > You received this message because you are subscribed to the Google Groups > "address-sanitizer" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "address-sanitizer" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
