On Tue, Mar 24, 2020 at 11:18:07AM -0700, Kees Cook wrote: > On Tue, Mar 24, 2020 at 11:44:33AM -0500, Josh Poimboeuf wrote: > > On Fri, Mar 20, 2020 at 11:35:37AM -0700, Kees Cook wrote: > > > On Fri, Mar 20, 2020 at 09:26:13AM +0100, Peter Zijlstra wrote: > > > > On Thu, Mar 19, 2020 at 12:45:50PM -0500, Josh Poimboeuf wrote: > > > > > > On Thu, Mar 19, 2020 at 12:33:31PM -0500, Josh Poimboeuf wrote: > > > > > > > > > > > Actually I suspect it's the __builtin_unreachable() annotation which is > > > > > > > making UBSAN add the __builtin_trap()... because I don't see any double > > > > > > > UD2s for WARNs. > > > > > > > > > Actually, removing __builtin_unreachable() *does* make the extra UD2 go > > > > > away -- I forgot I had some silly debug code. > > > > > > > > LOL, check this: > > > > > > > > "Built-in Function: void __builtin_unreachable (void) > > > > > > > > If control flow reaches the point of the __builtin_unreachable, the > > > > program is undefined. It is useful in situations where the compiler > > > > cannot deduce the unreachability of the code. " > > > > > > > > Which, I bet, is what makes UBSAN insert that __builtin_trap(). > > > > > > > > What a friggin mess :/ > > > > > > What I'd like is to be able to specify to UBSAN what function to call > > > for the trap. I'd prefer to specify a well-defined exception handler, > > > but at present, UBSAN just inserts __builtin_trap(). > > > > > > Can't objtool be told to ignore a ud2 that lacks an execution path to it? > > > > It can ignore unreachable UD2s, if we think that's the right fix. > > > > I was hoping we could find a way to get rid of the double UD2s, but I > > couldn't figure out a way to do that when I looked at it last week. > > As far as I could tell, this needs patches to the UBSAN support in gcc > and clang. I have opened bugs for each: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94307 > https://bugs.llvm.org/show_bug.cgi?id=45295 So it sounds like this would replace the second UD2 with a "call some_ubsan_function()"? That might be slightly better, though it would still need an objtool change to ignore unreachable warnings for such calls. In the meantime I can still change objtool to ignore unreachable UD2s if there aren't any better ideas. -- Josh
