Hi David, On Thu, Sep 14, 2023 at 10:01 PM David Gow <davidgow@xxxxxxxxxx> wrote: > > KUnit's deferred action API accepts a void(*)(void *) function pointer > which is called when the test is exited. However, we very frequently > want to use existing functions which accept a single pointer, but which > may not be of type void*. While this is probably dodgy enough to be on > the wrong side of the C standard, it's been often used for similar > callbacks, and gcc's -Wcast-function-type seems to ignore cases where > the only difference is the type of the argument, assuming it's > compatible (i.e., they're both pointers to data). > > However, clang 16 has introduced -Wcast-function-type-strict, which no > longer permits any deviation in function pointer type. This seems to be > because it'd break CFI, which validates the type of function calls. > > This rather ruins our attempts to cast functions to defer them, and > leaves us with a few options: > 1. Stick our fingers in our ears an ignore the warning. (It's worked so > far, but probably isn't the right thing to do.) > 2. Find some horrible way of casting which fools the compiler into > letting us do the cast. (It'd still break CFI, though.) > 3. Disable the warning, and CFI for this function. This isn't optimal, > but may make sense for test-only code. However, I think we'd have to > do this for every function called, not just the caller, so maybe it's > not practical. > 4. Manually write wrappers around any such functions. This is ugly (do > we really want two copies of each function, one of which has no type > info and just forwards to the other). It could get repetitive. > 5. Generate these wrappers with a macro. That's what this patch does. > > I'm broadly okay with any of the options above, though whatever we go > with will no doubt require some bikeshedding of details (should these > wrappers be public, do we dedupe them, etc). > > Thoughts? Using a macro to generate a wrapper is a reasonable approach IMO, and we've used it before in the kernel to fix type mismatches in indirectly called functions (v4l2-ioctl and cfg80211 come to mind at least). Sami
