On Tue, Apr 07, 2020 at 10:14:11PM +0100, David Howells wrote: > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > So the _real_ prototype for 'free()'-like operations should be something like > > > > void free(const volatile killed void *ptr); > > > > where that "killed" also tells the compiler that the pointer lifetime > > is dead, so that using it afterwards is invalid. So that the compiler > > could warn us about some of the most trivial use-after-free cases. > > It might be worth asking the compiler folks to give us an __attribute__ for > that - even if they don't do anything with it immediately. So we might have > something like: > > void free(const volatile void *ptr) __attribute__((free(1))); > > There are some for allocation functions, some of which we use, though I'm not > sure we do so as consistently as we should (should inline functions like > kcalloc() have them, for example?). GCC recognises free() as being a __builtin. I don't know if there's an __attribute__ for it. gcc/builtins.def:DEF_LIB_BUILTIN (BUILT_IN_FREE, "free", BT_FN_VOID_PTR, ATTR_NOTHROW_LEAF_LIST) It looks like the only two things this really does is warn you if you try to free a pointer that gcc can prove isn't in the heap, and elide the call if gcc can prove it's definitely NULL. Which are both things that a compiler should do, but aren't all that valuable.
