On Fri, Jan 19, 2018 at 02:49:55AM +0300, Kirill A. Shutemov wrote: > > So that's why you can't do pointer diffs between two arrays. Not > > because you can't subtract the two pointers, but because the > > *division* part of the C pointer diff rules leads to issues. > > Thanks a lot for the explanation! > > I wounder if this may be a problem in other places? > > For instance, perf uses address of a mutex to determinate the lock > ordering. See mutex_lock_double(). The mutex is embedded into struct > perf_event_context, which is allocated with kzalloc() so I don't see how > we can presume that alignment is consistent between them. > > I don't think it's the only example in kernel. Are we just lucky?
If you're just *comparing* the addresses of two objects, GCC doesn't care what the size of the object is. ie there's a difference between 'if (b < a)' and 'if ((a - b) < n)'. But yes, if you go by the strict wording of the standard: When two pointers are compared, the result depends on the relative locations in the address space of the objects pointed to. [...] In all other cases, the behavior is undefined http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf So really we should be casting 'b' and 'a' to uintptr_t to be fully compliant with the spec.
