[coldtea]

>The problem is the C code that relies on it: effectively you are using a dialect of C which gives stronger guarantees, so you lose the ability to use any other implementation which doesn't provide those guarantees.

How do you lose it in this case? You shouldn't been reading values from those padding bits anyway...

[boomlinde]

Tell that to a hacker. The problem here isn't that the code is functionally dependent on padding bytes, it's that when you copy those padding bytes around you are leaking information that you probably never meant to copy.

This can be problematic if you are copying kernel space memory to a user space process, for example. Let's say there's a call into the kernel that returns a copy of this 4+1 struct with three more bytes of padding. Maybe what was on the stack before the space was assigned to those last padding bytes are some information the kernel definitely shouldn't leak to user space, like some bytes of a password, and now any user space process could potentially read them simply by calling some unrelated kernel function.