Re: Propagating addresses from linker to the runtie

[Florian Weimer <fweimer@xxxxxxxxxx>]

* Josef Wolf:

> Strictly speaking, the symbols defined by the linker (_sidata, _sdata, _edata,
> _sbss and _ebss) are unrelated when seen from the perspective of the
> compiler. Therefore, it is not allowed by the standard to use their addresses
> for comparison.
>
> So what would be the proper way to pass this information from the linker to the
> compiler?

In glibc, we use this:

/* Perform vtable pointer validation.  If validation fails, terminate
   the process.  */
static inline const struct _IO_jump_t *
IO_validate_vtable (const struct _IO_jump_t *vtable)
{
  /* Fast path: The vtable pointer is within the __libc_IO_vtables
     section.  */
  uintptr_t section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;
  uintptr_t ptr = (uintptr_t) vtable;
  uintptr_t offset = ptr - (uintptr_t) __start___libc_IO_vtables;
  if (__glibc_unlikely (offset >= section_length))
    /* The vtable pointer is not in the expected section.  Use the
       slow path, which will terminate the process if necessary.  */
    _IO_vtable_check ();
  return vtable;
}

I do not know how effective this is.

In C++, you can use std::less, which was enhanced to cover your use
case.

Thanks,
Florian