std::rethrow_exception is broken

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

There is a lot of code in libsupc++/eh_* that relies on
__cxa_exception and __cxa_dependent_exception having similar layouts,
so tricks like this work:

static inline void*
__gxx_caught_object(_Unwind_Exception* eo)
{
  // Bad as it looks, this actually works for dependent exceptions too.
  __cxa_exception* header = __get_exception_header_from_ue (eo);
  return header->adjustedPtr;
}

The idea is that although the eo might be a pointer to the
unwindHeader member  of either __cxa_exception or
__cxa_dependent_exception, the adjustedPtr member will be always be at
the same location relative to the unwindHeader member, so it Just
Works.

Except it doesn't.

offsetof(__cxa_exception, unwindHeader) == 80
offsetof(__cxa_dependent_exception, unwindHeader) == 80
offsetof(__cxa_exception, adjustedPtr) == 72
offsetof(__cxa_dependent_exception, adjustedPtr) == 64

This is the cause of PR 60612, and means we need to fix a lot of code
by explicitly checking the exception class (see attached patch for a
partial fix doing that in two places) or we need to change the layout
of __cxa_dependent_exception (and then add some tests to verify the
assumptions implicit in the code!)

I don't think this is all fixable in time for 4.9.0, but the attached
patch fixes one part of the problem, so I'm probably going to commit
it and then work on a more complete fix later.

Attachment: patch.txt
Description: Text document