Re: [Qemu-devel] RFC: x86_64 Best way to fix 'cast to pointer from integer of different size' problems?

[Paul Brook <paul@codesourcery.com>]

On Saturday 03 November 2007, TJ wrote:
> I'm building on x86_64 GNU/Linux. There are *lots* of (1053) compiler
> warnings of the class:
>
> warning: cast to pointer from integer of different size

There are at due to the recent EFAULT/access_ok changes. There should be (and 
used to be) a clear separation between host and target addresses. The EFAULT 
changes have broken this. Before these ghanges it wa trivial to remap the 
target address space (e.g. place it at a high address on a 64-bit host), or 
even enabling softmmu. I'm fairly sure that wouldn't work if you tried it 
now.

> Fixing it looks to require a variety of fixes, from simple explicit
> casts in-line, to some complicated review of multiple levels of macros
> to decide where best to apply a fix.

Adding a cast is never the right thing to do. There should *always* be a clear 
distinction and explicit conversion between host pointers and target 
addresses. It is never safe to cast from one to the other.

I put quite a lot of effort into getting this separation right when I 
implemented the lock_user interfaces. It was fairly trivial to implement 
address space translation (even softmmu) using this inerface. I'm quite 
annoyed that we seem to have regresses so badly in this area. AFAICS the 
whole of syscalls.c needs re-auditing to figure out which values are host 
pointers and which are target addresses.

We should have either lock_user or {get,put}_user, not both.

Paul