Re: [Qemu-devel] RFC: [0/11] EFAULT patch

[Paul Brook]

On Wednesday 19 September 2007, Stuart Anderson wrote:
> On Wed, 19 Sep 2007, J. Mayer wrote:
> > Then, the changes you've done, changing long arguments (which should be
> > target_long to be correct, you can take a look at the last patch I sent
> > on the list) to pointers, for example in function prototypes, are
> > incorrect.
>
> I just went, and looked at the linux code again for 32 on 64 for x86_64 and
> powerpc.  In both of these cases (and I suspect the others as well), the
> parameters which are passed via registers are 0 extended from 32 bits to
> 64 bit in the syscall entry asm code. This way, once the C code is
> called via the sys_call_table, everything is dealt with as 64 bits. This
> actually keeps the rest of the code simpler as the rest of the kernel
> doesn't have to be extending & truncating pointers everywhere else.
>
> On x86_64 and powerpc, it appears that both user (ie target) and kernel
> pointers co-exist and that the code that maps structures assume that the
> __get_user()/__put_user() and copy_*_user() routines can handle any
> special situation. The pointers passed into functions like
> cp_compat_stat() are 64-bits for both the structure located in the
> kernel, and the one located in user space.
>
> My understanding is that we want to do as the kernel does as much as
> possible. In light of this, wouldn't we want to be decreasing the use
> of target_long where pointers may be involved instead of increasing it?

No. We're doing more than most 32-64 syscall thunks. To a first approximation 
the syscall thunks can bindly zero extend all values. In qemu we need to know 
whether something is a pointer or a value.

Kernel and userspace addresses are not interchangeable in the kernel. Any 
place that does so is probably a bug.

Paul