Preventing a tapscript from accessing user space.

[Frank Ch. Eigler]

David Wilder wrote (some time ago):

> [...] For example: if $arg is a pointer to a struct in user space
> (presumably passed to the kernel via some system call).  What
> prevents me from doing i=$arg->j in the tapscript?

Nothing - we can't.  The "__user" modifier doesn't make it into the
debugging data.  The old bug #2049 outlines a way of letting the
script explicitly designate user- vs. kernel-space pointer
dereferences.

> Also when I looked at the code generated by stap I was hoping to see
> deref() used but that is not what I found.

There should be one there, inside the generated "tvar_get" function.

> On i386 the above example works ok because we can just access the
> user space, it works but it is not correct.  On s390 I can't do that
> I must use something like get_user(), but I cant do that ether
> because get_user() calls might_sleep() and the probe is running with
> interrupts off.

The i386 code is predicated on the assumption that user-space accesses
are possible with ordinary loads/stores, but exceptions are caught and
returned as -EFAULT values rather than sleeping or whatnot.

- FChE