probes that access userspace

[Kevin Stafford <kevinrs at us dot ibm dot com>]

Especially when considering syscall tapset there are times when it is 
useful to access
the value referenced by a user space pointer. In probe script language, 
such an attempt
(i.e. $userspace_struct->member) will cause a pointer dereference fault. 
Thus it is
necessary to write a function to dig the value out of the pointer 
manually. Ex:

_get_user_ptr_val($userspace_struct,member) _get_user_ptr_val

function _get_user_ptr_val(u_addr,mem) %{
  [...]
  copy_from_user(..., THIS->u_addr, ...);
  [...]
%}

Is dwarf able to provide enough information to accurately detect a user 
space pointer
reference? If so, is it possible to teach the translator to bring 
references of user
space into the kernel automagically? Such a mechanism would relieve the 
user/tapset
author from writing an embedded C function for every pointer they wanted 
to look at.
The current implementation does not support such behavior.

If this is not possible, I think it would be useful to enhance the 
language such that
a probe script could explicitly notify the translator of a user space 
pointer access
(perhaps *var for scalars & *struct=>mem for structs). This will be 
especially
beneficial for scripts probing user-space apps. Eventually, when 
SystemTap supports
user-space probes, the translator will need to know how to follow 
pointers in user space.

--
Kevin Stafford
DES 2 | MS 2M3
Beaverton - OR
Linux Technology Center
IBM Systems & Technology
Phone: 1-503-578-3039
Email: kevinrs@us.ibm.com