Help getting pointer to a u structure of a process

[Chris Torek]

In article <1991Mar06.203225.20019 at lynx.CS.ORST.EDU> bridej at prism.CS.ORST.EDU
(Jim Bride) writes:
>Hi.  I am working on a program which requires me to be able to access the 
>u (user) structure of a process OTHER than the process which is currently 
>running.

>BTW, I am using a HP 9000 series 300 CPU running 4.3BSD Utah.
(this is an important constraint.)

>I think it involves translating the pointer inside the proc structure
>(p->p_addr) that points to the PTEs for the user structure into an
>address (which I can then cast to a (struct user *)).

This is correct as far as it goes.

p->p_addr points to the PTEs of pages mapping the u. area; there may be
more than one such PTE (and in fact there are 3 in the Utah HP BSD kernel).
But this is not the whole story.

Half of the raison d'etre of the `u. area' is that it can be sloughed off
when the process is not running.  If this has been done, p->p_flag&SLOAD
will be 0 and the u. area will reside only in swap space.  To alter it
you would have to read it in first.

>My program is running as a system call ...

This makes touching the u. area extra difficult.  (It is easier to do
in user code, except that it is unreliable since the process can move
while you are doing your thing.)

If the process is loaded, you can simply map in the corresponding pages
(there are some maps dedicated to mapping u. areas, Xswapmap and Xswap2map
with virtual addresses xswaputl and xswap2utl, and associated locks
xswaplock&1 and xswaplock&2, which you could steal for this purpose;
see vm_swap.c$swapout()).  If not, you must bring it in.  The easiest
way is to call swapin().  Swapin can fail, returning 0, so you must
loop; it may help to tickle the pager (see vm_sched.c$sched()).  Once
swapin succeeds, you can proceed as usual.

To make xswaputl or xswap2utl map the u. area of the other process, you
can simply call uaccess(); see swapout().  If you use the same algorithm,
utl->u_xxx will then refer to the u_xxx field of the u. area of the other
process.
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab EE div (+1 415 486 5427)
Berkeley, CA		Domain:	torek at ee.lbl.gov