Determining curthread pointer (ulwp_t) via libproc on i386

How would I determine the address of the ulwp_t structure
for the
current thread of a process from a 32-bit i386 executable
looking at a
32-bit executable?  (This is in support of a fix for
6593259:
"libdtrace should prefer pid probes to
breakpoints".)

For example, if I compile the following code 64-bit snippet,
I can get
that address from 32-bit or 64-bit executables:

------------------------------------------------------------
--------------------

        if ((Pr = Pgrab(pid, NULL, &gret)) == NULL) {
                fprintf(stderr, "cannot control process
%dn", pid);
                exit(1);
        }

        if ((lwp = &Pstatus(Pr)->pr_lwp) == NULL) {
                fprintf(stderr, "Couldn't grab
pr_lwpn");
                exit(1);
        }

#if defined(__amd64)
        if (Pstatus(Pr)->pr_dmodel == PR_MODEL_LP64) {
                addr = lwp->pr_reg[REG_FSBASE];
        } else {
                addr = lwp->pr_reg[REG_GSBASE];
        }
#else
#error "Need to figure this out"
#endif

        printf("ulwp_t is 0x%lxn", addr);
------------------------------------------------------------
--------------------

But things don't seem to be as simple as that for the
32-bit/32-bit
case.  From usr/src/uts/intel/dtrace/fasttrap_isa.c, we have
this code
for the i386 case (to get the address of the thread-local
scratch
space used for fasttrap probes, which is just after the
ul_self
pointer):

------------------------------------------------------------
--------------------
                /*
                 * Compute the address of the ulwp_t and
step over the
                 * ul_self pointer. The method used to store
the user-land
                 * thread pointer is very different on 32-
and 64-bit
                 * kernels.
                 */
[ ... ]
#elif defined(__i386)
                addr =
USEGD_GETBASE(&lwp->lwp_pcb.pcb_gsdesc);
                addr += sizeof (void *);
#endif

------------------------------------------------------------
--------------------

So we're using pcb_gsdesc, but it doesn't appear that this
is exposed
to userland via libproc (unless I've missed something.)

I've coded something similar to mdb's "::walk
ulwps" that suits my
purpose, but I'd rather be able to determine the ulwp_t for
the
current thread than walk that list.  (I'm trying to match
%pc against
the thread-local scratch space, so I have a way of searching
that
list.)

(I haven't come up to speed on understanding the Intel
architecture
segmentation stuff, so I'm hoping someone can help out
here.)

Thanks,
Chad Mynhier
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