dtrace discuss - Mar 2008 - 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 %d\n", pid);
                exit(1);
        }

        if ((lwp = &Pstatus(Pr)->pr_lwp) == NULL) {
                fprintf(stderr, "Couldn''t grab pr_lwp\n");
                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%lx\n", 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