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
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