FreeBSD 5.3 crash (core with debug symbols available)

[Doug White]

On Wed, 2 Mar 2005, David Xu wrote:

> I believe this is caused by swapped out of kernel thread stack.
> in /sys/vm/vm_glue.c, there is some code swapping out a sleeping process,
> this means any kernel code can not use thread local variable to communicate
> with other threads, this is a rather unsafe assumptions, the vm code really
> should be disabled.

I don't quite understand what you mean by "vm code really should be
disabled";  is virtual memory really that bad? :)

The consensus on IRC is that threads should not use their stacks for
anything but storage of their own variables. Anything used for
synchronization or state should be placed in malloc()d memory or some
other shared structure.

I'll start working on a patch to change these references in the sigwait()
family. And queue up a pointy hat to jeff at .  Pointers to other badly
behaved code gladly accepted :)

> David Xu
>
> Doug White wrote:
>
> >On Tue, 1 Mar 2005, Dariusz Kulinski wrote:
> >
> >
> >
> >>Hello Doug,
> >>
> >>Tuesday, March 1, 2005, 10:50:19 AM, you wrote:
> >>
> >>
> >>
> >>>>>Looks like it ran over a spammed thread, but I'll want to see the fault
> >>>>>address. Bets on whether its 0xdeadc0de+offset?
> >>>>>
> >>>>>
> >>>>0xdeadc0de, huh? :)
> >>>>
> >>>>
> >>>free()d memory regions get filled with 0xdeadc0de to hunt down
> >>>use-after-free conditions.
> >>>
> >>>
> >>Ok, I thought it was one of developer jokes =)
> >>
> >>
> >>
> >>>Thats what I want :-)
> >>>
> >>>
> >>>OK, it wasn't deadc0de, so can you load the crashdump up, go down to the
> >>>sigtd() frame, and "print *td"?  It'll be a huge spew.
> >>>
> >>>
> >>(kgdb) frame 20
> >>#20 0xc04e9d3f in sigtd (p=0xc16948d4, sig=14, prop=129) at /usr/src/sys/kern/kern_sig.c:1581
> >>1581   if (td->td_waitset != NULL &&
> >>(kgdb) print *td
> >>$1 = {td_proc = 0xc16948d4, td_ksegrp = 0xc26b9310, td_plist = {tqe_next = 0xc1b48190, tqe_prev = 0xc1b95198}, td_kglist = {
> >>    tqe_next = 0x0, tqe_prev = 0xc26b931c}, td_slpq = {tqe_next = 0x0, tqe_prev = 0xc1794b80}, td_lockq = {tqe_next = 0x0,
> >>    tqe_prev = 0x0}, td_runq = {tqe_next = 0x0, tqe_prev = 0xc26b9324}, td_selq = {tqh_first = 0x0, tqh_last = 0xc17c31c0},
> >>  td_sleepqueue = 0x0, td_turnstile = 0xc15d5dc0, td_tid = 100081, td_flags = 8, td_inhibitors = 6, td_pflags = 8,
> >>  td_dupfd = 0, td_wchan = 0xd12bfc20, td_wmesg = 0xc06cef0b "sigwait", td_lastcpu = 0 '\0', td_oncpu = 255 '˙',
> >>  td_locks = 0, td_blocked = 0x0, td_ithd = 0x0, td_lockname = 0x0, td_contested = {lh_first = 0x0}, td_sleeplocks = 0x0,
> >>  td_intr_nesting_level = 0, td_pinned = 0, td_mailbox = 0x9903010, td_ucred = 0xc2b41b00, td_standin = 0x0, td_prticks = 0,
> >>  td_upcall = 0xc17c0510, td_sticks = 2210, td_uuticks = 0, td_usticks = 0, td_intrval = 0, td_oldsigmask = {__bits = {0, 0,
> >>      0, 0}}, td_sigmask = {__bits = {159751, 0, 0, 0}}, td_siglist = {__bits = {0, 0, 0, 0}}, td_waitset = 0xd12bfc64,
> >>  td_umtx = {tqe_next = 0x0, tqe_prev = 0x0}, td_generation = 376536, td_sigstk = {ss_sp = 0x0, ss_size = 0, ss_flags = 0},
> >>  td_kflags = 0, td_xsig = 0, td_profil_addr = 0, td_profil_ticks = 0, td_base_pri = 104 'h', td_priority = 104 'h',
> >>  td_pcb = 0xd12bfda0, td_state = TDS_INHIBITED, td_retval = {0, 137620480}, td_slpcallout = {c_links = {sle = {
> >>        sle_next = 0x0}, tqe = {tqe_next = 0x0, tqe_prev = 0xc1cd68e4}}, c_time = 216540257, c_arg = 0xc17c3190, c_func = 0,
> >>    c_flags = 8}, td_frame = 0xd12bfd48, td_kstack_obj = 0xc1796318, td_kstack = 3509313536, td_kstack_pages = 2,
> >>  td_altkstack_obj = 0x0, td_altkstack = 0, td_altkstack_pages = 0, td_critnest = 1, td_md = {md_savecrit = 582},
> >>  td_sched = 0xc17c32e4}
> >>
> >>
> >
> >
> >This is quite helpful, thanks!  It appears the thread had called
> >sigtimedwait() and the timeout fired. The clock ithread goes to whack the
> >process with SIGALRM and checks if its waiting in sigtimedwait()
> >specifically.  That info is coded into the td_waitset member of struct
> >thread, which get set from the user. All of the frontends provide the set
> >from a stack variable.
> >
> >later, in kern_sigtimedwait()...
> >
> >926         td->td_waitset = &waitset;
> >927         error = msleep(&ps, &p->p_mtx, PPAUSE|PCATCH, "sigwait", hz);
> >
> >So now a pointer to stack variable is in the thread. Later on sigtd()
> >comes along and wants to dereference it and that stack page isn't
> >available according to the VM system and that trips the panic.
> >
> >Some more exploration is necessary. Can you make the crashdump and debug
> >kernel available?  Also, what was running when this panic tripped? ("info
> >threads" in kgdb may be useful.)
> >
> >
> >
>

-- 
Doug White                    |  FreeBSD: The Power to Serve
dwhite at gumbysoft.com          |  www.FreeBSD.org