On 12/16/15, Andy Lutomirski <l...@amacapital.net> wrote: > On Wed, Dec 16, 2015 at 4:31 PM, Jeff Merkey <linux....@gmail.com> wrote: >> On 12/16/15, Andy Lutomirski <l...@amacapital.net> wrote: >>> On Dec 16, 2015 3:12 PM, "Jeff Merkey" <linux....@gmail.com> wrote: >>>> >>>> Setting a hardware breakpoint at the >>>> >>>> rex64 sysret >>>> >>>> instruction at the end of int_ret_from_sys_call causes the system to >>>> triple fault >>>> and reboot when the breakpoint is triggered. Appears to be related >>>> the same problem >>>> as the lockup. >>>> >>>> This function can be stepped over and traced through with the TRAP >>>> FLAG set so long as a hardware breakpoint is set somewhere in the >>>> function. Otherwise upon exist the system hard hangs. If you break >>>> exactly on that instruction -- reboot. If you break a few >>>> instructions before it and single step through the call it works. If >>>> you step through the call with no breakpoint the system hard hangs. >>>> Same behavior as when you try to step from inside an nmi handler. >>>> Looks related. >>> >>> You're probably encountering the user mode RSP when SYSRET happens. >>> >>> --Andy >>> >> >> Hi Andy, >> >> Could be, but I am getting a double fault message with an error code >> of 0 that then scrolls off the screen when the triple fault hits. It >> flashes too quickly to get the function address -- wish I had a logic >> analyzer with an inverse assembler -- would already be there. A >> usermode RSP would I assume clear TRAP flag and that does not explain >> why it works if I set a breakpoint right above the instruction then >> step over it, which I can without the triple fault. >> >> Easy to reproduce, download the mdb debugger for 4.3.3 and apply it to >> 4.4-rc5, modprobe mdb, echo a > /proc/sysrq_trigger, u >> int_ret_from_syscall (scroll til you get to the swapgs then rex64 >> sysret, set a hardware breakpoint at that address , i.e. b >> ffffffff81673ae1 (or whatever address the swapgs instruction is at), >> then step through with t a few times (should just return after rex64 >> sysret since it returns to user space). The set a breakpoint at the >> rex64 sysret instruction, b <address>, let it break at the >> instruction, then hit g for go and watch the fireworks -- it will try >> to print a double fault message then reboot. >> >> I handle the whole user RSP thing, I just return if I see regs set to >> user space. This looks like some sort of problem in the exception >> handlers. > > It's kernel regs but user RSP. > > --Andy >
right, I handle that case and I have handled that case since about 2001. Used to before all the change I could just step from userspace to kernel space with mdb. Have not been able to do that for while since Linus fixed the VM in about 2002. So I handle that case. Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
