Messages in this thread | | | Date | Wed, 16 Dec 2015 17:31:06 -0700 | Subject | Re: 4.4-rc5 Setting hardware breakpoint in int_ret_from_sys_call causes triple fault/reboot | From | Jeff Merkey <> |
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On 12/16/15, Andy Lutomirski <luto@amacapital.net> wrote: > On Dec 16, 2015 3:12 PM, "Jeff Merkey" <linux.mdb@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.
Jeff
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