Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 23 May 2016 18:37:46 -0700 | Subject | Re: x86: A process doesn't stop on hw breakpoints sometimes |
| |
On Mon, May 23, 2016 at 4:05 PM, Andrei Vagin <avagin@gmail.com> wrote: > Hi, > > We use breakpoints on CRIU to stop a processes before calling > rt_sigreturn and we found that sometimes a process runs through a > break-point without stopping on it. > > https://github.com/xemul/criu/issues/162 > > > A small reproducer is attached. It forks a child, stops it, sets a > breakpoint, executes a child, waits when it stops on the breakpoint. I > execute it a few times concurrently and wait a few minutes. > > https://asciinema.org/a/006l3u5v82ubbkfy9fto07agd > > I know that it can be reproduced on: > AMD A10 Micro-6700T > Intel(R) Core(TM) i5-5200U CPU @ 2.20GHz > Intel(R) Core(TM) i7-4600U CPU @ 2.10GHz > > so It doesn't look like a bug in a processor.
I'm guessing you're either hitting a subtle bug in the mess that is breakpoint handling or you're hitting a bug in perf's context switch code.
Given that the breakpoint gets missed many times in a row, this is presumably either a bug in breakpoint programming (i.e. the thing isn't actually set in dr0/dr7) or a bug in the bp state tracking. If it were a bug in RF flag handling, I'd expect it to skip once and trip the second time through.
All that being said, I stared at the code for a while and I don't see the bug. I can trigger this quite rarely on a VM, and it's not fun to debug :(
--Andy
| |