Re: [Qemu-devel] Re: [linux-user] Fixed Qemu crash using Gdbstub

[Lionel Landwerlin <lionel.landwerlin@openwide.fr>]

Le samedi 13 décembre 2008 à 11:16 +0100, Jan Kiszka a écrit :
> Lionel Landwerlin wrote:
> > I just forgot to remove 2 printf ...
> > Here the good patch :
> > 
> > 
> > 
> > 
> >>From 2b3fe65ea3f2ee8dd3efbb52b66a2f4e53b788ea Mon Sep 17 00:00:00 2001
> > From: Lionel Landwerlin <lionel.landwerlin@openwide.fr>
> > Date: Sat, 13 Dec 2008 00:32:04 +0100
> > Subject: [PATCH] [linux-user] Fixed Qemu crash using Gdbstub
> > 
> >       When using gdb with qemu (via gdbstub), if your emulated
> >       application is multithreaded and does a segfault then qemu
> >       crashes.
> > 
> >       Qemu crashes because the break/watch points are shared between
> >       cpus. The TAILQ structure which handles the list of break/watch
> >       points is copied inside each CPUState structure. When the last
> >       breakpoint is removed (this happens on a segfault), it is
> >       removed across all cpus but because of the copied TAILQ
> >       structure a same breakpoint can be freed N times with N the
> >       current number of cpus.
> 
> OK, now I got the problem: user space emulation spawns additional VCPUs
> to emulate fork. Those VCPUs are cloned via cpu_copy which simply
> duplicates the CPUState of the parent, including the breakpoint and
> watchpoint TAILQ headers. This is doomed to fail.
> 
> But your approach to let the cloned VCPU point to the same TAILQ header
> as its parent is not correct as well. It will cause troubles to gdbstub
> which manages breakpoints on all VCPUs by adding duplicate instances on
> a per-VCPU base. If you inject a breakpoint before a fork and then
> remove it afterwards, gdbstub will report an error because it will only
> find the breakpoint once, not n times (n = number of VCPUs).
> 
> What you have to do is to cleanly duplicate the breakpoint and
> watchpoint lists on cpu_copy (filter out BP_CPU types for cleanness
> reasons, although they do not occur in user emulation ATM).

Hello Jan,

Thanks for reviewing my patch.

Duplication of all break/watchpoints will makes the patch bigger,
because it will required break/watchpoint_copy functions etc...

Another problem is that threads are also emulated by vcpus in user
emulation. But we also need to share break/watchpoints between threads.
This explain the way my patch do the thing.

Finally, this makes the modification a lot more complicated than what I
expected, because breakpoints on emulated forks should not apply.


-- 
Lione Landwerlin                                         

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