linux-kernel - Re: Bug: ptrace issues under x86

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Message-ID: <3e8340490904251510r5938b773r7b313555c91e214a@mail.gmail.com>
Date:	Sat, 25 Apr 2009 18:10:28 -0400
From:	Bryan Donlan <bdonlan@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, xen-devel@...ts.xensource.com
Subject: Re: Bug: ptrace issues under x86_64 Xen kernel 2.6.29

On Sun, Mar 29, 2009 at 11:01 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> Bryan Donlan wrote:
>>
>> Using 2.6.29 or 2.6.28 as a 64-bit Xen domU, a number of ptrace()
>> users seem to have issues with unexpected breakpoints. ltrace and gdb
>> both seem to be affected, under both 64-bit and 32-bit userspace.
>> 32-bit kernels do not seem to be affected. Typical symptoms look like:
>>
>
> It looks like this is because the kernel sets up int3 (breakpoint) and debug
> (watchpoints, etc) to be on a separate debug stack in the tss.  Xen doesn't
> do this (and doesn't appear to have a mechanism to do so), so I guess the
> on-stack format isn't what the kernel expects.  Does the patch below work?

Hi,

Sorry for the late reply; this message never made it to my inbox for
some reason...
I did try the patch, and saw similar results to Mark; ptrace works,
but lots of warnings:
------------[ cut here ]------------
WARNING: at arch/x86/xen/enlighten.c:447 cvt_gate_to_trap+0xe6/0xf0()
Modules linked in:
Pid: 0, comm: swapper Tainted: G        W
2.6.30-rc3-ptracefix-00330-g6d03473 #4
Call Trace:
 [<ffffffff8075e9b0>] ? stack_segment+0x0/0x30
 [<ffffffff8075e9b0>] ? stack_segment+0x0/0x30
 [<ffffffff8023ec4a>] ? warn_slowpath+0xea/0x160
 [<ffffffff8020dd79>] ? xen_force_evtchn_callback+0x9/0x10
 [<ffffffff8020e512>] ? check_events+0x12/0x20
 [<ffffffff8020dd79>] ? xen_force_evtchn_callback+0x9/0x10
 [<ffffffff8020e512>] ? check_events+0x12/0x20
 [<ffffffff8020e512>] ? check_events+0x12/0x20
 [<ffffffff8020e4ff>] ? xen_restore_fl_direct_end+0x0/0x1
 [<ffffffff8023f86f>] ? vprintk+0x1df/0x3f0
 [<ffffffff8020bb49>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
 [<ffffffff8020d928>] ? make_lowmem_page_readonly+0x28/0x40
 [<ffffffff8075e9b0>] ? stack_segment+0x0/0x30
 [<ffffffff8020a996>] ? cvt_gate_to_trap+0xe6/0xf0
 [<ffffffff8020a9f9>] ? xen_convert_trap_info+0x59/0xa0
 [<ffffffff8020b0cf>] ? xen_load_idt+0x3f/0x70
 [<ffffffff809b3b3f>] ? cpu_init+0xf0/0x2da
 [<ffffffff809b19a6>] ? cpu_bringup_and_idle+0x6/0x71
---[ end trace 4eaa2a86a8e2da36 ]---

Thanks,

Bryan Donlan
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