[Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[sathish kumar]

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hi,

Can someone explain why explain why ptrace system call is not implemented in QEMU?
if someone is working on it, please share some ideas.

Thanks & Regards,
Sathish



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Re: [Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[Paul Brook]

> Can someone explain why explain why ptrace system call is not implemented
> in QEMU? if someone is working on it, please share some ideas.

Because it's extremely hard (read: nearly impossible) to implement properly.
ptrace allows direct access to the state of a different process. Passing 
through ptrace would give you the state of qemu itself, not the program 
you're interested in, so you have to emulate everything via some othe IPC 
mechanism (which must also be hidden from the emulated application). This 
includes signals like SIGSTOP which cannot be trapped.

If you're not extremely careful you end up with a monstrosity like Cygwin, and 
all the problems that implies.

Paul

Re: [Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[Paul Brook]

On Tuesday 23 September 2008, Paul Brook wrote:
> > Can someone explain why explain why ptrace system call is not implemented
> > in QEMU? if someone is working on it, please share some ideas.
>
> Because it's extremely hard (read: nearly impossible) to implement
> properly. 

I'll also note that the usual use of ptrace if for gdb/gdbserver. You should 
use the builtin qemu gdb stub instead.

Paul

Re: [Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[Jamie Lokier]

Paul Brook wrote:
> On Tuesday 23 September 2008, Paul Brook wrote:
> > > Can someone explain why explain why ptrace system call is not implemented
> > > in QEMU? if someone is working on it, please share some ideas.
> >
> > Because it's extremely hard (read: nearly impossible) to implement
> > properly. 
> 
> I'll also note that the usual use of ptrace if for gdb/gdbserver. You should 
> use the builtin qemu gdb stub instead.

The other usual use is strace, which seems like it would be useful for
user mode emulation.

-- Jamie

Re: [Qemu-devel] Regarding PTRACE implementation in QEMU user mode

[Kirill A. Shutemov]

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On Tue, Sep 23, 2008 at 04:02:37PM +0100, Jamie Lokier wrote:
> Paul Brook wrote:
> > On Tuesday 23 September 2008, Paul Brook wrote:
> > > > Can someone explain why explain why ptrace system call is not implemented
> > > > in QEMU? if someone is working on it, please share some ideas.
> > >
> > > Because it's extremely hard (read: nearly impossible) to implement
> > > properly. 
> > 
> > I'll also note that the usual use of ptrace if for gdb/gdbserver. You should 
> > use the builtin qemu gdb stub instead.
> 
> The other usual use is strace, which seems like it would be useful for
> user mode emulation.

Set QEMU_STRACE environment variable.

-- 
Regards,  Kirill A. Shutemov
 + Belarus, Minsk
 + ALT Linux Team, http://www.altlinux.com/

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