Re: xend segfaults when starting

[Christoph Egger <Christoph.Egger@amd.com>]

On Wednesday 11 August 2010 15:11:54 Ian Campbell wrote:
> On Wed, 2010-08-04 at 15:55 +0100, Stefano Stabellini wrote:
> > > > On Wed, 2010-08-04 at 14:12 +0100, Christoph Egger wrote:
> > > > > Hi!
> > > > >
> > > > > xend causes python to segfault on startup.
> > > > > The changeset in error is: 21904:6a0dd2c29999
> >
> > It doesn't, in fact:
> >
> > changeset:   21907:6a0dd2c29999
> > parent:      21904:9f49667fec71
> > user:        Ian Campbell <ian.campbell@citrix.com>
> > date:        Fri Jul 30 16:20:48 2010 +0100
> > summary:     libxc: free thread specific hypercall buffer on
> > xc_interface_close
> >
> > I am going to revert this and leave it to Ian to fix it properly
> > (currently on vacation).
>
> I'm currently looking at this but I'm not seeing this issue, xend starts
> up fine and I can start a (PV) VM.
>
> When you said "segfault on startup" did you mean of xend or of a domain?
> (I think the former).
>
> Can you give me a little more information about your environment please?
> Is it NetBSD by any chance?
>
> Please could you reapply this changeset add some tracing to
> hcall_buf_prep and _xc_clean_hcall_buf to print out the hcall_buff and
> hcall_buff->buf as they are allocated and freed. The line numbers
> indicate that the free(hcall_buf->buf) is faulting. We've just called
> unlock_pages on the same address but since we seem to deliberately throw
> away any errors from munlock (see "safe_munlock") that doesn't really
> tell us much about its validity.

I tracked down where the error happens. In safe_munlock(),
the munlock() fails.

The trace is:

xc_interface_close -> _xc_clean_hcall_buf -> unlock_pages -> safe_munlock -> 
munlock

hcall_buf->buf has the address 0x7f7ffdfe7040

In unlock_pages, the address and length passed to munlock() is:

 laddr 0x7f7ffdfe7000, llen 0x2000

The reason why munlock() fails is that mlock() hasn't been called before.
The hcall_buf_prep() is not called at all before the first call to 
_xc_clean_hcall_buf().

Christoph


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