Re: [Xen-devel] [PATCH] Prevent xl save from segfaulting when control/shutdown key is removed

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-11-23 at 09:45 +0000, Paul Durrant wrote:
> # HG changeset patch
> # User Paul Durrant <paul.durrant@xxxxxxxxxx>
> # Date 1322041530 0
> # Node ID 3341e3e990568f459ae984cd9d2cac2d546eaa4e
> # Parent  0a0c02a616768bfab16c072788cb76be1893c37f
> Prevent xl save from segfaulting when control/shutdown key is removed
> 
> To acknowledge the tools' setting of control/shutdown it is normal for PV 
> drivers
> to rm the key. This leads to libxl__xs_read() returning NULL and thus a 
> subsequent
> strcmp on the return value will cause a segfault.

The Linux PV drivers actually write "" rather than removing the key
which is how this didn't get spotted already. We should be robust to PV
drivers which remove it as well.

At start of day .../control is created ro (to the guest)
whereas .../control/shutdown is created rw. Can you confirm that a
second operation still succeeds if the guest rm's the node on the first
action?

My concern is that while the first time the round the node will be rw
the second time round the write will actually re-create the node
(without setting the permissions) which might result in the node being
ro for the guest (xenstore perms confuse me, but I think new nodes
inherit the parent permissions).

That's assuming there's any chance of a second operation. I'm thinking
of a wedged reboot followed by an attempt to shutdown instead or
something like that. Perhaps in practice that wouldn't work anyway.

Apart form the above this change improves the robustness of the code so:

> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>



> 
> diff -r 0a0c02a61676 -r 3341e3e99056 tools/libxl/libxl_dom.c
> --- a/tools/libxl/libxl_dom.c Mon Nov 21 21:28:34 2011 +0000
> +++ b/tools/libxl/libxl_dom.c Wed Nov 23 09:45:30 2011 +0000
> @@ -444,6 +444,7 @@ static int libxl__domain_suspend_common_
>              usleep(100000);
>  
>              state = libxl__xs_read(si->gc, XBT_NULL, path);
> +            if (!state) state = "";
>  
>              watchdog--;
>          }
> @@ -463,6 +464,7 @@ static int libxl__domain_suspend_common_
>              t = xs_transaction_start(ctx->xsh);
>  
>              state = libxl__xs_read(si->gc, t, path);
> +            if (!state) state = "";
>  
>              if (!strcmp(state, "suspend"))
>                  libxl__xs_write(si->gc, t, path, "");
> 
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