[libvirt] [PATCH v2 2/4] rpc: gendispatch: Add a check for zero size client-side buffers

[Daniel P. Berrangé]

On Thu, Nov 21, 2019 at 09:58:30AM +0100, Erik Skultety wrote:
> After libvirt switched to GLib, we also started to use glib allocation
> primitives as of commit e85e34f3. Unlike malloc which is ambiguous with
> regards to size == 0 (which in our case returned a unique pointer safe
> to be passed to free), g_malloc0 strictly returns NULL on size == 0.
> 
> This change broke our legacy APIs which rely on the caller to
> pre-allocate the target buffer to hold the results and pass the buffer
> size as an argument. Since it makes very little sense to call an API
> with nowhere to store the results, fix this by returning 0 directly in
> such case in the RPC dispatch code - there are modern API equivalents
> allocating the target buffer automatically anyway.

As mentioned before, I don't think we should be short-circuiting the
driver APIs like this. It is still valid to call the APIs with zero
length list such that we get access control checks validated.

> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1772842
> 
> Signed-off-by: Erik Skultety <eskultet at redhat.com>
> ---
>  src/rpc/gendispatch.pl | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/src/rpc/gendispatch.pl b/src/rpc/gendispatch.pl
> index 8656c8f205..524d31f741 100755
> --- a/src/rpc/gendispatch.pl
> +++ b/src/rpc/gendispatch.pl
> @@ -1073,6 +1073,14 @@ elsif ($mode eq "server") {
>              print "        goto cleanup;\n";
>              print "    }\n";
>              print "\n";
> +
> +
> +            print "    if (args->$single_ret_list_max_var == 0) {\n";
> +            print "        ret->$single_ret_list_name.${single_ret_list_name}_len = 0;\n";
> +            print "        rv = 0;\n";
> +            print "        goto cleanup;\n";
> +            print "    }\n";
> +            print "\n";
>          }
>  
>          print join("\n", @getters_list);
> -- 
> 2.23.0
> 
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Regards,
Daniel
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