Re: [Qemu-devel] [PATCH] device_tree: load_device_tree(): Allow NULL sizep

[Peter Crosthwaite]

On Fri, Aug 10, 2012 at 11:42 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Aug 10, 2012 at 01:54:26PM +1000, Peter A. G. Crosthwaite wrote:
>> The sizep arg is populated with the size of the loaded device tree. Since 
>> this
>> is one of those informational "please populate" type arguments it should be
>> optional. Guarded writes to *sizep against NULL accordingly.
>>
>> Signed-off-by: Peter A. G. Crosthwaite <address@hidden>
>> Acked-by: Alexander Graf <address@hidden>
>> ---
>>  device_tree.c |    8 ++++++--
>>  1 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/device_tree.c b/device_tree.c
>> index d7a9b6b..641a48a 100644
>> --- a/device_tree.c
>> +++ b/device_tree.c
>> @@ -71,7 +71,9 @@ void *load_device_tree(const char *filename_path, int 
>> *sizep)
>>      int ret;
>>      void *fdt = NULL;
>>
>> -    *sizep = 0;
>> +    if (sizep) {
>> +        *sizep = 0;
>> +    }
>>      dt_size = get_image_size(filename_path);
>>      if (dt_size < 0) {
>>          printf("Unable to get size of device tree file '%s'\n",
>> @@ -104,7 +106,9 @@ void *load_device_tree(const char *filename_path, int 
>> *sizep)
>>              filename_path);
>>          goto fail;
>>      }
>> -    *sizep = dt_size;
>> +    if (sizep) {
>> +        *sizep = dt_size;
>> +    }
>
> What can the caller do with this void* buffer without knowing its size?
>

Sanity check the machine:

dtb = load_device_tree( ... ); //dont care how big it is
foo = fdt_gep_prop( dtb, ... );
if (foo != object_get_prop(foo_device, foo_prop, ... )) {
    hw_error("your dtb is bad because ... !\n", ... );
}
...
boot();

This happens at machine init, which is quite separate from boot.

> They cannot hand the buffer to the guest unless they duplicate the
> get_image_size() call, which is pointless, or we're assuming a fixed
> size, which is unsafe.  I'm asking what the legitimate use case for
> sizep == NULL is?

Sanity check the dtb without passing it to the guest. My CAD tools
emit a DTB for the hardware, but it wont always go through a linux
bootloader (E.G. I may want to boot random elfs). I still will pass
the CAD generated DTB to QEMU for sanity check however so qemu can
give me a nice report of what is and isn't modelled.

Regards,
Peter

>
> Stefan