Re: [Qemu-devel] [PATCH] fix loading tiny kernels

[Rene Rebe <rene@exactcode.de>]

Alexander Graf wrote:
> 
> On 03.02.2009, at 11:30, Daniel P. Berrange wrote:
> 
>> On Tue, Feb 03, 2009 at 10:06:10AM +0100, Ren? Rebe wrote:
>>> I babbled:
>>>> Further testing / polishing the multi-boot kernel loading support I 
>>>> found
>>>> the existing code fails to load unusual small kernels, less than 8192
>>>> bytes -
>>>> for example the example multi-boot kernel shipped within GRUB that
>>>> compiles to just 7121 bytes on my system.
>>>>
>>>> Signed-off-by: René Rebe <rene@exactcode.de>
>>>>
>>>> --- hw/pc.c     (revision 6501)
>>>> +++ hw/pc.c     (working copy)
>>>> @@ -554,7 +989,7 @@
>>>>   /* load the kernel header */
>>>>   f = fopen(kernel_filename, "rb");
>>>>   if (!f || !(kernel_size = get_file_size(f)) ||
>>>> -    fread(header, 1, 1024, f) != 1024) {
>>>> +    fread(header, 1, MIN(8192, kernel_size), f) != MIN(8192,
>>>> kernel_size)) {
>>>>   fprintf(stderr, "qemu: could not load kernel '%s'\n",
>>>>       kernel_filename);
>>>>   exit(1);
>>>>
>>> Ah,  sorry - mix in the series. This only applies to the multi-boot 
>>> series
>>> which increases the header read to 8192 bytes.
>>
>> Regardless, this code should not hardcode the size like this. It should
>> use sizeof(header) instead of 1024 or 8192, thus avoiding the potential
>> bug.
> 
> You don't really know sizeof(header), do you? Header could be the Linux 
> header or the Multiboot header which is by definition allowed to sit 
> somewhere within the first 8192 bytes.

Maybe he ment just sizeof(header) to avoid letting future changes
of the code let the definition and code get out of sync if the
header size to be read is changed again.

-- 
   René Rebe - ExactCODE GmbH - Europe, Germany, Berlin
   http://exactcode.de | http://t2-project.org | http://rene.rebe.name