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Re: [Xen-devel] libxc: Question on kernel image unzipping

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] libxc: Question on kernel image unzipping
From: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Date: Thu, 09 Jul 2009 16:03:43 +0200
Cc: Andreas Sommer <AndiDog@xxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 07/09/09 15:53, Keir Fraser wrote:
On 09/07/2009 13:34, "Andreas Sommer"<AndiDog@xxxxxx>  wrote:

libxc contains the following function which is used when uncompressing zipped
kernel images:
size_t xc_dom_check_gzip(void *blob, size_t ziplen)
     return unziplen + 16;
The returned unziplen+16 is used for the size of the destination buffer given
to inflate. But it is then also written to the kernel_size attribute of the
xc_dom_image struct. Hence kernel_size does not contain the uncompressed
kernel size but that plus 16.
So why do you always add 16 bytes to the real uncompressed kernel size?? That
doesn't make much sense to me but I need to know it because it is related to
my current work.

Gerd Hoffman would be the person to ask. The +16 doesn't appear to me to
have any purpose.

Oh, has been quite a while. IIRC that is related to zlib needing some extra space. So I *think* you can drop it there to get a correct kernel_size, but then you'll have to care somewhere else (probably when allocating the unzip target buffer) about the 16 extra bytes to make sure zlib doesn't overrun the buffer.

But better double-check that ...


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