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Re: [linux-lvm] LVM metadata checksums



On Fri, 2006-09-08 at 14:55 +0000, Mark Krenz wrote:
>   Before you say, DON'T DO IT THAT WAY, let me assure you that I have my
> reasons and I'm aware of the risks. This is mostly related to my
> question about changing UUIDs of physical volumes and volume groups.
> Anyways, I thought that perhaps I could try to use hexedit to modify the
> UUID within the image file.  However, after I did this I ran into all
> the lvm commands complaining about checksum errors in the metadata.  So
> my question is, where is the checksum stored and how is it calculated.
> I noticed that there is an "LVM header near the beginning of the disk
> followed by a long string:
> 
>  LABELONE........?... ...LVM2  001M0U7CEZK8M7l2Z4KnOtI1uSVjAu0cCFd
> 
>  Is that last part the checksum?  What kind is it?
> 
> 
after a fast read of sources, i think checksum is computed here :
lib/misc/crc.c :
/* Calculate an endian-independent CRC of supplied buffer */
uint32_t calc_crc(uint32_t initial, void *buf, uint32_t size)
{
        static const uint32_t crctab[] = {
                0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
                0x76dc4190, 0x6b6b51f4, 0x4db26158, 0x5005713c,
                0xedb88320, 0xf00f9344, 0xd6d6a3e8, 0xcb61b38c,
                0x9b64c2b0, 0x86d3d2d4, 0xa00ae278, 0xbdbdf21c
        };
        uint32_t i, crc = initial;
        uint8_t *data = (uint8_t *) buf;

        for (i = 0; i < size; i++) {
                crc ^= *data++;
                crc = (crc >> 4) ^ crctab[crc & 0xf];
                crc = (crc >> 4) ^ crctab[crc & 0xf];
        }
        return crc;
}

Here is the call :
lib/format_text/format-text.c,139 :
        if (mdah->checksum_xl != xlate32(calc_crc(INITIAL_CRC,
mdah->magic,
                                                  MDA_HEADER_SIZE -

sizeof(mdah->checksum_xl)))) {
                log_error("Incorrect metadata area header checksum");
                return NULL;
        }

-- 
Fabien Jakimowicz <fabien jakimowicz com>

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