Re: [Qemu-devel] [PATCH v14 10/13] Add xbzrle_encode_buffer and xbzrle_decode_buffer functions

[Eric Blake <eblake@redhat.com>]

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On 07/03/2012 03:39 PM, Eric Blake wrote:
> On 07/03/2012 03:32 PM, Eric Blake wrote:
> 
>>> +        ret = uleb128_decode_small(src + i, &count);
>>> +        if (ret < 0) {
>>
>> An nzrun should be a non-zero value; I'd write this as (ret <= 0) to
>> rule out an attempt to pass a zero-length nzrun.
> 
> Correcting myself,
> 
> if (ret < 0 || !count) {
> 
> At this point, I think I will just bite the bullet and post a version of
> this code that incorporates my review.

Something like this (lightly tested):

/*
  page = zrun nzrun
       | zrun nzrun page

  zrun = length

  nzrun = length byte...

  length = uleb128 encoded integer
 */
int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t *new_buf, int slen,
                         uint8_t *dst, int dlen)
{
    uint32_t zrun_len = 0, nzrun_len = 0;
    int d = 0, i = 0;
    long res, xor;
    uint8_t *nzrun_start = NULL;

    g_assert(!(((uintptr_t)old_buf | (uintptr_t)new_buf | slen) %
	       sizeof(long)));

    while (i < slen) {
        /* overflow */
        if (d + 2 > dlen) {
            return -1;
        }

        /* not aligned to sizeof(long) */
        res = (slen - i) % sizeof(long);
        while (res && old_buf[i] == new_buf[i]) {
            zrun_len++;
            i++;
            res--;
        }

        if (!res) {
            while (i < slen &&
                   (*(long *)(old_buf + i)) == (*(long *)(new_buf + i))) {
                i += sizeof(long);
                zrun_len += sizeof(long);
            }

            /* go over the rest */
            while (i < slen && old_buf[i] == new_buf[i]) {
                zrun_len++;
		i++;
            }
        }

        /* buffer unchanged */
        if (zrun_len == slen) {
            return 0;
        }

        /* skip last zero run */
        if (i == slen) {
            return d;
        }

        d += uleb128_encode_small(dst + d, zrun_len);

        zrun_len = 0;
        nzrun_start = new_buf + i;

        /* overflow */
        if (d + 2 > dlen) {
            return -1;
        }

        /* not aligned to sizeof(long) */
        res = (slen - i) % sizeof(long);
        while (res && old_buf[i] != new_buf[i]) {
            nzrun_len++;
            i++;
            res--;
        }

        if (!res) {
            /* truncation to 32-bit long okay */
            long mask = 0x0101010101010101ULL;
            while (i < slen) {
                xor = *(long *)(old_buf + i) ^ *(long *)(new_buf + i);
	        if ((xor - mask) & ~xor & (mask << 7)) {
		    /* found the end of an nzrun within the current long */
		    while (old_buf[i] != new_buf[i]) {
                        nzrun_len++;
                        i++;
                    }
                    break;
                } else {
                    i += sizeof(long);
                    nzrun_len += sizeof(long);
                }
            }
        }

        d += uleb128_encode_small(dst + d, nzrun_len);

        /* overflow */
        if (d + nzrun_len > dlen) {
            return -1;
        }

        memcpy(dst + d, nzrun_start, nzrun_len);
        d += nzrun_len;
        nzrun_len = 0;
    }

    return d;
}

int xbzrle_decode_buffer(uint8_t *src, int slen, uint8_t *dst, int dlen)
{
    int i = 0, d = 0;
    int ret;
    uint32_t count = 0;

    while (i < slen) {

        /* zrun */
        if (slen - i < 2) {
            return -1;
        }

        ret = uleb128_decode_small(src + i, &count);
        if (ret < 0 || (i && !count)) {
            return -1;
        }
        i += ret;
        d += count;

        /* overflow */
        if (d > dlen) {
            return -1;
        }

        /* nzrun */
        if (slen - i < 2) {
            return -1;
        }
        ret = uleb128_decode_small(src + i, &count);
        if (ret < 0 || !count) {
            return -1;
        }
        i += ret;

        /* overflow */
        if (d + count > dlen || i + count > slen) {
            return -1;
        }

        memcpy(dst + d , src + i, count);
        d += count;
        i += count;
    }

    return d;
}

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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