bsd_comp.c

讲述linux的初始化过程

    int olen;

#define PUTBYTE(v)			\
  {					\
    ++olen;				\
    if (wptr)				\
      {					\
	*wptr++ = (unsigned char) (v);	\
	if (olen >= osize)		\
	  {				\
	    wptr = NULL;		\
	  }				\
      }					\
  }

#define OUTPUT(ent)			\
  {					\
    bitno -= n_bits;			\
    accm |= ((ent) << bitno);		\
    do					\
      {					\
	PUTBYTE(accm >> 24);		\
	accm <<= 8;			\
	bitno += 8;			\
      }					\
    while (bitno <= 24);		\
  }

  /*
   * If the protocol is not in the range we're interested in,
   * just return without compressing the packet.  If it is,
   * the protocol becomes the first byte to compress.
   */

    ent = PPP_PROTOCOL(rptr);
    if (ent < 0x21 || ent > 0xf9)
      {
	return 0;
      }

    db      = (struct bsd_db *) state;
    hshift  = db->hshift;
    max_ent = db->max_ent;
    n_bits  = db->n_bits;
    bitno   = 32;
    accm    = 0;
    mxcode  = MAXCODE (n_bits);

    /* Initialize the output pointers */
    wptr  = obuf;
    olen  = PPP_HDRLEN + BSD_OVHD;

    if (osize > isize)
      {
	osize = isize;
      }

    /* This is the PPP header information */
    if (wptr)
      {
	*wptr++ = PPP_ADDRESS(rptr);
	*wptr++ = PPP_CONTROL(rptr);
	*wptr++ = 0;
	*wptr++ = PPP_COMP;
	*wptr++ = db->seqno >> 8;
	*wptr++ = db->seqno;
      }

    /* Skip the input header */
    rptr  += PPP_HDRLEN;
    isize -= PPP_HDRLEN;
    ilen   = ++isize;	/* Low byte of protocol is counted as input */

    while (--ilen > 0)
      {
	c     = *rptr++;
	fcode = BSD_KEY  (ent, c);
	hval  = BSD_HASH (ent, c, hshift);
	dictp = dict_ptr (db, hval);
	
	/* Validate and then check the entry. */
	if (dictp->codem1 >= max_ent)
	  {
	    goto nomatch;
	  }

	if (dictp->f.fcode == fcode)
	  {
	    ent = dictp->codem1 + 1;
	    continue;	/* found (prefix,suffix) */
	  }
	
	/* continue probing until a match or invalid entry */
	disp = (hval == 0) ? 1 : hval;

	do
	  {
	    hval += disp;
	    if (hval >= db->hsize)
	      {
		hval -= db->hsize;
	      }
	    dictp = dict_ptr (db, hval);
	    if (dictp->codem1 >= max_ent)
	      {
		goto nomatch;
	      }
	  }
	while (dictp->f.fcode != fcode);

	ent = dictp->codem1 + 1;	/* finally found (prefix,suffix) */
	continue;
	
nomatch:
	OUTPUT(ent);		/* output the prefix */
	
	/* code -> hashtable */
	if (max_ent < db->maxmaxcode)
	  {
	    struct bsd_dict *dictp2;
	    struct bsd_dict *dictp3;
	    int    indx;

	    /* expand code size if needed */
	    if (max_ent >= mxcode)
	      {
		db->n_bits = ++n_bits;
		mxcode     = MAXCODE (n_bits);
	      }
	    
	    /* Invalidate old hash table entry using
	     * this code, and then take it over.
	     */

	    dictp2 = dict_ptr (db, max_ent + 1);
	    indx   = dictp2->cptr;
	    dictp3 = dict_ptr (db, indx);

	    if (dictp3->codem1 == max_ent)
	      {
		dictp3->codem1 = BADCODEM1;
	      }

	    dictp2->cptr   = hval;
	    dictp->codem1  = max_ent;
	    dictp->f.fcode = fcode;
	    db->max_ent    = ++max_ent;

	    if (db->lens)
	      {
		unsigned short *len1 = lens_ptr (db, max_ent);
		unsigned short *len2 = lens_ptr (db, ent);
		*len1 = *len2 + 1;
	      }
	  }
	ent = c;
      }
    
    OUTPUT(ent);		/* output the last code */

    db->bytes_out    += olen - PPP_HDRLEN - BSD_OVHD;
    db->uncomp_bytes += isize;
    db->in_count     += isize;
    ++db->uncomp_count;
    ++db->seqno;

    if (bitno < 32)
      {
	++db->bytes_out; /* must be set before calling bsd_check */
      }

    /*
     * Generate the clear command if needed
     */

    if (bsd_check(db))
      {
	OUTPUT (CLEAR);
      }
    
    /*
     * Pad dribble bits of last code with ones.
     * Do not emit a completely useless byte of ones.
     */

    if (bitno != 32)
      {
	PUTBYTE((accm | (0xff << (bitno-8))) >> 24);
      }
    
    /*
     * Increase code size if we would have without the packet
     * boundary because the decompressor will do so.
     */

    if (max_ent >= mxcode && max_ent < db->maxmaxcode)
      {
	db->n_bits++;
      }

    /* If output length is too large then this is an incomplete frame. */
    if (wptr == NULL)
      {
	++db->incomp_count;
	db->incomp_bytes += isize;
	olen              = 0;
      }
    else /* Count the number of compressed frames */
      {
	++db->comp_count;
	db->comp_bytes += olen;
      }

    /* Return the resulting output length */
    return olen;
#undef OUTPUT
#undef PUTBYTE
  }

/*
 * Update the "BSD Compress" dictionary on the receiver for
 * incompressible data by pretending to compress the incoming data.
 */

static void bsd_incomp (void *state, unsigned char *ibuf, int icnt)
  {
    (void) bsd_compress (state, ibuf, (char *) 0, icnt, 0);
  }

/*
 * Decompress "BSD Compress".
 *
 * Because of patent problems, we return DECOMP_ERROR for errors
 * found by inspecting the input data and for system problems, but
 * DECOMP_FATALERROR for any errors which could possibly be said to
 * be being detected "after" decompression.  For DECOMP_ERROR,
 * we can issue a CCP reset-request; for DECOMP_FATALERROR, we may be
 * infringing a patent of Motorola's if we do, so we take CCP down
 * instead.
 *
 * Given that the frame has the correct sequence number and a good FCS,
 * errors such as invalid codes in the input most likely indicate a
 * bug, so we return DECOMP_FATALERROR for them in order to turn off
 * compression, even though they are detected by inspecting the input.
 */

static int bsd_decompress (void *state, unsigned char *ibuf, int isize,
			   unsigned char *obuf, int osize)
  {
    struct bsd_db *db;
    unsigned int max_ent;
    unsigned long accm;
    unsigned int bitno;		/* 1st valid bit in accm */
    unsigned int n_bits;
    unsigned int tgtbitno;	/* bitno when we have a code */
    struct bsd_dict *dictp;
    int explen;
    int seq;
    unsigned int incode;
    unsigned int oldcode;
    unsigned int finchar;
    unsigned char *p;
    unsigned char *wptr;
    int adrs;
    int ctrl;
    int ilen;
    int codelen;
    int extra;

    db       = (struct bsd_db *) state;
    max_ent  = db->max_ent;
    accm     = 0;
    bitno    = 32;		/* 1st valid bit in accm */
    n_bits   = db->n_bits;
    tgtbitno = 32 - n_bits;	/* bitno when we have a code */
    
    /*
     * Save the address/control from the PPP header
     * and then get the sequence number.
     */

    adrs  = PPP_ADDRESS (ibuf);
    ctrl  = PPP_CONTROL (ibuf);

    seq   = (ibuf[4] << 8) + ibuf[5];

    ibuf += (PPP_HDRLEN + 2);
    ilen  = isize - (PPP_HDRLEN + 2);
    
    /*
     * Check the sequence number and give up if it differs from
     * the value we're expecting.
     */

    if (seq != db->seqno)
      {
	if (db->debug)
	  {
	    printk("bsd_decomp%d: bad sequence # %d, expected %d\n",
		   db->unit, seq, db->seqno - 1);
	  }
	return DECOMP_ERROR;
      }

    ++db->seqno;
    db->bytes_out += ilen;

    /*
     * Fill in the ppp header, but not the last byte of the protocol
     * (that comes from the decompressed data).
     */

    wptr    = obuf;
    *wptr++ = adrs;
    *wptr++ = ctrl;
    *wptr++ = 0;
    
    oldcode = CLEAR;
    explen  = 3;

    /*
     * Keep the checkpoint correctly so that incompressible packets
     * clear the dictionary at the proper times.
     */

    for (;;)
      {
	if (ilen-- <= 0)
	  {
	    db->in_count += (explen - 3); /* don't count the header */
	    break;
	  }

	/*
	 * Accumulate bytes until we have a complete code.
	 * Then get the next code, relying on the 32-bit,
	 * unsigned accm to mask the result.
	 */

	bitno -= 8;
	accm  |= *ibuf++ << bitno;
	if (tgtbitno < bitno)
	  {
	    continue;
	  }

	incode = accm >> tgtbitno;
	accm <<= n_bits;
	bitno += n_bits;

	/*
	 * The dictionary must only be cleared at the end of a packet.
	 */
	
	if (incode == CLEAR)
	  {
	    if (ilen > 0)
	      {
		if (db->debug)
		  {
		    printk("bsd_decomp%d: bad CLEAR\n", db->unit);
		  }
		return DECOMP_FATALERROR;	/* probably a bug */
	      }
	    
	    bsd_clear(db);
	    break;
	  }

	if ((incode > max_ent + 2) || (incode > db->maxmaxcode)
	    || (incode > max_ent && oldcode == CLEAR))
	  {
	    if (db->debug)
	      {
		printk("bsd_decomp%d: bad code 0x%x oldcode=0x%x ",
		       db->unit, incode, oldcode);
		printk("max_ent=0x%x explen=%d seqno=%d\n",
		       max_ent, explen, db->seqno);
	      }
	    return DECOMP_FATALERROR;	/* probably a bug */
	  }
	
	/* Special case for KwKwK string. */
	if (incode > max_ent)
	  {
	    finchar = oldcode;
	    extra   = 1;
	  }
	else
	  {
	    finchar = incode;
	    extra   = 0;
	  }
	
	codelen = *(lens_ptr (db, finchar));
	explen += codelen + extra;
	if (explen > osize)
	  {
	    if (db->debug)
	      {
		printk("bsd_decomp%d: ran out of mru\n", db->unit);
#ifdef DEBUG
		printk("  len=%d, finchar=0x%x, codelen=%d, explen=%d\n",
		       ilen, finchar, codelen, explen);
#endif
	      }
	    return DECOMP_FATALERROR;
	  }
	
	/*
	 * Decode this code and install it in the decompressed buffer.
	 */

	wptr += codelen;
	p     = wptr;
	while (finchar > LAST)
	  {
	    struct bsd_dict *dictp2 = dict_ptr (db, finchar);
	    
	    dictp = dict_ptr (db, dictp2->cptr);
#ifdef DEBUG
	    if (--codelen <= 0 || dictp->codem1 != finchar-1)
	      {
		if (codelen <= 0)
		  {
		    printk("bsd_decomp%d: fell off end of chain ", db->unit);
		    printk("0x%x at 0x%x by 0x%x, max_ent=0x%x\n",
			   incode, finchar, dictp2->cptr, max_ent);
		  }
		else
		  {
		    if (dictp->codem1 != finchar-1)
		      {
			printk("bsd_decomp%d: bad code chain 0x%x "
			       "finchar=0x%x ",
			       db->unit, incode, finchar);

			printk("oldcode=0x%x cptr=0x%x codem1=0x%x\n",
			       oldcode, dictp2->cptr, dictp->codem1);
		      }
		  }
		return DECOMP_FATALERROR;
	      }
#endif
	    *--p    = dictp->f.hs.suffix;
	    finchar = dictp->f.hs.prefix;
	  }
	*--p = finchar;
	
#ifdef DEBUG
	if (--codelen != 0)
	  {
	    printk("bsd_decomp%d: short by %d after code 0x%x, max_ent=0x%x\n",
		   db->unit, codelen, incode, max_ent);
	  }
#endif
	
	if (extra)		/* the KwKwK case again */
	  {
	    *wptr++ = finchar;
	  }
	
	/*
	 * If not first code in a packet, and
	 * if not out of code space, then allocate a new code.
	 *
	 * Keep the hash table correct so it can be used
	 * with uncompressed packets.
	 */

	if (oldcode != CLEAR && max_ent < db->maxmaxcode)
	  {
	    struct bsd_dict *dictp2, *dictp3;
	    unsigned short  *lens1,  *lens2;
	    unsigned long fcode;
	    int hval, disp, indx;
	    
	    fcode = BSD_KEY(oldcode,finchar);
	    hval  = BSD_HASH(oldcode,finchar,db->hshift);
	    dictp = dict_ptr (db, hval);
	    
	    /* look for a free hash table entry */
	    if (dictp->codem1 < max_ent)
	      {
		disp = (hval == 0) ? 1 : hval;
		do
		  {
		    hval += disp;
		    if (hval >= db->hsize)
		      {
			hval -= db->hsize;
		      }
		    dictp = dict_ptr (db, hval);
		  }
		while (dictp->codem1 < max_ent);
	      }
	    
	    /*
	     * Invalidate previous hash table entry
	     * assigned this code, and then take it over
	     */

	    dictp2 = dict_ptr (db, max_ent + 1);
	    indx   = dictp2->cptr;
	    dictp3 = dict_ptr (db, indx);

	    if (dictp3->codem1 == max_ent)
	      {
		dictp3->codem1 = BADCODEM1;
	      }

	    dictp2->cptr   = hval;
	    dictp->codem1  = max_ent;
	    dictp->f.fcode = fcode;
	    db->max_ent    = ++max_ent;

	    /* Update the length of this string. */
	    lens1  = lens_ptr (db, max_ent);
	    lens2  = lens_ptr (db, oldcode);
	    *lens1 = *lens2 + 1;
	    
	    /* Expand code size if needed. */
	    if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode)
	      {
		db->n_bits = ++n_bits;
		tgtbitno   = 32-n_bits;
	      }
	  }
	oldcode = incode;
      }

    ++db->comp_count;
    ++db->uncomp_count;
    db->comp_bytes   += isize - BSD_OVHD - PPP_HDRLEN;
    db->uncomp_bytes += explen;

    if (bsd_check(db))
      {
	if (db->debug)
	  {
	    printk("bsd_decomp%d: peer should have cleared dictionary on %d\n",
		   db->unit, db->seqno - 1);
	  }
      }
    return explen;
  }
     
/*************************************************************
 * Table of addresses for the BSD compression module
 *************************************************************/

static struct compressor ppp_bsd_compress = {
    CI_BSD_COMPRESS,		/* compress_proto */
    bsd_comp_alloc,		/* comp_alloc */
    bsd_free,			/* comp_free */
    bsd_comp_init,		/* comp_init */
    bsd_reset,			/* comp_reset */
    bsd_compress,		/* compress */
    bsd_comp_stats,		/* comp_stat */
    bsd_decomp_alloc,		/* decomp_alloc */
    bsd_free,			/* decomp_free */
    bsd_decomp_init,		/* decomp_init */
    bsd_reset,			/* decomp_reset */
    bsd_decompress,		/* decompress */
    bsd_incomp,			/* incomp */
    bsd_comp_stats		/* decomp_stat */
};

/*************************************************************
 * Module support routines
 *************************************************************/

int bsdcomp_init(void)
{
	int answer = ppp_register_compressor(&ppp_bsd_compress);
	if (answer == 0)
		printk(KERN_INFO "PPP BSD Compression module registered\n");
	return answer;
}

void bsdcomp_cleanup(void)
{
	ppp_unregister_compressor(&ppp_bsd_compress);
}

module_init(bsdcomp_init);
module_exit(bsdcomp_cleanup);