Commit message: Use post-increment only in inffast.c.

An old inffast.c optimization turns out to not be optimal anymore
with modern compilers, and furthermore was not compliant with the
C standard, for which decrementing a pointer before its allocated
memory is undefined. Per the recommendation of a security audit of
the zlib code by Trail of Bits and TrustInSoft, in support of the
Mozilla Foundation, this "optimization" was removed, in order to
avoid the possibility of undefined behavior.
CVE: CVE-2016-9841
CWE: CWE-189
Vulnerability description: inffast.c in zlib 1.2.8 might allow context-dependent attackers to have unspecified impact by leveraging improper pointer arithmetic.

Project: zlib
File: inffast.c

void ZLIB_INTERNAL inflate_fast(strm, start)
z_streamp strm;
unsigned start;         /* inflate()'s starting value for strm->avail_out */
{
    struct inflate_state FAR *state;
    z_const unsigned char FAR *in;      /* local strm->next_in */
    z_const unsigned char FAR *last;    /* have enough input while in < last */
    unsigned char FAR *out;     /* local strm->next_out */
    unsigned char FAR *beg;     /* inflate()'s initial strm->next_out */
    unsigned char FAR *end;     /* while out < end, enough space available */
#ifdef INFLATE_STRICT
    unsigned dmax;              /* maximum distance from zlib header */
#endif
    unsigned wsize;             /* window size or zero if not using window */
    unsigned whave;             /* valid bytes in the window */
    unsigned wnext;             /* window write index */
    unsigned char FAR *window;  /* allocated sliding window, if wsize != 0 */
    unsigned long hold;         /* local strm->hold */
    unsigned bits;              /* local strm->bits */
    code const FAR *lcode;      /* local strm->lencode */
    code const FAR *dcode;      /* local strm->distcode */
    unsigned lmask;             /* mask for first level of length codes */
    unsigned dmask;             /* mask for first level of distance codes */
    code here;                  /* retrieved table entry */
    unsigned op;                /* code bits, operation, extra bits, or */
                                /*  window position, window bytes to copy */
    unsigned len;               /* match length, unused bytes */
    unsigned dist;              /* match distance */
    unsigned char FAR *from;    /* where to copy match from */
 
     /* copy state to local variables */
     state = (struct inflate_state FAR *)strm->state;
    in = strm->next_in;
     last = in + (strm->avail_in - 5);
    out = strm->next_out;
     beg = out - (start - strm->avail_out);
     end = out + (strm->avail_out - 257);
 #ifdef INFLATE_STRICT
    dmax = state->dmax;
#endif
    wsize = state->wsize;
    whave = state->whave;
    wnext = state->wnext;
    window = state->window;
    hold = state->hold;
    bits = state->bits;
    lcode = state->lencode;
    dcode = state->distcode;
    lmask = (1U << state->lenbits) - 1;
    dmask = (1U << state->distbits) - 1;

    /* decode literals and length/distances until end-of-block or not enough
        input data or output space */
     do {
         if (bits < 15) {
            hold += (unsigned long)(*in++) << bits;
             bits += 8;
            hold += (unsigned long)(*in++) << bits;
             bits += 8;
         }
         here = lcode[hold & lmask];
      dolen:
        op = (unsigned)(here.bits);
        hold >>= op;
        bits -= op;
        op = (unsigned)(here.op);
        if (op == 0) {                          /* literal */
             Tracevv((stderr, here.val >= 0x20 && here.val < 0x7f ?
                     "inflate:         literal '%c'\n" :
                     "inflate:         literal 0x%02x\n", here.val));
            *out++ = (unsigned char)(here.val);
         }
         else if (op & 16) {                     /* length base */
             len = (unsigned)(here.val);
             op &= 15;                           /* number of extra bits */
             if (op) {
                 if (bits < op) {
                    hold += (unsigned long)(*in++) << bits;
                     bits += 8;
                 }
                 len += (unsigned)hold & ((1U << op) - 1);
                hold >>= op;
                bits -= op;
             }
             Tracevv((stderr, "inflate:         length %u\n", len));
             if (bits < 15) {
                hold += (unsigned long)(*in++) << bits;
                 bits += 8;
                hold += (unsigned long)(*in++) << bits;
                 bits += 8;
             }
             here = dcode[hold & dmask];
          dodist:
            op = (unsigned)(here.bits);
            hold >>= op;
            bits -= op;
            op = (unsigned)(here.op);
            if (op & 16) {                      /* distance base */
                 dist = (unsigned)(here.val);
                 op &= 15;                       /* number of extra bits */
                 if (bits < op) {
                    hold += (unsigned long)(*in++) << bits;
                     bits += 8;
                     if (bits < op) {
                        hold += (unsigned long)(*in++) << bits;
                         bits += 8;
                     }
                 }
                dist += (unsigned)hold & ((1U << op) - 1);
#ifdef INFLATE_STRICT
                if (dist > dmax) {
                    strm->msg = (char *)"invalid distance too far back";
                    state->mode = BAD;
                    break;
                }
#endif
                hold >>= op;
                bits -= op;
                Tracevv((stderr, "inflate:         distance %u\n", dist));
                op = (unsigned)(out - beg);     /* max distance in output */
                if (dist > op) {                /* see if copy from window */
                    op = dist - op;             /* distance back in window */
                    if (op > whave) {
                        if (state->sane) {
                            strm->msg =
                                (char *)"invalid distance too far back";
                            state->mode = BAD;
                            break;
                        }
 #ifdef INFLATE_ALLOW_INVALID_DISTANCE_TOOFAR_ARRR
                         if (len <= op - whave) {
                             do {
                                *out++ = 0;
                             } while (--len);
                             continue;
                         }
                         len -= op - whave;
                         do {
                            *out++ = 0;
                         } while (--op > whave);
                         if (op == 0) {
                             from = out - dist;
                             do {
                                *out++ = *from++;
                             } while (--len);
                             continue;
                         }
 #endif
                     }
                    from = window;
                     if (wnext == 0) {           /* very common case */
                         from += wsize - op;
                         if (op < len) {         /* some from window */
                             len -= op;
                             do {
                                *out++ = *from++;
                             } while (--op);
                             from = out - dist;  /* rest from output */
                         }
                    }
                    else if (wnext < op) {      /* wrap around window */
                        from += wsize + wnext - op;
                        op -= wnext;
                         if (op < len) {         /* some from end of window */
                             len -= op;
                             do {
                                *out++ = *from++;
                             } while (--op);
                            from = window;
                             if (wnext < len) {  /* some from start of window */
                                 op = wnext;
                                 len -= op;
                                 do {
                                    *out++ = *from++;
                                 } while (--op);
                                 from = out - dist;      /* rest from output */
                             }
                        }
                    }
                    else {                      /* contiguous in window */
                        from += wnext - op;
                         if (op < len) {         /* some from window */
                             len -= op;
                             do {
                                *out++ = *from++;
                             } while (--op);
                             from = out - dist;  /* rest from output */
                         }
                     }
                     while (len > 2) {
                        *out++ = *from++;
                        *out++ = *from++;
                        *out++ = *from++;
                         len -= 3;
                     }
                     if (len) {
                        *out++ = *from++;
                         if (len > 1)
                            *out++ = *from++;
                     }
                 }
                 else {
                     from = out - dist;          /* copy direct from output */
                     do {                        /* minimum length is three */
                        *out++ = *from++;
                        *out++ = *from++;
                        *out++ = *from++;
                         len -= 3;
                     } while (len > 2);
                     if (len) {
                        *out++ = *from++;
                         if (len > 1)
                            *out++ = *from++;
                     }
                 }
             }
            else if ((op & 64) == 0) {          /* 2nd level distance code */
                here = dcode[here.val + (hold & ((1U << op) - 1))];
                goto dodist;
            }
            else {
                strm->msg = (char *)"invalid distance code";
                state->mode = BAD;
                break;
            }
        }
        else if ((op & 64) == 0) {              /* 2nd level length code */
            here = lcode[here.val + (hold & ((1U << op) - 1))];
            goto dolen;
        }
        else if (op & 32) {                     /* end-of-block */
            Tracevv((stderr, "inflate:         end of block\n"));
            state->mode = TYPE;
            break;
        }
        else {
            strm->msg = (char *)"invalid literal/length code";
            state->mode = BAD;
            break;
        }
    } while (in < last && out < end);

    /* return unused bytes (on entry, bits < 8, so in won't go too far back) */
    len = bits >> 3;
    in -= len;
    bits -= len << 3;
     hold &= (1U << bits) - 1;
 
     /* update state and return */
    strm->next_in = in;
    strm->next_out = out;
     strm->avail_in = (unsigned)(in < last ? 5 + (last - in) : 5 - (in - last));
     strm->avail_out = (unsigned)(out < end ?
                                  257 + (end - out) : 257 - (out - end));
    state->hold = hold;
    state->bits = bits;
    return;
}