[erlang-bugs] Bug in ei_decode_big, implementation for ei_*encode_big

[Björn-Egil Dahlberg]

Hi Paul,

Thank you for reporting this.
I will look into it and try to make the appropriate corrections
for this and the fun problems.

Regards,
Björn-Egil
Erlang/OTP

Paul Guyot wrote:
> Hello,
> 
> ei.h has the following FIXME notice before the prototype of the big  
> functions:
> 
> /* FIXME: is this really the best way to handle bignums? */
> 
> The thing is that ei_decode_big has a bug that makes it unusable,  
> around line 54 of decode_big.c:
> 
>        for (i = 0; i < b->arity; ++i) {
> 	  dt[i] = u[i*2];
> 	  dt[i] |= ((unsigned short) u[(i*2)+1]) << 8;           <--- this  
> is line 54
>        }
> 
> The high order bits of the 16-bits digit (short) are read, even if  
> the byte (at u[(i*2)+1]) doesn't belong to the big. In other words,  
> big numbers such as 16#1234567890 with an odd number of bytes are  
> decoded with a garbage byte (i.e. 16#XX1234567890).
> 
> The following change fixes the problem:
> 
>        for (i = 0; i < b->arity; ++i) {
> 	  dt[i] = u[i*2];
> 	  if ((i*2 + 1) < digit_bytes)
> 	  {
>                dt[i] |= ((unsigned short) u[(i*2)+1]) << 8;
> 	  }
>        }
> 
> Also, ei_*encode_big is missing. Here is a sample implementation:
> 
> int ei_encode_big(char *buf, int *index, erlang_big* big)
> {
>      unsigned short* digits = big->digits;
>      char *s = buf + *index;
>      char *s0 = s;
>      int arity = big->arity;
>      long digit_bytes = (arity * 2);
> 
>      if (digit_bytes < 256)
>      {
>          if (buf != NULL)
>          {
>              put8(s, ERL_SMALL_BIG_EXT);
>              put8(s, digit_bytes);
>          } else {
>              s += 2;
>          }
>      } else {
>          if (buf != NULL)
>          {
>              put8(s, ERL_LARGE_BIG_EXT);
>              put32be(s, digit_bytes);
>          } else {
>              s += 5;
>          }
>      }
>      if (buf != NULL)
>      {
>          put8(s, big->is_neg);
>          int i;
>          for (i = 0; i < arity; ++i)
>          {
>              put16le(s, digits[i]);
>          }
>      } else {
>          s += 1 + (arity * 2);
>      }
> 
>      *index += s-s0;
> 
>      return 0;
> }
> 
> int ei_x_encode_big(ei_x_buff* x, erlang_big* big)
> {
>      int i = x->index;
>      ei_encode_big(NULL, &i, big);
>      if (!x_fix_buff(x, i))
> 	return -1;
>      return ei_encode_big(x->buff, &x->index, big);
> }
> 
> Regards,
> 
> Paul
> 
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