[Speex-dev] [PATCH]Add address overflow check

[Nicholas Wilson]

On 09 February 2018 15:56 Jean-Marc Valin wrote:
> Pointers are unsigned so this shouldn't be an issue. I suspect you're
> being hit by something else. That or your compiler is really broken.

I don't know how important it is in this case (probably pretty minor) but in general Ruikai's right.

It doesn't matter that pointers are unsigned; that fact that a pointer could have a "large" value like 0xffff_ff00 means that they can wrap if you do length checks the wrong way. The behaviour is completely defined - it just causes the code not to work as intended.

The "bad" way of doing a length check is

char* buf_start, buf_end;
unsigned len_to_check;
if (buf_start + len_to_check > buf_end)
    fail()

Because the length is to-be-checked, it could have an unsafe large value, causing an (unsigned) overflow. For example, with buf_start = 0xffff_ff00 and buf_end = 0xffff_ff10, the maximum allowed length is 0x10, but a length of 0x100 will cause an overflow and bypass the check.

The safe way of doing a length check is

if (buf_end - buf_start < len_to_check)
    fail()

The buffer bounds are known safe, so the arithmetic is OK to do that way round.

Nick