phc-discussions - RE: Specification of a modular crypt format (2)

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Date: Sat, 24 Oct 2015 15:06:48 +0000
From: Peter Gutmann <pgut001@...auckland.ac.nz>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: Specification of a modular crypt format (2)

Just thought I'd post an update to this, based on the discussion earlier in
the thread I rewrote the code to avoid gcc's issues, the result ended up
almost identical to Thomas' decode_decimal() (I didn't look at it when I
updated my code, it just ended up that way :-):

    for( value = 0, i = 0; i < strLen; i++ )
        {
        const int ch = byteToInt( str[ i ] ) - '0';

        if( ch < 0 || ch > 9 )
            return( CRYPT_ERROR_BADDATA );
        if( value < 0 || value >= MAX_INTLENGTH / 10 )
            return( CRYPT_ERROR_BADDATA );
        value *= 10;                                    // Line 19
        if( value >= MAX_INTLENGTH - ch )               // Line 20
            return( CRYPT_ERROR_BADDATA );
        value += ch;
        ENSURES( value >= 0 && value < MAX_INTLENGTH );
        }

What's scary about this is that without the unnecessary 'value < 0' condition,
the STACK analyser still reports it as being problematic:

bug: anti-simplify
model: |
  %cmp10 = icmp sge i32 %mul, %sub9, !dbg !37
  -->  false
stack:
  - test.c:20:0
ncore: 1
core:
  - test.c:19:0
    - signed multiplication overflow

Depending on how closely STACK follows gcc's reasoning, if they do perform the
same analysis then gcc would "see" UB there and decide that it could break the
code.

Peter.