Request Comments on an Overflow Coverage Case

[Joel Sherrill]

Hi,

I am looking at trying to deal with an
uncovered piece of code in coremsg.c (line 86)

http://www.rtems.org/ftp/pub/rtems/people/joel/coverage/erc32/erc32-Ospd-20110704-1327/annotated.html#range3

This is trying to detect overflow when
determining the amount of memory to allocate
for the buffers.  As best I can tell, it
misses a lot of overflow cases for the following
multiplication.

needed = num msgs * size of a msg

Looking around on the web for suggestions, it
looks like the most reliable technique is
to do a 32x32 multiply into a 64-bit result.
Then see if the answer is too large for 32 bits.
 From http://www.fefe.de/intof.html

int umult32(uint32 a,uint32 b,uint32* c) {
   unsigned long long x=(unsigned long long)a*b;
   if (x>0xffffffff) return 0;
   *c=x&0xffffffff;
   return 1;
}

Since this is only an issue during object
creation, I think using this technique
via a helper routine like the above would
make sense.

Any thoughts? suggestions?

-- 
Joel Sherrill, Ph.D.             Director of Research&  Development
joel.sherrill at OARcorp.com        On-Line Applications Research
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