[Fontconfig] MD5 checksum? Why? And why is there stuff to switch little endien?

[Akira TAGOH]

As you can see that from the code, those code are only used to make a
hash for the cache filename.

The caches are architecture-dependent because those are mmap'ed to
reduce the memory footprint. that is irrelevant to MD5 checksum thing.

On Fri, Jun 20, 2014 at 1:10 PM, L. A. Walsh <fonts at tlinx.org> wrote:
> I don't understand why font caches are being MD5 checksummed?
>
> It's not like they are that security sensitive, and I don't see why such
> an expensive technique is being used.
>
> Also, in looking at the code, I see a byte order reversal for little ending
> machines -- which I don't understand.
>
> The caches aren't even portable between 32 and 64 bit caches on same
> machine, so why is there byte-reversing going on for some compat w/
> big endian machines when the caches aren't portable between archs?
>
> I'd never seen byte order switching w/an MD5 algorithm before, and
> checked against wikipedia.  The one there doesn't seem to do any
> endian switching -- AND of more concern, the constants, (the same
> ones in the font-config code), the input and the output are all described
> as being little endian.  So why is there code to switch endianness?:
>
>
> #ifndef HIGHFIRST
> #define byteReverse(buf, len)    /* Nothing */
> #else
> /*
> * Note: this code is harmless on little-endian machines.
> */
> void byteReverse(unsigned char *buf, unsigned longs) {
>    FcChar32 t;
>    do {
>        t = (FcChar32) ((unsigned) buf[3] << 8 | buf[2]) << 16 |
>            ((unsigned) buf[1] << 8 | buf[0]);
>        *(FcChar32 *) buf = t;
>        buf += 4;
>    } while (--longs);
> }
> #endif
> --------------
>
> Is that only for big endian machines?   But does it make sense even there
> given that the caches aren't portable?
>
> Why are they being checksumed -- is it just to check for need to update?
> Can't modification times be used for that?
>
>
>
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-- 
Akira TAGOH