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Re: [PATCH v3, 15/16] xfsprogs: metadump: use printable characters for o

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH v3, 15/16] xfsprogs: metadump: use printable characters for obfuscated names
From: Alex Elder <aelder@xxxxxxx>
Date: Fri, 25 Feb 2011 12:13:56 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110224084545.GC3166@dastard>
References: <201102182121.p1ILL2Ti029193@xxxxxxxxxxxxxxxxxxxxxx> <20110224084545.GC3166@dastard>
Reply-to: aelder@xxxxxxx
On Thu, 2011-02-24 at 19:45 +1100, Dave Chinner wrote:
> On Fri, Feb 18, 2011 at 03:21:02PM -0600, Alex Elder wrote:
> > There is probably not much need for an extreme amount of randomness
> > in the obfuscated names produced in metadumps.  Limit the character
> > set used for (most of) these names to printable characters rather
> > than every permittable byte.  The result makes metadumps a bit more
> > natural to work with.
> > 
> > I chose the set of all upper- and lower-case letters, digits, and
> > the dash and underscore for the alphabet.  It could easily be
> > expanded to include others (or reduced for that matter).
> > 
> > This change also avoids ever having to retry after picking an
> > unusable character.
> > 
> > Signed-off-by: Alex Elder <aelder@xxxxxxx>
> > 
> > No significant changes in this version from the last version posted.
> > 
> > ---
> >  db/metadump.c |    9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > Index: b/db/metadump.c
> > ===================================================================
> > --- a/db/metadump.c
> > +++ b/db/metadump.c
> > @@ -412,12 +412,11 @@ nametable_add(xfs_dahash_t hash, int nam
> >  static inline uchar_t
> >  random_filename_char(void)
> >  {
> > -   uchar_t                 c;
> > +   static uchar_t filename_alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
> > +                                           "abcdefghijklmnopqrstuvwxyz"
> > +                                           "0123456789-_";
> >  
> > -   do {
> > -           c = random() % 127 + 1;
> > -   } while (c == '/');
> > -   return c;
> > +   return filename_alphabet[random() % (sizeof filename_alphabet - 1)];
> >  }
> 
> Why not just:
> 
>       do {
>               c = random() % 127 + 1;
>       } while (!isalnum(c));
> 
>       return c;
> 

Mainly because I wasn't sure what people would want as an acceptable
alphabet to select from.  We could just use [a-z], for example, and
this way that could easily be changed without changing how the
function worked.  It's also locale-independent (which may or may not
be good I suppose).

Plus as an added bonus, it will never need to compute any
unnecessary random numbers, thereby saving about 12 CPU
cycles. :)

I don't really care much, but would lean toward leaving
it the way I have it.  Do you feel strongly that I should
change it?  Do you think [a-z] (islower()) would be even
better?

                                        -Alex


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