linux-kernel - Re: [PATCH] FS-Cache: print hexadecimal value for special cookies type

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Message-ID: <20170425173700.124ee74d@blatinox-laptop.localdomain>
Date:   Tue, 25 Apr 2017 17:37:00 -0400
From:   Jérémy Lefaure <jeremy.lefaure@....epita.fr>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     David Howells <dhowells@...hat.com>, linux-cachefs@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] FS-Cache: print hexadecimal value for special cookies
 type

On Tue, 25 Apr 2017 00:47:41 +0300
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Sat, Apr 22, 2017 at 2:09 AM, Jérémy Lefaure
> <jeremy.lefaure@....epita.fr> wrote:
> > On Fri, 21 Apr 2017 22:41:54 +0300
> > Andy Shevchenko <andy.shevchenko@...il.com> wrote:  
> >> On Wed, Apr 19, 2017 at 11:38 PM, Jérémy Lefaure
> >> <jeremy.lefaure@....epita.fr> wrote:  
> >> > When building object-list.o, gcc 6 raises a warning on the sprintf call
> >> > in fscache_objlist_show:
> >> >
> >> >   CC      fs/fscache/object-list.o
> >> > fs/fscache/object-list.c: In function ‘fscache_objlist_show’:
> >> > fs/fscache/object-list.c:265:19: warning: ‘sprintf’ may write a
> >> > terminating nul past the end of the destination [-Wformat-overflow=]
> >> >     sprintf(_type, "%02u", cookie->def->type);
> >> >                    ^~~~~~
> >> > fs/fscache/object-list.c:265:4: note: ‘sprintf’ output between 3 and 4
> >> > bytes into a destination of size 3
> >> >     sprintf(_type, "%02u", cookie->def->type);
> >> >     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> >
> >> > Moreover, the documentation says that we should have an hex value for
> >> > special cookies (see Documentation/filesystems/caching/fscache.txt).
> >> >
> >> > Printing hexadecimal value for special cookies fixes the overflow
> >> > warning and complies with the documentation.  
> >>
> >> If this is used by some user space tool the safest fix is to print in
> >> BCD and fix documentation.
> >>  
> > Are you talking about the Binary Coded Decimal format ?  
> 
> Yes.
> 
> > I don't get why
> > it would be better. If we change the printed format, it would break
> > user space tool anyway, right ?  
> 
> If userspace tool is bound to the format it might go crazy about hex
> digits in the cookie.
> 
> Is that cookie important somehow? (for example, user space might
> request something based on it?) It might be a case when you got let
> say '75' and then ask for 117 (0x75), however real value is 75.
> 
Ok I get it. But changing the format can break the userland tool
anyway, can't it ? For example if a userland tool expects the value 75,
it won't read it properpy in BCD.

> So, if it's okay in this, perhaps paragraph explaining side effects in
> commit message would be good to have/
> 
Maybe the best fix would be to increase the size of the _type buffer to
4 so the unsigned byte can fit in it. And change the documentation to
decimal value. What do you think about this solution ?

Thanks,