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

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Date:   Fri, 21 Apr 2017 19:09:29 -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 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 ? I don't get why
it would be better. If we change the printed format, it would break
user space tool anyway, right ?

> > Signed-off-by: Jérémy Lefaure <jeremy.lefaure@....epita.fr>
> > ---
> >  fs/fscache/object-list.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/fscache/object-list.c b/fs/fscache/object-list.c
> > index 67f940892ef8..d51303124889 100644
> > --- a/fs/fscache/object-list.c
> > +++ b/fs/fscache/object-list.c
> > @@ -262,7 +262,7 @@ static int fscache_objlist_show(struct seq_file *m, void *v)
> >                         type = "DT";
> >                         break;
> >                 default:
> > -                       sprintf(_type, "%02u", cookie->def->type);
> > +                       sprintf(_type, "%02x", cookie->def->type);
> >                         type = _type;
> >                         break;
> >                 }
> > --
> > 2.12.2
> >  
> 
> 
>