Re: [PATCH] xfs_io: fix gcc-7 related printf warnings

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 09, 2017 at 07:43:59PM -0600, Eric Sandeen wrote:
> On 11/9/17 7:39 PM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > New compiler, new checks, new warnings.
> > 
> > Fix the new [-Wformat-truncation=] warnings that io/fsmap.c is
> > throwing w/ gcc-7.2 because "%lld..%lld" requires a buffer 40
> > characters long, not 32.
> > 
> > Signed-Off-By: Dave Chinner <dchinner@redhat.com>
> 
> 2^63 is 19 chars, yup.
> 
> Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
> 
> > ---
> >  io/fsmap.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/io/fsmap.c b/io/fsmap.c
> > index 448fb5356466..e46fdbaa148c 100644
> > --- a/io/fsmap.c
> > +++ b/io/fsmap.c
> > @@ -184,8 +184,8 @@ dump_map_verbose(
> >  	off64_t			agoff, bperag;
> >  	int			foff_w, boff_w, aoff_w, tot_w, agno_w, own_w;
> >  	int			nr_w, dev_w;
> > -	char			rbuf[32], bbuf[32], abuf[32], obuf[32];
> > -	char			nbuf[32], dbuf[32], gbuf[32];
> > +	char			rbuf[40], bbuf[40], abuf[40], obuf[40];
> > +	char			nbuf[40], dbuf[40], gbuf[40];

Waitaminute.  The longest format string is bbuf, which gets
"[%lld..%lld]:".  The longest number %lld can print is
"-9223372036854775807" which requires 20 bytes.  Therefore, bbuf needs
1 + 20 + 2 + 20 + 2 + 1 == 46 bytes, not 40.  rbuf and abuf have similar
problems, though I guess a 48-byte string will work for all.

Granted, it's snprintf so it'll just truncate the second number if it
has to, but seeing as this came from a gcc warning:

"fsmap.c:228:3: note: ‘snprintf’ output between 8 and 40 bytes into a
destination of size 32"

So am I just bad at math, or is gcc wrong here?

--D

> >  	char			owner[OWNER_BUF_SZ];
> >  	int			sunit, swidth;
> >  	int			flg = 0;
> > 
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