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

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

On Thu, Nov 09, 2017 at 07:21:04PM -0800, Darrick J. Wong wrote:
> 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?

Ah, dchinner points out on irc that gcc knows we (>> 9) the numbers
before printing them so the maximum value is 36028797018963967, so
40 bytes works.

gcc: 1, maintainer: 0

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

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