netdev - Re: [PATCH iproute2 v2] json_print: Fix hidden 64-bit type promotion

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Date:   Wed, 25 Apr 2018 08:12:10 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Toke Høiland-Jørgensen <toke@...e.dk>
Cc:     netdev@...r.kernel.org,
        Kevin Darbyshire-Bryant <ldir@...byshire-bryant.me.uk>
Subject: Re: [PATCH iproute2 v2] json_print: Fix hidden 64-bit type
 promotion

On Wed, 25 Apr 2018 16:57:52 +0200
Toke Høiland-Jørgensen <toke@...e.dk> wrote:

> Stephen Hemminger <stephen@...workplumber.org> writes:
> 
> > On Wed, 25 Apr 2018 16:30:22 +0200
> > Toke Høiland-Jørgensen <toke@...e.dk> wrote:
> >  
> >> print_uint() will silently promote its variable type to uint64_t, but there
> >> is nothing that ensures that the format string specifier passed along with
> >> it fits (and the function name suggest to pass "%u").
> >> 
> >> Fix this by changing print_uint() to use a native 'unsigned int' type, and
> >> introduce a separate print_u64() function for printing 64-bit values. All
> >> call sites that were actually printing 64-bit values using print_uint() are
> >> converted to use print_u64() instead.
> >> 
> >> Since print_int() was already using native int types, just add a
> >> print_s64() to match, but don't convert any call sites.
> >> 
> >> Cc: Kevin Darbyshire-Bryant <ldir@...byshire-bryant.me.uk>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...e.dk>  
> >
> > Yes, this makes sense. Maybe there should be a print_luint for
> > consistency.  
> 
> I just realised I missed a few call sites, so I'll resend. Should I
> call the new function print_luint() instead of print_u64()?

Ideally, there would be both functions, and use based on what is being printed.

> > Also, I tried (in vain) to make a version that allows GCC to check the
> > format string.  But it was a struggle and just gave up.  
> 
> Yeah, no idea how to do this either...

Maybe some magic smatch or multi-line regex it would be possible to
find all instances of print_uint, then look at format string of each and see if there is
a single %u.  Some added complexity since some places only print json and don't care
and pass NULL for format.