netdev - Re: [PATCH bpf-next v2 2/3] bpf: btf: add btf print functionality

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Message-ID: <20180703233350.aa33jyzrc6hmaelq@kafai-mbp.dhcp.thefacebook.com>
Date:   Tue, 3 Jul 2018 16:33:50 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
CC:     Okash Khawaja <osk@...com>, Daniel Borkmann <daniel@...earbox.net>,
        "Alexei Starovoitov" <ast@...nel.org>, Yonghong Song <yhs@...com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        "David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/3] bpf: btf: add btf print functionality

On Tue, Jul 03, 2018 at 03:38:43PM -0700, Jakub Kicinski wrote:
> On Tue, 3 Jul 2018 15:23:31 -0700, Jakub Kicinski wrote:
> > > > > +			else
> > > > > +				jsonw_printf(jw, "%hhd", *((char *)data));    
> > > > 
> > > > ... I think you need to always print a string, and express it as
> > > > \u00%02hhx for non-printable.    
> > > Okay that makes sense  
> > 
> > Yeah, IDK, char can be used as a byte as well as a string.  In eBPF
> > it may actually be more likely to just be used as a raw byte buffer...
> 
> Actually, what is the definition/purpose of BTF_INT_CHAR?  There seems
> to be no BTF_INT_SHORT and BTF_INT_SIGNED can simply be of size 8...
> Is normal int only used for bitfields of size 8 and BTF_INT_CHAR for
> char variables?
> 
> The kernel seems to be rejecting combinations of those flags, is
> unsigned char going to not be marked as char then?
BTF_INT_ENOCODING (CHAR/SIGNED/BOOL) is for formatting (e.g. pretty
print).  It is mainly how CTF is using it also.  Hence, BTF_INT_ENCODINGs
is not a 1:1 mapping to C integer types.
The size of an interger is described by BTF_INT_BITS instead.  

> 
> > Either way I think it may be nice to keep it consistent, at least for
> > the JSON output could we do either always ints or always characters?
>