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

[Martin KaFai Lau <kafai@fb.com>]

On Fri, Jun 22, 2018 at 04:32:00PM -0700, Jakub Kicinski wrote:
> On Fri, 22 Jun 2018 15:54:08 -0700, Martin KaFai Lau wrote:
> > > > > > > > > > >         "value": ["0x02","0x00","0x00","0x00","0x00","0x00","0x00","0x00"
> > > > > > > > > > >         ],
> > > > > > > > > > > 	"value_struct":{
> > > > > > > > > > > 		"src_ip":2,      
> > > > > > If for the same map the user changes the "src_ip" to an array of int[4]
> > > > > > later (e.g. to support ipv6), it will become "src_ip": [1, 2, 3, 4].
> > > > > > Is it breaking backward compat?
> > > > > > i.e.
> > > > > > struct five_tuples {
> > > > > > -	int src_ip;
> > > > > > +	int src_ip[4];
> > > > > > /* ... */
> > > > > > };    
> > > > > 
> > > > > Well, it is breaking backward compat, but it's the program doing it,
> > > > > not bpftool :)  BTF changes so does the output.    
> > > > As we see, the key/value's btf-output is inherently not backward compat.
> > > > Hence, "-j" and "-p" will stay as is.  The whole existing json will
> > > > be backward compat instead of only partly backward compat.  
> > > 
> > > No.  There is a difference between user of a facility changing their
> > > input and kernel/libraries providing different output in response to
> > > that, and the libraries suddenly changing the output on their own.
> > > 
> > > Your example is like saying if user started using IPv6 addresses
> > > instead of IPv4 the netlink attributes in dumps will be different so
> > > kernel didn't keep backwards compat.  While what you're doing is more
> > > equivalent to dropping support for old ioctl interfaces because there
> > > is a better mechanism now.  
> > Sorry, I don't follow this.  I don't see netlink suffer json issue like
> > the one on "key" and "value".
> > 
> > All I can grasp is, the json should normally be backward compat but now
> > we are saying anything added by btf-output is an exception because
> > the script parsing it will treat it differently than "key" and "value"
> 
> Backward compatibility means that if I run *the same* program against
> different kernels/libraries it continues to work.  If someone decides
> to upgrade their program to work with IPv6 (which was your example)
> obviously there is no way system as a whole will look 1:1 the same.
> 
> > > BTF in JSON is very useful, and will help people who writes simple
> > > orchestration/scripts based on bpftool *a* *lot*.  I really appreciate  
> > Can you share what the script will do?  I want to understand why
> > it cannot directly use the BTF format and the map data.
> 
> Think about a python script which wants to read a counter in a map.
> Right now it would have to get the BTF, find out which bytes are the
> counter, then convert the bytes into a larger int.  With JSON BTF if
> just does entry["formatted"]["value"]["counter"].
> 
> Real life example from my test code (conversion of 3 element counter
> array):
> 
> def str2int(strtab):
>     inttab = []
>     for i in strtab:
>         inttab.append(int(i, 16))
>     ba = bytearray(inttab)
>     if len(strtab) == 4:
>         fmt = "I"
>     elif len(strtab) == 8:
>         fmt = "Q"
>     else:
>         raise Exception("String array of len %d can't be unpacked to an int" %
>                         (len(strtab)))
>     return struct.unpack(fmt, ba)[0]
> 
> def convert(elems, idx):
>     val = []
>     for i in range(3):
>         part = elems[idx]["value"][i * length:(i + 1) * length]
>         val.append(str2int(part))
>     return val
> 
> With BTF it would be:
> 
> 	elems[idx]["formatted"]["value"]
> 
> Which is fairly awesome.
Thanks for the example.  Agree that with BTF, things are easier in general.

btw, what more awesome is,
#> bpftool map find id 100 key 1
{
	"counter_x": 1,
	"counter_y": 10
}

> 
> > > this addition to bpftool and will start using it myself as soon as it
> > > lands.  I'm not sure why the reluctance to slightly change the output
> > > format?  
> > The initial change argument is because the json has to be backward compat.
> > 
> > Then we show that btf-output is inherently not backward compat, so
> > printing it in json does not make sense at all.
> > 
> > However, now it is saying part of it does not have to be backward compat.
> 
> Compatibility of "formatted" member is defined as -> fields broken out
> according to BTF.  So it is backward compatible.  The definition of
> "value" member is -> an array of unfortunately formatted array of
> ugly hex strings :(
> 
> > I am fine putting it under "formatted" for "-j" or "-p" if that is the
> > case, other than the double output is still confusing.  Lets wait for
> > Okash's input.
> >
> > At the same time, the same output will be used as the default plaintext
> > output when BTF is available.  Then the plaintext BTF output
> > will not be limited by the json restrictions when we want
> > to improve human readability later.  Apparently, the
> > improvements on plaintext will not be always applicable
> > to json output.
>