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

[Jakub Kicinski <jakub.kicinski@netronome.com>]

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.

> > 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.