[DTrace-devel] [PATCH 2/2] Support two representations for embedded IPv4 addresses

[Kris Van Hees]

On Fri, Sep 15, 2023 at 05:30:43PM -0400, Eugene Loh via DTrace-devel wrote:
> Yipes.  Once again, I didn't get the email in my inbox.  Yick.
> 
> Anyhow, the big question is whether we are changing the interface.  The
> commit message says the patch is introducing an undocumented arg.  Well, the
> test suite has a number of tests that check that not too many args are being
> passed to a function.  (Not every function is so tested, but arguably that's
> a short-coming of the test suite rather than an intentional policy of
> allowing too many args for all those untested functions.)  So, it seems to
> me that either we are changing the inet_ntoa6() interface (and should
> document that) or we are breaking a test (that we to date have declined to
> write).  Is it legal for a D user to write inet_ntop(af, addr, 0)?  How
> about inet_ntop(af, addr, 0, 0)?  How should D handle those two cases?  Or
> are we using an optional arg that a D user will not be able to use?

We are not changing the interface in a substantial way.  We are extending it
a tiny bit by allowing inet_ntoa6(addr, int).  If int is 0, the regular
inet_ntoa6() behaviour results (no :: prefix) whereas if int is not 0, then
we yield a result with :: prefix.

So, this is a completely backwards compatible change, and if anyone wants to
use the new feature, they certainly can.

I have no problem documenting it, but I also do not see that there is any
issue if this remains undocumented until documentation gets updated for other
things as well.

I also do not mind adding tests that ensure that only two arguments can be
supplied, and that the proper behaviour results from not supplying this 2nd
argument, or from supplying 0 vs non-0 values.

I do not see any reason why we would prevent users from using this, nor is
that really something we can easily do.

> Incidentally,
> 
> +.Lipv4_2:
> +       /* Output IPv4 address prefixed by ::ffff: (if is_ntop is set). */
> +       ldxw    %r0, [%fp + -8]         /* restore is_ntop */
> +       jeq     %r0, 0, .Lipv4
> +       stb     [%r9 + 0], ':'
> +       stb     [%r9 + 1], ':'
> +       add     %r9, 2
> +       mov     %r1, -1
> +       mov     %r2, %r9
> +       call    write_hex16
> +       add     %r9, %r0
> +       stb     [%r9 + 0], ':'
> +       add     %r9, 1
> +       ja      .Lipv4
> +
>  .Lipv4:
> 
> No need for the ja since you fall through to that label anyhow. Also,
> instead of the "add %r9,..." instructions and everything in-between, how
> about simply

True, good point.

> stb [%r9+2], 'f'
> stb [%r9+3], 'f'
> stb [%r9+4], 'f'
> stb [%r9+5], 'f'
> stb [%r9+6], ':'
> add %r9, 7
> 
> Fewer instructions, easier to read.

Also on this.

> Maybe this weird difference between ntoa6 and ntop is just not worth solving
> (until we run out of other things to do)?

That becomes a bit irrelevant given that it is already implemented...