Dump is a terrible word. Does this mean discard or list? I'd suggest replacing the ambiguous term with whichever this is.

I think it might be a bit clearer to rewrite this as a switch on the what. I think it's then more obvious that your MPASS is really an '_assert_unreachable()'. Looking at this even further though, I think I'd really like to just split out this early section into a helper function:

static int
kenv_dump(char **envp, int what, char *value, int len)
{
   char *senv;

   senv = **envp;
   ....
}

And then in sys_kenv() you add this to the start of the switch statement below:

    switch (uap->what) {
    case KENV_DUMP:
        return (kenv_dump(kenvp, uap->what, uap->value, uap->len));
    case KENV_DUMP_LOADER:
    case KENV_DUMP_STATIC:
#ifdef PRESERVE_EARLY_KENV
        return (kenv_dump(uap->what == KENV_DUMP_LOADER ? &md_envp : &kern_envp,
            uap->what, uap->value, uap->len);
#else
        return (ENOENT);
#endif
`