kmodule: absolute symbols

[Maxime Villard <max%m00nbsd.net@localhost>]

Throwing this here in case someone cares. In the kernel module relocator we
don't support absolute symbols coming from the kernel, and it means that we
can't modload a module containing for example:

	extern char sigcode[];
	printf("sigcode = %p\n", sigcode);

Currently, absolute symbols work like this:

    +-----------+    +---------+    +---------+
    | sigcode   |    | sigcode |    | sigcode |
    | SHN_UNDEF |    | SHBSS   |<~~~| SHN_ABS |
    | Val = 0   |<---| Val = 0 |    | Val = 0 |
    +-----------+    +---------+    +---------+
       MODULE       KERNEL (MEM)    KERNEL (BIN)

The kernel binary has a symbol entry for "sigcode" with st_shndx = SHN_ABS.
When loaded in memory, st_shndx gets overwritten with SHBSS. Then, when
modloading a kernel module, only the st_value field gets copied in the
module version of the symbol.

But for "sigcode" st_value evaluates to zero. Therefore kobj_sym_lookup
returns a zero st_value, which is interpreted as an error in the kobj_reloc
functions.

To fix that:
 * Don't overwrite SHN_ABS with SHBSS.
 * Copy the st_shndx field into the module symbol.
 * Change the return value of kobj_sym_lookup to be errno-like, giving the
   address as argument.
 * Still in kobj_sym_lookup, if st_shndx == SHN_ABS, take the value as-is and
   don't return an error if it equals zero.

Patch here [1]. Unless told otherwise I will commit this soon - fixing the
other kobj_reloc functions along the way, obviously. There are also a few other
things to be fixed but I'll do that later.

Maxime

(note: we've always had this issue, so what, no one has ever tried to
modload COMPAT_16 on amd64?)

[1] http://m00nbsd.net/garbage/module/abs.diff