follows=, vfollows= and nobits sections

Urk.

I just ran into a *very* real-life problem which shows that some of the 
assumptions we've made about nobits sections are just plain wrong.

This is an excerpt from my current SYSLINUX development tree:


; Memory below 0800h is reserved for the BIOS and the MBR
BSS_START       equ 0800h

; PXELINUX needs lots of BSS, so it relocates itself on startup
%if IS_PXELINUX
TEXT_START      equ 9000h
%else
TEXT_START      equ 7C00h
%endif

;
; The various sections and their relationship
;
                org TEXT_START

                section .earlybss nobits start=BSS_START
                section .bcopy32  align=16 follows=.data vfollows=.earlybss
                section .bss      nobits align=256 vfollows=.bcopy32

                section .text     start=TEXT_START
                section .data     align=16 follows=.text

                section .latebss  nobits align=16 follows=.bcopy32


The section .bcopy32 is relocated at runtime, and so has LMA != VMA. 
There are two different sections following it, quite literally; one 
after its VMA and one after its LMA.

Anyway, this doesn't currently work; NASM rejects the vfollows= 
statement on .bss.  I'm going to try to fix this, but I haven't groped 
around too much inside the outbin code.

A few more things: it would be a very useful things to have 
auto-generated symbols for the vstart as well as the start, and the 
total length.  Although this can be accomplished with carefully 
positioned symbols, it would be more foolproof if it didn't need it.

Also a thought for people: how hard do you guys think it would be to put 
a hack in to show the VMA in the list file when using -f bin, instead of 
the section offset?  It would definitely make debugging of 
NASM-generated code a lot easier, especially when making lots of sections.

Finally, I really think [map] needs to be command-line optionable.  It's 
really awkward for it to be specified inline.

        -hpa


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