Re: Relative expressions and ASSERT

[Alan Modra <amodra at gmail dot com>]

On Thu, Dec 16, 2010 at 03:14:38PM -0800, H.J. Lu wrote:
> Your patch doesn't work. I got
> 
> ./ld: kernel image bigger than KERNEL_IMAGE_SIZE
> 
> due to arch/x86/kernel/vmlinux.lds:
> 
> ----
> /*
>  * Build-time check on the image size:
>  */
> . = ASSERT((_end - _text <= (512 * 1024 * 1024)),
>     "kernel image bigger than KERNEL_IMAGE_SIZE");

Extracting the relevant parts from the kernel link script:

SECTIONS
{
  . = 0xc000;
  .text :
  {
    _text = .;
    *(.text)
  }
  _end = .;
}
ASSERT (_end - _text <= 0x100, "fail");

You'd think that under the new rules, this ought to work.  "_end" is
an absolute address, so when evaluating "_end - _text", "_text" ought
to be converted from relative to absolute, the subtraction resuling in
an absolute address.  Then 0x100 gets converted from a number to
absolute address before being compared.

The trouble is that "_end" is *not* treated as an absolute address but
as a plain number.  That makes ld convert "_end" to a .text relative
address when evaluation "_end - _text", which doesn't cause a problem
at that stage, but the result is a relative address.  That means 0x100
is converted to a .text relative address too, which gives the wrong
result.

The underlying problem is that we don't distinguish between plain
numbers and absolute addresses once values are stored in symbols.

-- 
Alan Modra
Australia Development Lab, IBM