Re: Relative expressions and ASSERT

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Dec 16, 2010 at 4:06 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Dec 16, 2010 at 4:01 PM, Alan Modra <amodra@gmail.com> wrote:
>> 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.
>>
>
> I am putting this into the Linux binutils.
>
>
> --
> H.J.
> ---
> ld/
>
> 2010-12-16 ?H.J. Lu ?<hongjiu.lu@intel.com>
>
> ? ? ? ?PR ld/12327
> ? ? ? ?* ldexp.c (exp_fold_tree_1): Only make absolute symbol value
> ? ? ? ?absolute.
>
> ld/testsuite/
>
> 2010-12-16 ?H.J. Lu ?<hongjiu.lu@intel.com>
>
> ? ? ? ?PR ld/12327
> ? ? ? ?* ld-scripts/defined.exp: Run defined4.
>
> ? ? ? ?* ld-scripts/defined4.d: New.
> ? ? ? ?* ld-scripts/defined4.s: Likewise.
> ? ? ? ?* ld-scripts/defined4.t: Likewise.
>

The updated patch with a new testcase.

-- 
H.J.
---
ld/

2010-12-17  H.J. Lu  <hongjiu.lu@intel.com>

	PR ld/12327
	* ldexp.c (exp_fold_tree_1): Only make absolute symbol value
	absolute.

ld/testsuite/

2010-12-17  H.J. Lu  <hongjiu.lu@intel.com>

	PR ld/12327
	* ld-scripts/defined4.d: New.
	* ld-scripts/defined4.s: Likewise.
	* ld-scripts/defined4.t: Likewise.
	* ld-scripts/expr2.d: Likewise.
	* ld-scripts/expr2.s: Likewise.
	* ld-scripts/expr2.t: Likewise.

	* ld-scripts/defined.exp: Run defined4.

	* ld-scripts/expr.exp: Run expr2.

Attachment: binutils-pr12327-2.patch
Description: Text document