What changed in ld?

What changed in ld?

[G. Adam Stanislav]

Hello, I posted this in the FreeBSD hackers list and was referred
to you. I was told that when I upgraded to FreeBSD 4.3, I also
upgraded binutils from 2.9.1 to 2.10.1.

I am the author of the assembly language tutorial for FreeBSD
( http://www.int80h.org/bsdasm/ ), but something has changed in ld
that causes programs assembled with nasm 0.98 and linked with ld
crash with the bus error when having uninitialized data in the
.bss section. This worked with no problems with ld from binutils
2.9.1.

My original message follows. Please advise (I am not subscribed
to this list, so please CC your advice to me). Thank you,

Adam

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>Date: Fri, 1 Jun 2001 02:42:34 -0500
>From: "G. Adam Stanislav" <adam@whizkidtech.net>
>To: hackers@FreeBSD.org
>Subject: What changed in ld?
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>I have recently upgraded from FreeBSD 3.1 to 4.3-20010525-STABLE.
>
>I wrote a very simple assembly language program that was giving me
>a bus error. For several hours I have been trying to find what was
>wrong with it, but could not.
>
>Finally, out of desperation, I moved uninitialized data from .bss
>to .data, and suddenly the program worked without a problem.
>
>This puzzled me, so I used ld on an older program of mine, one of
>the programs from my assembly language tutorial
>( http://www.int80h.org/bsdasm/ ). It started giving me the same
>bus errors. The version I created under 3.1 continues to work
>fine.
>
>That tells me something has changed in the way ld handles the .bss
>section between 3.1 and 4.3 - either on purpose or as a bug.
>This completely invalidates my assembly language tutorial. If the
>change was on purpose, can someone please tell me what new switch
>I need to use with ld to recognize the .bss section as being bss?
>
>Thank you,
>Adam
>-- 
>Perfection is for neurotics
>
>
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>
>

Re: What changed in ld?

[H . J . Lu]

On Fri, Jun 01, 2001 at 12:50:28PM -0500, G. Adam Stanislav wrote:
> Hello, I posted this in the FreeBSD hackers list and was referred
> to you. I was told that when I upgraded to FreeBSD 4.3, I also
> upgraded binutils from 2.9.1 to 2.10.1.
> 
> I am the author of the assembly language tutorial for FreeBSD
> ( http://www.int80h.org/bsdasm/ ), but something has changed in ld
> that causes programs assembled with nasm 0.98 and linked with ld

Are you using ELF? nasm usually generates bad ELF object files.


H.J.

Re: What changed in ld?

[G. Adam Stanislav]

At 10:57 01-06-2001 -0700, H . J . Lu wrote:
>On Fri, Jun 01, 2001 at 12:50:28PM -0500, G. Adam Stanislav wrote:
>> Hello, I posted this in the FreeBSD hackers list and was referred
>> to you. I was told that when I upgraded to FreeBSD 4.3, I also
>> upgraded binutils from 2.9.1 to 2.10.1.
>> 
>> I am the author of the assembly language tutorial for FreeBSD
>> ( http://www.int80h.org/bsdasm/ ), but something has changed in ld
>> that causes programs assembled with nasm 0.98 and linked with ld
>
>Are you using ELF? nasm usually generates bad ELF object files.

Yes, I'm using elf. But I did not have any problem when using the ld
from binutils 2.9.1. I was using the same version of nasm.

What can I do?

Adam

Re: What changed in ld?

[H . J . Lu]

On Fri, Jun 01, 2001 at 01:28:12PM -0500, G. Adam Stanislav wrote:
> >
> >Are you using ELF? nasm usually generates bad ELF object files.
> 
> Yes, I'm using elf. But I did not have any problem when using the ld
> from binutils 2.9.1. I was using the same version of nasm.
> 
> What can I do?

If you can provide a testcase which I can use under Linux, I will
take a look.


H.J.

Re: What changed in ld?

[Andrew Haley]

G. Adam Stanislav writes:
 > At 10:57 01-06-2001 -0700, H . J . Lu wrote:
 > >On Fri, Jun 01, 2001 at 12:50:28PM -0500, G. Adam Stanislav wrote:
 > >> Hello, I posted this in the FreeBSD hackers list and was referred
 > >> to you. I was told that when I upgraded to FreeBSD 4.3, I also
 > >> upgraded binutils from 2.9.1 to 2.10.1.
 > >> 
 > >> I am the author of the assembly language tutorial for FreeBSD
 > >> ( http://www.int80h.org/bsdasm/ ), but something has changed in ld
 > >> that causes programs assembled with nasm 0.98 and linked with ld
 > >
 > >Are you using ELF? nasm usually generates bad ELF object files.
 > 
 > Yes, I'm using elf. But I did not have any problem when using the ld
 > from binutils 2.9.1. I was using the same version of nasm.

Read the NASM page.  It says:

http://www.web-sites.co.uk/nasm/bugs.html

"Relocations between multiple code segments in ELF are wrong. NASM's
ELF files don't conform to the ELF standard properly. "

 > What can I do?

I would have thought it would be a good time to fix nasm.

Andrew.

Re: What changed in ld?

[H . J . Lu]

On Fri, Jun 01, 2001 at 07:49:21PM +0100, Andrew Haley wrote:
> 
> Read the NASM page.  It says:
> 
> http://www.web-sites.co.uk/nasm/bugs.html
> 
> "Relocations between multiple code segments in ELF are wrong. NASM's
> ELF files don't conform to the ELF standard properly. "
> 
>  > What can I do?
> 
> I would have thought it would be a good time to fix nasm.

FWIW, I have fixed all the nasm ELF bugs which I can reproduce.


H.J.

Re: What changed in ld?

[G. Adam Stanislav]

At 12:02 01-06-2001 -0700, H . J . Lu wrote:
>>  > What can I do?
>> 
>> I would have thought it would be a good time to fix nasm.
>
>FWIW, I have fixed all the nasm ELF bugs which I can reproduce.

Thank you, everybody. I found the problem and fixed it:

The messages here that mentioned there was something non-standard
in nasm reminded me there was something non-standard in my code:
Instead of using .text I was using .code. This worked fine before.
But it was non-standard. I have now changed it to .text, and suddenly
everything works fine again.

I had not thought of it until I saw your messages: So I do have you
to thank for solving this mystery.

Cheers,
Adam

Re: What changed in ld?

[Philip Blundell]

>>That tells me something has changed in the way ld handles the .bss
>>section between 3.1 and 4.3 - either on purpose or as a bug.
>>This completely invalidates my assembly language tutorial. If the
>>change was on purpose, can someone please tell me what new switch
>>I need to use with ld to recognize the .bss section as being bss?

This question is rather broad.  I think you will need to provide a testcase 
and explain what you think is wrong about the new behaviour.

p.


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Re: What changed in ld?

[G. Adam Stanislav]

At 19:24 01-06-2001 +0100, Philip Blundell wrote:
>>>That tells me something has changed in the way ld handles the .bss
>>>section between 3.1 and 4.3 - either on purpose or as a bug.
>>>This completely invalidates my assembly language tutorial. If the
>>>change was on purpose, can someone please tell me what new switch
>>>I need to use with ld to recognize the .bss section as being bss?
>
>This question is rather broad.  I think you will need to provide a testcase 
>and explain what you think is wrong about the new behaviour.

Very well. Here is some example code that performed flawlessly
before, but causes a bus error whenever the "write" procedure
is called. If I comment out the "section .bss" directive, it
works, but wastes disk space as it allocates the buffers in the
.data section instead of just reserving it in the .bss section:

%include	'system.inc'

%define	BUFSIZE	2048

section	.data
fd.in	dd
stdin
fd.out	dd	stdout

section .bss
ibuffer	resb	BUFSIZE
obuffer	resb
BUFSIZE

section	.code
[...]
read:
	push	dword BUFSIZE
	mov	esi, ibuffer

push	esi
	push	dword [fd.in]
	sys.read
	add	esp, byte 12
	mov	ebx, eax
	or
eax, eax
	je	.done
	sub	eax, eax
	ret

align 4
.done:
	call	write		; flush
output buffer
[...]
align 4
write:
	sub	edi, ecx	; start of buffer
	push
ecx
	push	edi
	push	dword [fd.out]
	sys.write
	add	esp, byte 12
	sub	eax,
eax
	sub	ecx, ecx	; buffer is empty now
	ret

Re: What changed in ld?

[Philip Blundell]

>Very well. Here is some example code that performed flawlessly
>before, but causes a bus error whenever the "write" procedure
>is called. If I comment out the "section .bss" directive, it
>works, but wastes disk space as it allocates the buffers in the
>.data section instead of just reserving it in the .bss section:

Thanks, but a test in this form is not much use to me; I don't run FreeBSD, 
and I don't have nasm.  For linker bugs you really need to provide an object 
file, the output generated by the linker, and a detailed explanation of what 
is wrong with the latter (e.g. "the destination for the CALL instruction at 
0x666 is incorrect", or "the .frobozz section is missing from the output 
file").  Simply reporting that your program crashes is not especially useful 
since that might be caused by bugs in your code, or in the assembler you are 
using, or in some library.

p.


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