Debugger problems

[Martijn Bos]

All,
It has been 30+ years since I played around with some assembly.
Just for learning purposes I'm doing some more playing :-)

First some information :
The system I'm on:
[***@fedora asm]$ uname -a
Linux fedora 6.2.15-100.fc36.x86_64 #1 SMP PREEMPT_DYNAMIC Thu May 11 16:51:53 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
[***@fedora asm]$

Then for completeness also the program (for this particular problem I do not think it's that relevant :-) ):
[***@fedora asm]$ cat een.asm
section .data

section .text

global _start

_start:
mov al, 1 ; mov 1 into the al register
add al, 1 ; add one to the al register

mov rax, 60 ; syscall for exit
mov rdi, 0 ; Return code
syscall
[***@fedora asm]$


Then I compile, link and run the progrm:
[***@fedora asm]$ nasm -f elf64 -gdwarf een.asm
[***@fedora asm]$ ld -m elf_x86_64 -o een een.o
[***@fedora asm]$ ./een
[***@fedora asm]$

So far so good (I think):

But I want to see what is actually happening, so I want to see it in my debugger :

[***@fedora asm]$ gdb een
GNU gdb (GDB) Fedora 12.1-2.fc36
....
....
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from een...
(gdb) list
1 section .data
2
3 section .text
4
5 global _start
6
7 _start:
8 mov al, 1 ; mov 1 into the al register
9
10 add al, 1 ; add one to the al register
(gdb) break 8
No line 8 in the current file.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb)


As you can see I can not set a breakpoint. And that's my problem.
Whitout breakpoints I can not step through my program.

Does anyone have any pointers/tips/trics on how to resolve this issue.

(I know myself...I probably oversee the obvious...do not hesitate to point that out)

Best Regards,
Martijn Bos

[Tavis Ormandy]

Post by Martijn Bos
Does anyone have any pointers/tips/trics on how to resolve this issue.
(I know myself...I probably oversee the obvious...do not hesitate to point that out)

I would just use the disassembler, for example `x/10i _start` instead of
`list _start`!

I don't think nasm will generate line info automatically (just names),
so you would set breakpoints on symbols `b _start` or addresses `b
*0x401000)

However... gas can do it if you prefer this style. Your code is
basically valid gas syntax already. Just add `.intel_syntax noprefix`
and add a `.` before directives, e.g. .section, .global, etc, and use #
for comments instead of ;.

Then you can use `as -g foo.asm -o foo.o` instead.

Now `b foo.asm:123` should work.

Nasm and gas are both great assemblers, it doesn't make much difference
which one you choose while you're getting started.

Tavis.

[Borax Man]

Post by Tavis Ormandy

Post by Martijn Bos
Does anyone have any pointers/tips/trics on how to resolve this issue.
(I know myself...I probably oversee the obvious...do not hesitate to point that out)

I would just use the disassembler, for example `x/10i _start` instead of
`list _start`!
I don't think nasm will generate line info automatically (just names),
so you would set breakpoints on symbols `b _start` or addresses `b
*0x401000)
However... gas can do it if you prefer this style. Your code is
basically valid gas syntax already. Just add `.intel_syntax noprefix`
and add a `.` before directives, e.g. .section, .global, etc, and use #
for comments instead of ;.
Then you can use `as -g foo.asm -o foo.o` instead.
Now `b foo.asm:123` should work.
Nasm and gas are both great assemblers, it doesn't make much difference
which one you choose while you're getting started.
Tavis.

NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

ie.
nasm -f elf32 -F dwarf hello.asm


P.S., I assume you're the same Tavis who is known for your FVWM
config?

[Martijn Bos]

Hi,

Thanks for taking the tmie to answer.
See my remarks inline

Best regards,
Martijn

Post by Borax Man

Post by Tavis Ormandy

Post by Martijn Bos
Does anyone have any pointers/tips/trics on how to resolve this issue.
(I know myself...I probably oversee the obvious...do not hesitate to point that out)

I would just use the disassembler, for example `x/10i _start` instead of
`list _start`!
I don't think nasm will generate line info automatically (just names),
so you would set breakpoints on symbols `b _start` or addresses `b
*0x401000)
However... gas can do it if you prefer this style. Your code is
basically valid gas syntax already. Just add `.intel_syntax noprefix`
and add a `.` before directives, e.g. .section, .global, etc, and use #
for comments instead of ;.
Then you can use `as -g foo.asm -o foo.o` instead.
Now `b foo.asm:123` should work.
Nasm and gas are both great assemblers, it doesn't make much difference
which one you choose while you're getting started.
Tavis.

NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.
ie.
nasm -f elf32 -F dwarf hello.asm

From 'man nasm' I quote:
==quote==
-F format
Specifies the debug information format. To see a list of valid output formats, use the -y option (for example -felf -y).

-g
Causes nasm to generate debug information.

-gformat
Equivalent to -g -F format.>
==unquote==

When I tried your suggestion I got the same result:
(gdb) break 8
No line 8 in the current file.

So i tried -gdwarf

Same result.

Maybe I should also give the linker some options to handle debug information (?)


Anyway....I'm off for familiy matters.

I'll try further tomorrow

Post by Borax Man
P.S., I assume you're the same Tavis who is known for your FVWM
config?

[Tavis Ormandy]

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.

As far as I know nasm won't do that?

Post by Borax Man
P.S., I assume you're the same Tavis who is known for your FVWM
config?

Hah, yes, that was me :)

Tavis.

[Borax Man]

Post by Tavis Ormandy

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.
As far as I know nasm won't do that?

I did a test using nasm with the '-F dwarf' and '-g' parameters, and I
was able to add breakpoints in GDB specifying the line using the
syntax you mentioned.

I didn't mention "-g" before, oops.

Post by Tavis Ormandy

Post by Borax Man
P.S., I assume you're the same Tavis who is known for your FVWM
config?

Hah, yes, that was me :)
Tavis.

Cool. Avid and longtime FVWM user here.

[Tavis Ormandy]

Post by Borax Man

Post by Tavis Ormandy

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.
As far as I know nasm won't do that?

I did a test using nasm with the '-F dwarf' and '-g' parameters, and I
was able to add breakpoints in GDB specifying the line using the
syntax you mentioned.

Weird.... and this is for elf64?

Perhaps you have a newer version than me, good to know it will arrive eventually!

Tavis.

[Borax Man]

On 3 Dec 2023 16:43:39 GMT

Post by Tavis Ormandy

Post by Borax Man

Post by Tavis Ormandy

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.
As far as I know nasm won't do that?

I did a test using nasm with the '-F dwarf' and '-g' parameters, and I
was able to add breakpoints in GDB specifying the line using the
syntax you mentioned.

Weird.... and this is for elf64?
Perhaps you have a newer version than me, good to know it will arrive eventually!
Tavis.

I tested this on Debian 12... 32 bit (my laptop is old).

[Borax Man]

Post by Borax Man
On 3 Dec 2023 16:43:39 GMT

Post by Tavis Ormandy

Post by Borax Man

Post by Tavis Ormandy

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.
As far as I know nasm won't do that?

I did a test using nasm with the '-F dwarf' and '-g' parameters, and I
was able to add breakpoints in GDB specifying the line using the
syntax you mentioned.

Weird.... and this is for elf64?
Perhaps you have a newer version than me, good to know it will arrive eventually!
Tavis.

I tested this on Debian 12... 32 bit (my laptop is old).

Just to clarify, NASM version 2.16.01.

[Martijn Bos]

All,

Reporting back was taking a little longer then I expexted ...sorry.

Anyway.
I still cannot get the debugger to break on a linenumber. If I read correctly you had more luck (or wisdom).

However the method mentioned earlier : x/10i _start does work!

Did really helps me, I was not aware of x being able to display instructions.
Then I was able to set a breakpoint on the adress that I need/want.

And now I can start debugging.
Stepping through the program with "s" was running through the program immediately, so I found that I had to use "si" (Step Instruction)

It still seams/seems silly to me that I can list a piece of a program in gdb, but can not set a break on the kines I think trhe debugger knows:

(gdb) list
1 section .data
2
3 section .text
4
5 global _start
6
7 _start:
8 mov al, 1 ; mov 1 into the al register
9
10 mov rax, 60 ; syscall for exit
(gdb) b 8
No line 8 in the current file.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb)

Anyway....I can dust off some more old assembly knowledge now.


Thanks all for you help and time.

Best Regards,
Martijn

Post by Borax Man

Post by Borax Man
On 3 Dec 2023 16:43:39 GMT

Post by Tavis Ormandy

Post by Borax Man

Post by Tavis Ormandy

Post by Borax Man
NASM can generate debugging information which I believe will generate
line info. Pass '-F dwarf' option to nasm if under Linux, and you should see each
line of source in the debugger.

You're thinking of line markers I think (like %line 123), but I think
Martijn wants the assembler to do that automatically so that b
foo.asm:123 works, like it does in gas.
As far as I know nasm won't do that?

I did a test using nasm with the '-F dwarf' and '-g' parameters, and I
was able to add breakpoints in GDB specifying the line using the
syntax you mentioned.

Weird.... and this is for elf64?
Perhaps you have a newer version than me, good to know it will arrive eventually!
Tavis.

I tested this on Debian 12... 32 bit (my laptop is old).

Just to clarify, NASM version 2.16.01.

[Martijn Bos]

Thank you so much for this information.

Today I have to visit my parents (my father turned 83 :-))
So...no testing today.

I'll report back hopefully tomorrow.

Best Regards,
Martijn

Post by Tavis Ormandy

Post by Martijn Bos
Does anyone have any pointers/tips/trics on how to resolve this issue.
(I know myself...I probably oversee the obvious...do not hesitate to point that out)

I would just use the disassembler, for example `x/10i _start` instead of
`list _start`!
I don't think nasm will generate line info automatically (just names),
so you would set breakpoints on symbols `b _start` or addresses `b
*0x401000)
However... gas can do it if you prefer this style. Your code is
basically valid gas syntax already. Just add `.intel_syntax noprefix`
and add a `.` before directives, e.g. .section, .global, etc, and use #
for comments instead of ;.
Then you can use `as -g foo.asm -o foo.o` instead.
Now `b foo.asm:123` should work.
Nasm and gas are both great assemblers, it doesn't make much difference
which one you choose while you're getting started.
Tavis.
--
_o) $ lynx lock.cmpxchg8b.com