gdb questions

gdb questions

[Andrew Zimmerman]

I am considering the use of gdb on a project and have some questions. The
project consists of porting to Linux an application written in assembly
language for a 32-bit DOS extender environment.

1. If gdb is used with assembly language what kind of debugging symbols are
required? I'd like to use NASM, but it doesn't seem to be able to add
symbols.

2. Does gdb have a way to examine the LDT (local descriptor table)? My
application creates some additional selectors for segments that reside
within the flat memory space allocated to the application. With the Phar Lap
Dos Extender we could easily display a list of the segments. And yes, we
were in 32-bit protected mode, way back in 1989.

Thank you.

Andrew Zimmerman

Re: gdb questions

[Eli Zaretskii]

> From: "Andrew Zimmerman" <andrew@zimmerman.net>
> Date: Wed, 18 Jul 2001 13:54:07 -0500
> 
> 1. If gdb is used with assembly language what kind of debugging symbols are
> required? I'd like to use NASM, but it doesn't seem to be able to add
> symbols.

If you use Gas, the GNU assembler, you can invoke it with -gstabs+ or
-gdwarf-2 switches, and get stabs or DWARF2 debug info.

> 2. Does gdb have a way to examine the LDT (local descriptor table)? My
> application creates some additional selectors for segments that reside
> within the flat memory space allocated to the application.

Do you intend to port that code to Linux?  I don't know much about the
protection scheme employed by Linux, but from what I know, I'd doubt
very much that it will allow you to futz with the LDT.

> With the Phar Lap
> Dos Extender we could easily display a list of the segments. And yes, we
> were in 32-bit protected mode, way back in 1989.

That's funny: I've just committed a few days ago a bunch of changes to
the DJGPP port of GDB which introduce commands to display LDT, GDT,
and other system-related info.  You can now say "info dos ldt $cs" and
see the LDT entry for the debuggee's CS selector, including base
address, limit, and the access right bits.

But these command only work in GDB when it is compiled for native
debugging of DJGPP programs.  DJGPP programs are 32-bit protected-mode
programs which can run on DOS and all versions of Windows as DPMI
clients.

Re: gdb questions

[Andrew Zimmerman]

----- Original Message ----- 
From: "Eli Zaretskii" <eliz@is.elta.co.il>
To: <andrew@zimmerman.net>
Cc: <gdb@sources.redhat.com>
Sent: Wednesday, July 18, 2001 3:16 PM
Subject: Re: gdb questions


> > 2. Does gdb have a way to examine the LDT (local descriptor table)? My
> > application creates some additional selectors for segments that reside
> > within the flat memory space allocated to the application.
> 
> Do you intend to port that code to Linux?  I don't know much about the
> protection scheme employed by Linux, but from what I know, I'd doubt
> very much that it will allow you to futz with the LDT.
> 
What is the purpose of the MODIFY_LDT system call?

Re: gdb questions

[Eli Zaretskii]

On Wed, 18 Jul 2001, Andrew Zimmerman wrote:

> > > 2. Does gdb have a way to examine the LDT (local descriptor table)? My
> > > application creates some additional selectors for segments that reside
> > > within the flat memory space allocated to the application.
> > 
> > Do you intend to port that code to Linux?  I don't know much about the
> > protection scheme employed by Linux, but from what I know, I'd doubt
> > very much that it will allow you to futz with the LDT.
> > 
> What is the purpose of the MODIFY_LDT system call?

I have no idea.  The main issue is: can an application issue that
system call without triggering a GPF?

Re: gdb questions

[Jamie Guinan]

On Wed, 18 Jul 2001, Eli Zaretskii wrote:

> > From: "Andrew Zimmerman" <andrew@zimmerman.net>
> > Date: Wed, 18 Jul 2001 13:54:07 -0500
> >
> > 1. If gdb is used with assembly language what kind of debugging symbols are
> > required? I'd like to use NASM, but it doesn't seem to be able to add
> > symbols.
>
> If you use Gas, the GNU assembler, you can invoke it with -gstabs+ or
> -gdwarf-2 switches, and get stabs or DWARF2 debug info.

My Gas [1] only supports: -gstabs, -gdwarf, and -gdwarf2.  gcc accepts
-gstabs+ and -gdwarf-2, but does not appear to pass them down to the
assembler (aside from using -Wa).

But -gstabs (and -gdwarf{,2}) do indeed work with assembly source,

  $ as -gstabs foo.S -o foo.o
  $ gcc foo.o -o foo
  $ gdb foo
  (gdb) list foo_func
  ...

-Jamie

[1] stock Red Hat 7.1, gas 2.10.91.  I just note this because I don't know
if latest CVS Gas is any different.


> > 2. Does gdb have a way to examine the LDT (local descriptor table)? My
> > application creates some additional selectors for segments that reside
> > within the flat memory space allocated to the application.
>
> Do you intend to port that code to Linux?  I don't know much about the
> protection scheme employed by Linux, but from what I know, I'd doubt
> very much that it will allow you to futz with the LDT.
>
> > With the Phar Lap
> > Dos Extender we could easily display a list of the segments. And yes, we
> > were in 32-bit protected mode, way back in 1989.
>
> That's funny: I've just committed a few days ago a bunch of changes to
> the DJGPP port of GDB which introduce commands to display LDT, GDT,
> and other system-related info.  You can now say "info dos ldt $cs" and
> see the LDT entry for the debuggee's CS selector, including base
> address, limit, and the access right bits.
>
> But these command only work in GDB when it is compiled for native
> debugging of DJGPP programs.  DJGPP programs are 32-bit protected-mode
> programs which can run on DOS and all versions of Windows as DPMI
> clients.
>

-- 
================================================================
Jamie Guinan                         Blue Button Solutions, Inc.
guinan@bluebutton.com                 http://www.bluebutton.com/
================================================================

Re: gdb questions

[Eli Zaretskii]

On Wed, 18 Jul 2001, Jamie Guinan wrote:

> > If you use Gas, the GNU assembler, you can invoke it with -gstabs+ or
> > -gdwarf-2 switches, and get stabs or DWARF2 debug info.
> 
> My Gas [1] only supports: -gstabs, -gdwarf, and -gdwarf2.  gcc accepts
> -gstabs+ and -gdwarf-2

Sorry, I should have checked that.

But this sounds like something that's worth reporting to the Binutils
maintainers: users shouldn't be required to remember two sets of
similar but subtly different options.

gdb questions

[Mickael Gicquaire]

Hi all,
I have a few enquiries concerning GDB.

It seems that there is no way at the moment to see what commands are being
issued to gdb while sourcing a file using the source command. I am right?
I think it could be useful in some instances.

Another thing is why is it not possible to issue mi commands inside a file that
is being sourced or why is there not an equivalent command to the source
command in the mi set of commands ?

cheers mickael

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Re: gdb questions

[Fernando Nasser]

Mickael Gicquaire wrote:
> 
> Hi all,
> I have a few enquiries concerning GDB.
> 
> It seems that there is no way at the moment to see what commands are being
> issued to gdb while sourcing a file using the source command. I am right?
> I think it could be useful in some instances.
> 

Sure.  We just need a volunteer to submit a patch.  

However, this will require that a switch is added to the "source"
command syntax
and that will lead us to a long discussion on the syntax of switches
("\" or "-").

> Another thing is why is it not possible to issue mi commands inside a file that
> is being sourced or why is there not an equivalent command to the source
> command in the mi set of commands ?
> 

IMO, that is conceptually wrong.  The MI is not another command line
syntax -- it is
a machine protocol between two programs.  It was chosen to be in ASCII
and readable
for easy of debugging and implementation.

The idea is that the MI source command reads and executes commands (from
a file) that
are in the normal GDB command line (cli) script language.  It was not
implemented yet
because GDB is slowly being modified to allow switching the format of
commands on the
fly.  This requires adding parameters to functions and passing them down
so it takes
some time to do.


Thanks for the comments and suggestions.

-- 
Fernando Nasser
Red Hat Canada Ltd.                     E-Mail:  fnasser@redhat.com
2323 Yonge Street, Suite #300
Toronto, Ontario   M4P 2C9