Debugging gdb under gdb

Debugging gdb under gdb

[Michael Eager]

Hi --

When I try to run a test gdb under gdb, the host gdb hangs.

The hang occurs when the test gdb is executing pex_unix_exec_child().
As soon as the test gdb executes fork(), the host gdb hangs.
I've tried all of the permutations of follow-fork-mode
and detach-on-fork, with no luck.  The host neither follows
the parent nor the child processes.

The host gdb is "GNU gdb Fedora (6.8-32.fc10)".

Suggestions?

-- 
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077

Re: Debugging gdb under gdb

[Michael Snyder]

Michael Eager wrote:
> Hi --
> 
> When I try to run a test gdb under gdb, the host gdb hangs.
> 
> The hang occurs when the test gdb is executing pex_unix_exec_child().
> As soon as the test gdb executes fork(), the host gdb hangs.
> I've tried all of the permutations of follow-fork-mode
> and detach-on-fork, with no luck.  The host neither follows
> the parent nor the child processes.
> 
> The host gdb is "GNU gdb Fedora (6.8-32.fc10)".
> 
> Suggestions?

Are you stepping or continuing when this occurs?

There is a point, somewhere around where gdb is forking the child,
where stepping always hangs for me.  I don't remember exactly where.
I work around it by setting a breakpoint somewhere just after, and
continuing.

It's been a while since I've done this... but see if that helps.

Michael

Re: Debugging gdb under gdb

[Michael Eager]

Michael Snyder wrote:
> Michael Eager wrote:
>> Hi --
>>
>> When I try to run a test gdb under gdb, the host gdb hangs.
>>
>> The hang occurs when the test gdb is executing pex_unix_exec_child().
>> As soon as the test gdb executes fork(), the host gdb hangs.
>> I've tried all of the permutations of follow-fork-mode
>> and detach-on-fork, with no luck.  The host neither follows
>> the parent nor the child processes.
>>
>> The host gdb is "GNU gdb Fedora (6.8-32.fc10)".
>>
>> Suggestions?
> 
> Are you stepping or continuing when this occurs?
> 
> There is a point, somewhere around where gdb is forking the child,
> where stepping always hangs for me.  I don't remember exactly where.
> I work around it by setting a breakpoint somewhere just after, and
> continuing.

I've tried both stepping and continuing.  I set breakpoints in
the code shared between parent & child (where it checks the pid)
and also on both paths.  None of the breakpoints are hit.

The host gdb also hangs when there are no breakpoints set and
I just run the test gdb.

> It's been a while since I've done this... but see if that helps.

The call to pex_unix_exec_child() is in find_charset_names().
My workaround is to have the host gdb force a return from
find_charset_names() in the test gdb, so it never tries to
do a fork().

-- 
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077

Re: Debugging gdb under gdb

[Jan Kratochvil]

On Thu, 09 Jul 2009 01:52:27 +0200, Michael Eager wrote:
> When I try to run a test gdb under gdb, the host gdb hangs.
...
> The host gdb is "GNU gdb Fedora (6.8-32.fc10)".

From the GDB version assuming you did hit a Fedora kernel problem tracked at:
	https://bugzilla.redhat.com/show_bug.cgi?id=486564

It is fixed in Fedora-11 kernels, Fedora-10 will get a backport - or may
already got one).  From my experience Fedora-11 kernels also work fine with
Fedora-10 OS (but be sure to have /usr/bin/rpm from the latest Fedora-10
update before installing any Fedora-11 package)


Sorry,
Jan

Re: Debugging gdb under gdb

[Michael Eager]

Jan Kratochvil wrote:
> On Thu, 09 Jul 2009 01:52:27 +0200, Michael Eager wrote:
>> When I try to run a test gdb under gdb, the host gdb hangs.
> ...
>> The host gdb is "GNU gdb Fedora (6.8-32.fc10)".
> 
>>From the GDB version assuming you did hit a Fedora kernel problem tracked at:
> 	https://bugzilla.redhat.com/show_bug.cgi?id=486564
> 
> It is fixed in Fedora-11 kernels, Fedora-10 will get a backport - or may
> already got one).  From my experience Fedora-11 kernels also work fine with
> Fedora-10 OS (but be sure to have /usr/bin/rpm from the latest Fedora-10
> update before installing any Fedora-11 package)

Thanks for the info.

I'll hope for a kernel-2.6.30 backport to F10, or maybe I'll build it myself.
I'm still recovering from the trauma of upgrading from Fedora 8.

-- 
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077