Can I debug busybox program running on a qemu virtual machine using gdb?

Can I debug busybox program running on a qemu virtual machine using gdb?

[Chan Kim]

Hi all,

I'm trying to track how tftp program runs when I run tftp provided by
busybox on qemu virtual machine.
I can debug the linux source but I cannot debug tftp (busybox) program. 
Can I debug busybox program using gdb in this case?
Any hint or advice will be very appreciated.
Thank you!

Chan Kim





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Re: Can I debug busybox program running on a qemu virtual machine using gdb?

[Richard]

Hi,

If the goal is only to understand tftp and it's not something hardware 
dependend I would debug it on my normal desktop system/notebook. My 
distro (debian) has a busybox package. It does not provide the symlinks 
to change the ARGV[0] but you can create them manually and then debug 
locally as usually with your prefered tool (emacs recommended :) ).

If it has to be in quemu, I see no reason against it, the normal rules 
for remote debugging apply.

-- Richard Sailer

On 07/07/2022 11:27, Chan Kim wrote:
> Hi all,
> 
> I'm trying to track how tftp program runs when I run tftp provided by
> busybox on qemu virtual machine.
> I can debug the linux source but I cannot debug tftp (busybox) program.
> Can I debug busybox program using gdb in this case?
> Any hint or advice will be very appreciated.
> Thank you!
> 
> Chan Kim
> 
> 
> 
> 
> 
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

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RE: Can I debug busybox program running on a qemu virtual machine using gdb?

[Chan Kim]

Hi, 

Thanks for replying.
Actually, I just wanted to know what kernel config variables are needed to
make tftp work in vanilla linux.
I get the message "tftp: sendto: Network is unreachable". 
With another default config, I can use tftp on qemu machine but still the
kernel image size is big so I tried to reduce the size and reduce the
experiment cycle time. But now I know I can build my test platform driver
into a kernel module I can do away with it. Maybe I'll just try this
"finding minimal config for tftp " later.
(I though because the shell itself is the busybox, maybe I could find where
the program is loaded and set breakpoint in the tftp entrance or condition,
but couldn't find where busybox was loaded.)

Thank you.

Chan Kim

>-----Original Message-----
>From: Richard <richard_siegfried@systemli.org>
>Sent: Friday, July 8, 2022 8:46 AM
>To: kernelnewbies@kernelnewbies.org
>Subject: Re: Can I debug busybox program running on a qemu virtual machine
>using gdb?
>
>Hi,
>
>If the goal is only to understand tftp and it's not something hardware
>dependend I would debug it on my normal desktop system/notebook. My distro
>(debian) has a busybox package. It does not provide the symlinks to change
>the ARGV[0] but you can create them manually and then debug locally as
>usually with your prefered tool (emacs recommended :) ).
>
>If it has to be in quemu, I see no reason against it, the normal rules for
>remote debugging apply.
>
>-- Richard Sailer
>
>On 07/07/2022 11:27, Chan Kim wrote:
>> Hi all,
>>
>> I'm trying to track how tftp program runs when I run tftp provided by
>> busybox on qemu virtual machine.
>> I can debug the linux source but I cannot debug tftp (busybox) program.
>> Can I debug busybox program using gdb in this case?
>> Any hint or advice will be very appreciated.
>> Thank you!
>>
>> Chan Kim
>>
>>
>>
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies@kernelnewbies.org
>> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>_______________________________________________
>Kernelnewbies mailing list
>Kernelnewbies@kernelnewbies.org
>https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies





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Re: Can I debug busybox program running on a qemu virtual machine using gdb?

[Valdis Klētnieks]

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On Fri, 08 Jul 2022 10:11:53 +0900, "Chan Kim" said:

> I get the message "tftp: sendto: Network is unreachable".

Unless you did something *really* drastic, like figure out how to build
the kernel without IPv4 support (is that even possible yet?), or leave out
support for whatever virtual network device qemu is providing to the
emulated kernel, you're probably just missing an 'ifconfig' and/or a 'route add'
to tell the kernel where the bits need to go.

Less likely, you did something silly like tell the kernel to enforce SELinux
or AppArmor or whatever, but you fed it a policy file that doesn't allow tftp
to have network access....

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Re: Can I debug busybox program running on a qemu virtual machine using gdb?

[Tom Mitchell]

On Thu, Jul 7, 2022 at 2:27 AM Chan Kim <ckim@etri.re.kr> wrote:
>
> Hi all,
>
> I'm trying to track how tftp program runs when I run tftp provided by
> busybox on qemu virtual machine.
> I can debug the linux source but I cannot debug tftp (busybox) program.
> Can I debug busybox program using gdb in this case?
> Any hint or advice will be very appreciated.
> Thank you!

Check the tftp options for verbose flags.   The granularity of gdb is finer
than most connection problems.

tftp in busybox may be a subset of stand alone tftp.
fire it up and see what "help" and other commands tell you.

As always tftp has two parts.
Near and far.    Both the server and the client have verbos
the server may log authentication or other errors.

It would help to run standalone tftp on a laptop and also install busybox in
a scratch location to see if there are differences you care about.
As a security risk tftp is full of troubles so firewalls and switches
can drop traffic.
gdb is too fine.   If you are building it add classic print statements
to the verbose mode.
Two programs, two firewalls, is the network local or wild west big
with multiple hops and firewalls.

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Re: Can I debug busybox program running on a qemu virtual machine using gdb?

[Richard]

On 08/07/2022 10:36, Tom Mitchell wrote:
> On Thu, Jul 7, 2022 at 2:27 AM Chan Kim <ckim@etri.re.kr> wrote:
>>
>> Hi all,
>>
>> I'm trying to track how tftp program runs when I run tftp provided by
>> busybox on qemu virtual machine.
>> I can debug the linux source but I cannot debug tftp (busybox) program.
>> Can I debug busybox program using gdb in this case?
>> Any hint or advice will be very appreciated.
>> Thank you!
> 
> Check the tftp options for verbose flags.   The granularity of gdb is finer
> than most connection problems.
> 
> tftp in busybox may be a subset of stand alone tftp.
> fire it up and see what "help" and other commands tell you.
> 
> As always tftp has two parts.
> Near and far.    Both the server and the client have verbos
> the server may log authentication or other errors.
> 
> It would help to run standalone tftp on a laptop and also install busybox in
> a scratch location to see if there are differences you care about.
> As a security risk tftp is full of troubles so firewalls and switches
> can drop traffic.
> gdb is too fine.   If you are building it add classic print statements
> to the verbose mode.
> Two programs, two firewalls, is the network local or wild west big
> with multiple hops and firewalls.


I agree, that's all pretty good advice


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