bpf_debug output

bpf_debug output

[Ethy H. Brito]

Hi! 

I am working on some minor modifications in xdp-cpumap-tc to accommodate NAT at WAN side.
But the things are not working as expected.

How can bpf_debug be used to help debugging?

I read that its output ends-up in /sys/kernel/debug/tracing/trace_pipe.
But how can it be read? 
I tried "cat trace_pipe" but this produced no output whatsoever.

Any hints will be appreciated.

Thanx

Regards

Ethy

Re: bpf_debug output

[Ivan Koveshnikov]

Hi Ethy,

Correct. The output is available via file
`/sys/kernel/debug/tracing/trace_pipe`. There are other helper
programs that can also read it, like `bpftool prog tracelog` or `tc
exec bpf dbg`.

But you need to pay attention, what you're trying to print, since
`bpf_printk()` supports much fewer formats than usual `printk()`. I.e.
only  `%d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %s`
conversion specifiers are allowed. If an unsupported specifier is
spotted, the string is silently dropped and never appears in trace
log.

Here you can find some examples:
https://github.com/xdp-project/xdp-tutorial/tree/master/tracing03-xdp-debug-print


Best regards,
Ivan Koveshnikov

On Wed, 14 Jul 2021 at 08:08, Ethy H. Brito <ethy.brito@inexo.com.br> wrote:
>
>
> Hi!
>
> I am working on some minor modifications in xdp-cpumap-tc to accommodate NAT at WAN side.
> But the things are not working as expected.
>
> How can bpf_debug be used to help debugging?
>
> I read that its output ends-up in /sys/kernel/debug/tracing/trace_pipe.
> But how can it be read?
> I tried "cat trace_pipe" but this produced no output whatsoever.
>
> Any hints will be appreciated.
>
> Thanx
>
> Regards
>
> Ethy

Re: bpf_debug output

[Ethy H. Brito]

On Wed, 14 Jul 2021 22:06:48 +0500
Ivan Koveshnikov <ikoveshnik@gmail.com> wrote:

> Hi Ethy,

Hi Ivan (and others behind the bushes)

> 
> Correct. The output is available via file
> `/sys/kernel/debug/tracing/trace_pipe`. There are other helper
> programs that can also read it, like `bpftool prog tracelog` or `tc
> exec bpf dbg`.
> 
> But you need to pay attention, what you're trying to print, since
> `bpf_printk()` supports much fewer formats than usual `printk()`. I.e.
> only  `%d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %s`
> conversion specifiers are allowed. If an unsupported specifier is
> spotted, the string is silently dropped and never appears in trace
> log.

Very useful info. 
I couldn't see my logs just because of this: unsupported format.
Now it is OK.

> 
> Here you can find some examples:
> https://github.com/xdp-project/xdp-tutorial/tree/master/tracing03-xdp-debug-print

Thanx

Regards

Ethy



> 
> 
> Best regards,
> Ivan Koveshnikov
> 
> On Wed, 14 Jul 2021 at 08:08, Ethy H. Brito <ethy.brito@inexo.com.br> wrote:
> >
> >
> > Hi!
> >
> > I am working on some minor modifications in xdp-cpumap-tc to accommodate NAT at WAN side.
> > But the things are not working as expected.
> >
> > How can bpf_debug be used to help debugging?
> >
> > I read that its output ends-up in /sys/kernel/debug/tracing/trace_pipe.
> > But how can it be read?
> > I tried "cat trace_pipe" but this produced no output whatsoever.
> >
> > Any hints will be appreciated.
> >
> > Thanx
> >
> > Regards
> >
> > Ethy  


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