netdev - Re: [PATCH bpf-next] tools: bpftool: add a command to dump the trace pipe

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Message-ID: <20181206193638.vdktw23hqzlfi7ld@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 6 Dec 2018 11:36:40 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Quentin Monnet <quentin.monnet@...ronome.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next] tools: bpftool: add a command to dump the trace
 pipe

On Thu, Dec 06, 2018 at 05:20:54PM +0000, Quentin Monnet wrote:
> 2018-12-05 19:18 UTC-0800 ~ Alexei Starovoitov
> <alexei.starovoitov@...il.com>
> > On Wed, Dec 05, 2018 at 06:15:23PM +0000, Quentin Monnet wrote:
> >>>> +
> >>>> +	/* Allow room for NULL terminating byte and pipe file name */
> >>>> +	snprintf(format, sizeof(format), "%%*s %%%zds %%99s %%*s %%*d %%*d\\n",
> >>>> +		 PATH_MAX - strlen(pipe_name) - 1);
> >>>
> >>> before scanning trace_pipe could you add a check that trace_options are compatible?
> >>> Otherwise there will be a lot of garbage printed.
> >>> afaik default is rarely changed, so the patch is ok as-is.
> >>> The followup some time in the future would be perfect.
> >>
> >> Sure. What do you mean exactly by compatible options? I can check that
> >> "trace_printk" is set, is there any other option that would be relevant?
> > 
> > See Documentation/trace/ftrace.rst
> > a lot of the flags will change the format significantly.
> > Like 'bin' will make it binary.
> > I'm not suggesting to support all possible output formats.
> > Only to check that trace flags match scanf.
> 
> fscanf() is only used to retrieve the name of the sysfs directory where
> the pipe is located, when listing all the mount points on the system. It
> is not used to dump the content from the pipe (which is done with
> getline(), so formatting does not matter much).
> 
> If the "bin" option is set, "bpftool prog tracelog" will dump the same
> binary content as "cat /sys/kernel/debug/tracing/trace_pipe", which is
> the expected behaviour (at least with the current patch). Let me know if
> you would like me to change this somehow.

I misread the patch :) thanks for explaining. all good then.