linux-kernel - Re: [PATCH] tracing: Fix printing ptrs in preempt/irq enable/disable events

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Message-ID: <20191221234741.GA116648@google.com>
Date:   Sat, 21 Dec 2019 18:47:41 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Antonio Borneo <antonio.borneo@...com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH] tracing: Fix printing ptrs in preempt/irq enable/disable
 events

On Wed, Dec 04, 2019 at 09:21:15AM -0500, Steven Rostedt wrote:
> 
> Joel,
> 
> Any comments on this patch?

Steve, it looks like this issue happens with trace-cmd not knowing what
_stext is. If I do cat trace_pipe , then I don't see the issue as _stext is
looked up correctly but the reporter of the bug is using trace-cmd. Is there
a way to solve this within trace-cmd? Not knowing much about trace-cmd
internals, I will have to defer to you on this though..

Other than this, I need to make the offset to _stext as s32 instead of u32
type so that the problem of the symbol location being before _stext does not
cause overflow.

Lastly, I am not super convinced that we need to store the full pointer just
to handle a case where the offset of the symbol might be more than +-2G from
_stext. Once we see such issue, then we can handle it. But right now the size
of the trace buffer is utilized better by just storing the offset IMHO.

thanks,

 - Joel


> 
> -- Steve
> 
> On Wed, 27 Nov 2019 16:44:28 +0100
> Antonio Borneo <antonio.borneo@...com> wrote:
> 
> > This tracing event class is the only instance in kernel that logs
> > in the trace buffer the instruction pointer as offset to _stext,
> > instead of logging the full pointer.
> > This looks like a nice optimization for 64 bits platforms, where a
> > 32 bit offset can take less space than a full 64 bits pointer. But
> > the symbol _stext is incorrectly resolved as zero in the expansion
> > of TP_printk(), which then prints only the hex offset instead of
> > the name of the caller function. Plus, on arm arch the kernel
> > modules are loaded at address lower than _stext, causing the u32
> > offset arithmetics to overflow and wrap at 32 bits.
> > I did not identified a 64 bit arch where the modules are loaded at
> > offset from _stext that exceed u32 range, but I also did not
> > identified any constraint to feel safe with a u32 offset.
> > 
> > Log directly the instruction pointer instead of the offset to
> > _stext.
> > 
> > Signed-off-by: Antonio Borneo <antonio.borneo@...com>
> > Fixes: d59158162e03 ("tracing: Add support for preempt and irq enable/disable events")
> > ---
> >  include/trace/events/preemptirq.h | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/trace/events/preemptirq.h b/include/trace/events/preemptirq.h
> > index 95fba0471e5b..d548a6aafa18 100644
> > --- a/include/trace/events/preemptirq.h
> > +++ b/include/trace/events/preemptirq.h
> > @@ -18,18 +18,18 @@ DECLARE_EVENT_CLASS(preemptirq_template,
> >  	TP_ARGS(ip, parent_ip),
> >  
> >  	TP_STRUCT__entry(
> > -		__field(u32, caller_offs)
> > -		__field(u32, parent_offs)
> > +		__field(unsigned long, caller_ip)
> > +		__field(unsigned long, parent_ip)
> >  	),
> >  
> >  	TP_fast_assign(
> > -		__entry->caller_offs = (u32)(ip - (unsigned long)_stext);
> > -		__entry->parent_offs = (u32)(parent_ip - (unsigned long)_stext);
> > +		__entry->caller_ip = ip;
> > +		__entry->parent_ip = parent_ip;
> >  	),
> >  
> >  	TP_printk("caller=%pS parent=%pS",
> > -		  (void *)((unsigned long)(_stext) + __entry->caller_offs),
> > -		  (void *)((unsigned long)(_stext) + __entry->parent_offs))
> > +		  (void *)__entry->caller_ip,
> > +		  (void *)__entry->parent_ip)
> >  );
> >  
> >  #ifdef CONFIG_TRACE_IRQFLAGS
>