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

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Date:   Wed, 4 Dec 2019 09:21:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Antonio Borneo <antonio.borneo@...com>
Cc:     Ingo Molnar <mingo@...hat.com>, Joel Fernandes <joelaf@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>
Subject: Re: [PATCH] tracing: Fix printing ptrs in preempt/irq
 enable/disable events


Joel,

Any comments on this patch?

-- 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