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SubjectRe: [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@st.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@st.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
>

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