linux-kernel - Re: [PATCH] trace: Make trace

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Message-ID: <20170327121111.242bc7b4@gandalf.local.home>
Date:   Mon, 27 Mar 2017 12:11:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>, Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] trace: Make trace_hwlat timestamp y2038 safe

On Mon, 27 Mar 2017 17:35:13 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Mon, Mar 27, 2017 at 5:30 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Mon, 27 Mar 2017 16:53:09 +0200  
> >> We could probably introduce a %pts format string for timespec64
> >> and have that pretty-printed.  
> >
> > Hmm, probably don't want a %p as that suggests its a pointer, which it
> > should not be. Unless we pass in the address of the number.  
> 
> The special format strings that the kernel defines all start with %p and
> require passing by reference so we don't get a warning from gcc. We can't
> just make up new format strings otherwise, but we can create new meaning
> for special pointers as we do for struct resource and others.
> 

That's fine, but we need to be careful when it comes to tracing.
Passing in the address of a structure in the ring buffer may be fine,
but we need to make sure that an address pointing to something other
than the ring buffer is forbidden.

I'll need to update libtraceevent to handle such cases too.

-- Steve