linux-kernel - Re: [PATCH v3 3/3] tracing: Add ptr-hash option to show the hashed pointer value

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Message-ID: <20210212103310.791c1f48@gandalf.local.home>
Date:   Fri, 12 Feb 2021 10:33:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3 3/3] tracing: Add ptr-hash option to show the hashed
 pointer value


Hi Masami,

I noticed theses sitting in my patchwork and I said I was going to hold off
to the next merge window, and these got pushed down in my stack :-/


On Thu, 15 Oct 2020 23:55:25 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Add tracefs/options/hash-ptr option to show hashed pointer
> value by %p in event printk format string.
> 
> For the security reason, normal printk will show the hashed
> pointer value (encrypted by random number) with %p to printk
> buffer to hide the real address. But the tracefs/trace always
> shows real address for debug. To bridge those outputs, add an
> option to switch the output format. Ftrace users can use it
> to find the hashed value corresponding to the real address
> in trace log.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  Documentation/trace/ftrace.rst |    6 ++++++
>  kernel/trace/trace.c           |    3 +++
>  kernel/trace/trace.h           |    1 +
>  3 files changed, 10 insertions(+)
> 
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index 87cf5c010d5d..62c98e9bbdd9 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -1159,6 +1159,12 @@ Here are the available options:
>  	This simulates the original behavior of the trace file.
>  	When the file is closed, tracing will be enabled again.
>  
> +  hash-ptr
> +        When set, "%p" in the event printk format displays the
> +        hashed pointer value instead of real address.
> +        This will be useful if you want to find out which hashed
> +        value is corresponding to the real value in trace log.
> +

I'm thinking of making this the default. I'll add a patch to make it
enabled by default "for security reasons", but still allow people to clear
it this value.

Are you OK with that?

-- Steve



>    record-cmd
>  	When any event or tracer is enabled, a hook is enabled
>  	in the sched_switch trace point to fill comm cache
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 75395293d8df..b88cccf224cd 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3543,6 +3543,9 @@ const char *trace_event_format(struct trace_iterator *iter, const char *fmt)
>  	if (WARN_ON_ONCE(!fmt))
>  		return fmt;
>  
> +	if (iter->tr->trace_flags & TRACE_ITER_HASH_PTR)
> +		return fmt;
> +
>  	p = fmt;
>  	new_fmt = q = iter->fmt;
>  	while (*p) {
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 524502d1f60a..c34187bd22a9 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1347,6 +1347,7 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
>  		C(MARKERS,		"markers"),		\
>  		C(EVENT_FORK,		"event-fork"),		\
>  		C(PAUSE_ON_TRACE,	"pause-on-trace"),	\
> +		C(HASH_PTR,		"hash-ptr"),	/* Print hashed pointer */ \
>  		FUNCTION_FLAGS					\
>  		FGRAPH_FLAGS					\
>  		STACK_FLAGS					\