Re: XFS tracepoint warning due to NULL string

[Brian Foster <bfoster@redhat.com>]

On Wed, Mar 23, 2022 at 10:02:00AM -0400, Steven Rostedt wrote:
> On Wed, 23 Mar 2022 08:47:10 -0400
> Brian Foster <bfoster@redhat.com> wrote:
> 
> > What's the best way to address this going forward with the memory usage
> > verification in place? ISTM we could perhaps consider dropping the
> > custom %.*s thing in favor of using %s with __string_len() and friends,
> > or perhaps just replace the open-coded NULL parameter with the "(null)"
> > string that the trace subsystem code seems to use on NULL pointer
> > checks. The latter seems pretty simple and straightforward of a change
> > to me, but I want to make sure I'm not missing something more obvious.
> > Thoughts?
> 
> Can you see if the following (totally untested) patch fixes your issue?
> 

Yup, that restores historical behavior. Thanks!

Brian

> -- Steve
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 08ea781540b5..f4de111fa18f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3673,12 +3673,17 @@ static char *trace_iter_expand_format(struct trace_iterator *iter)
>  }
>  
>  /* Returns true if the string is safe to dereference from an event */
> -static bool trace_safe_str(struct trace_iterator *iter, const char *str)
> +static bool trace_safe_str(struct trace_iterator *iter, const char *str,
> +			   bool star, int len)
>  {
>  	unsigned long addr = (unsigned long)str;
>  	struct trace_event *trace_event;
>  	struct trace_event_call *event;
>  
> +	/* Ignore strings with no length */
> +	if (star && !len)
> +		return true;
> +
>  	/* OK if part of the event data */
>  	if ((addr >= (unsigned long)iter->ent) &&
>  	    (addr < (unsigned long)iter->ent + iter->ent_size))
> @@ -3864,7 +3869,7 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
>  		 * instead. See samples/trace_events/trace-events-sample.h
>  		 * for reference.
>  		 */
> -		if (WARN_ONCE(!trace_safe_str(iter, str),
> +		if (WARN_ONCE(!trace_safe_str(iter, str, star, len),
>  			      "fmt: '%s' current_buffer: '%s'",
>  			      fmt, show_buffer(&iter->seq))) {
>  			int ret;
>