Subject: Re: [PATCH v2 50/63] tracing: Use memset_startat() to zero struct trace_iterator

From: Kees Cook <[email-masked]>
Date: Wed, 18 Aug 2021 09:21:29 -0700

On Wed, Aug 18, 2021 at 09:33:49AM -0400, Steven Rostedt wrote:
> On Tue, 17 Aug 2021 23:05:20 -0700
> Kees Cook <[email-masked]> wrote:
> 
> > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > field bounds checking for memset(), avoid intentionally writing across
> > neighboring fields.
> > 
> > Use memset_startat() to avoid confusing memset() about writing beyond
> > the target struct member.
> > 
> > Cc: Steven Rostedt <[email-masked]>
> > Cc: Ingo Molnar <[email-masked]>
> > Signed-off-by: Kees Cook <[email-masked]>
> > ---
> >  kernel/trace/trace.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 13587e771567..9ff8c31975cd 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -6691,9 +6691,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
> >  		cnt = PAGE_SIZE - 1;
> >  
> >  	/* reset all but tr, trace, and overruns */
> > -	memset(&iter->seq, 0,
> > -	       sizeof(struct trace_iterator) -
> > -	       offsetof(struct trace_iterator, seq));
> > +	memset_startat(iter, 0, seq);
> 
> I can't find memset_startat() in mainline nor linux-next. I don't see it
> in this thread either, but since this has 63 patches, I could have
> easily missed it.

Sorry, it isn't called out in the Subject, but it's part of this patch:
https://lore.kernel.org/lkml/[email-masked]/

> This change really should belong to a patch set that just introduces
> memset_startat() (and perhaps memset_after()) and then updates all the
> places that should use it. That way I can give it a proper review. In
> other words, you should break this patch set up into smaller, more
> digestible portions for the reviewers.

I will split memset_after() and memset_startat() introduction patches. I
already split up each use into individual cases, so that those changes
could be checked one step at a time for differences in pahole struct
layout and object code.

Thanks for taking a look!

-Kees

-- 
Kees Cook