Re: [PATCH] tracing: fprobe: fix the length of unused fgraph_data

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 24 Mar 2026 09:05:12 +0100
Martin Kaiser <martin@kaiser.cx> wrote:

> Thus wrote Masami Hiramatsu (mhiramat@kernel.org):
> 
> > On Mon, 23 Mar 2026 10:48:18 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > > On Mon, 23 Mar 2026 11:19:36 +0100
> > > Martin Kaiser <martin@kaiser.cx> wrote:
> 
> > > > If fprobe_entry does not fill the allocated fgraph_data completely, the
> > > > unused part is zeroed with memset.
> 
> > > > Fix the length for this memset call. Both reserved_words and used are in
> > > > units of return stack words, but memset needs the number of bytes.
> 
> > > > Cc: stable@vger.kernel.org
> > > > Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> > > > Signed-off-by: Martin Kaiser <martin@kaiser.cx>
> > > > ---
> > > >  kernel/trace/fprobe.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> > > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > > > index dcadf1d23b8a..6a1192515afd 100644
> > > > --- a/kernel/trace/fprobe.c
> > > > +++ b/kernel/trace/fprobe.c
> > > > @@ -451,7 +451,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
> > > >  		}
> > > >  	}
> > > >  	if (used < reserved_words)
> > > > -		memset(fgraph_data + used, 0, reserved_words - used);
> > > > +		memset(fgraph_data + used, 0, (reserved_words - used) * sizeof(long));
> 
> > > So fgraph_data is only used internally between the fprobe_fgraph_entry()
> > > and fprobe_return() as it only exists on the fgraph shadow stack. I'm not
> > > even sure if the unused portion needs to be zeroed out.
> 
> > > Thus, this may be correct, but it doesn't look like a true bug that needs a
> > > stable tag.
> 
> > Hmm, indeed. Maybe we'd better just remove this memset from for-next.
> 
> Ok, I see your point. I'll send a v2 that removes the memset.

Yeah, thanks!

> 
> Best regards,
> Martin
> 
> > Thanks,
> 
> 
> > > -- Steve
> 
> 
> 
> > > >  	/* If any exit_handler is set, data must be used. */
> > > >  	return used != 0;
> 
> 
> 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>