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

[Martin Kaiser]

Thus wrote Masami Hiramatsu (mhiramat@xxxxxxxxxx):

> On Mon, 23 Mar 2026 10:48:18 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> > On Mon, 23 Mar 2026 11:19:36 +0100
> > Martin Kaiser <martin@xxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> > > Signed-off-by: Martin Kaiser <martin@xxxxxxxxx>
> > > ---
> > >  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.

Best regards,
Martin

> Thanks,


> > -- Steve



> > >  	/* If any exit_handler is set, data must be used. */
> > >  	return used != 0;



> -- 
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>