Re: [PATCH bpf] ftrace: fix incorrect hash size in register_ftrace_direct()

[Menglong Dong]

On Sat, Apr 12, 2025 at 10:32 PM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> On Sat, Apr 12, 2025 at 10:09 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Sat, 12 Apr 2025 21:33:48 +0800
> > Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > > The max ftrace hash bits is made fls(32) in register_ftrace_direct(),
> > > which seems illogical, and it seems to be a spelling mistake.
> > >
> > > Just fix it.
> > >
> > > (Do I misunderstand something?)
> >
> > I think the logic is incorrect and this patch doesn't fix it.
> >
> > >
> > > Fixes: d05cb470663a ("ftrace: Fix modification of direct_function hash while in use")
> > > Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> > > ---
> > >  kernel/trace/ftrace.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > > index 1a48aedb5255..7697605a41e6 100644
> > > --- a/kernel/trace/ftrace.c
> > > +++ b/kernel/trace/ftrace.c
> > > @@ -5914,7 +5914,7 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > >
> > >       /* Make a copy hash to place the new and the old entries in */
> > >       size = hash->count + direct_functions->count;
> > > -     if (size > 32)
> > > +     if (size < 32)
> > >               size = 32;
> > >       new_hash = alloc_ftrace_hash(fls(size));
> > >       if (!new_hash)
> >
> > The above probably should be:
> >
> >         size = hash->count + direct_functions->count
> >         size = fls(size);
> >         if (size > FTRACE_HASH_MAX_BITS)
> >                 size = FTRACE_HASH_MAX_BITS;
> >         new_hash = alloc_ftrace_hash(size);
>
> Yeah, this seems to make more sense. And I'll send a V2
> later.
>
> BTW, Should we still keep the "size = min(size, 32)" logic

Oops, I mean "size =  max(size, 32); size = fls(size);" here :/

> to avoid the hash bits being too small, just like the origin
> logic in "dup_hash"?
>
> Thanks!
> Menglong Dong
>
> >
> > -- Steve
> >
> >