Re: Potential static analysis ideas

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Arnd,

On Mon, Jul 26, 2021 at 10:21 AM Arnd Bergmann <arnd@arndb.de> wrote:
> On Mon, Jul 26, 2021 at 9:53 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Mon, Jul 26, 2021 at 9:26 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Sun, Jul 25, 2021 at 1:45 AM NeilBrown <neilb@suse.de> wrote:
> > > > On Sun, 25 Jul 2021, Laurent Pinchart wrote:
> > > > > > To make it work well, you need to know if frob() and/or the current
> > > > > > function return an error code or not.  While you can use some heuristics
> > > > > > (e.g. is there any return -Exxx), perhaps we can add an annotation to
> > > > > > indicate if a function returns an error code, or an error pointer?
> > > > >
> > > > > https://lore.kernel.org/linux-media/YNMvarFl%2FKU1pGCG@pendragon.ideasonboard.com/
> > > > >
> > > > > I think it would be useful, if not for the tools, at least for
> > > > > developers.
> > > >
> > > > Agreed.  I added some code to smatch so that I could annotate pointers to
> > > > say if they are allowed to be NULL.  The implementation isn't perfect,
> > > > but I love having that extra documentation about when I do or don't have
> > > > to check for NULL.
> > >
> > > I can think of four different annotations that limit what a pointer return from
> > > a function can be:
> > >
> > > a) either a valid pointer or NULL, but never an error pointer,
> > > b) either a valid pointer or an error pointer, but not NULL,
> > > c) always a valid pointer, never NULL or an error,
> > > d) always NULL, but callers are expected to check for error pointers.
> >
> > e) either a valid pointer, NULL, or an error pointer
> >
> > The last pattern is seen with the various *get*_optional() functions.
>
> I would always consider those the exact bug that I meant with "because
> everyone gets those wrong". I think the idea of the "optional" functions is
> that you have two implementations b) and d) and pick one of them
> at compile time. To the caller this means either an error pointer or
> success, but checking for NULL is a bug in the caller, while conditionally
> returning NULL or ERR_PTR() would be a bug in the interface.

e) is not for optional kernel features, but for optional hardware
functionality.  So it is not known at compile time, but depends on
e.g. the DTB.  See e.g. devm_clk_get_optional().

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds