Re: [PATCH v2] seq_file: fix clang warning for NULL pointer arithmetic

[Arnd Bergmann]

On Tue, Oct 27, 2020 at 8:22 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> > index f277d023ebcd..eafeb8bf4fe4 100644
> > --- a/fs/kernfs/file.c
> > +++ b/fs/kernfs/file.c
> > @@ -121,10 +121,10 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
> >               return next;
> >       } else {
> >               /*
> > -              * The same behavior and code as single_open().  Returns
> > -              * !NULL if pos is at the beginning; otherwise, NULL.
> > +              * The same behavior and code as single_open().  Continues
> > +              * if pos is at the beginning; otherwise, EOF.
> >                */
> > -             return NULL + !*ppos;
> > +             return *ppos ? SEQ_OPEN_SINGLE : SEQ_OPEN_EOF;
>
> Why the somewhat obsfucating unary expression instead of a good
> old if?
>
> e.g.
>
>                 return next;
>         }
>         if (*ppos)
>                 retun SEQ_OPEN_SINGLE;
>         return NULL;
>
>
> >               ++*ppos;
> > -             return NULL;
> > +             return SEQ_OPEN_EOF;
>
> I don't think SEQ_OPEN_EOF is all that useful.  NULL is the documented
> end case already.

Right, Al already pointed out the same thing on IRC.

> > diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
> > index 813614d4b71f..26f0758b6551 100644
> > --- a/include/linux/seq_file.h
> > +++ b/include/linux/seq_file.h
> > @@ -37,6 +37,9 @@ struct seq_operations {
> >
> >  #define SEQ_SKIP 1
> >
> > +#define SEQ_OPEN_EOF (void *)0
> > +#define SEQ_OPEN_SINGLE      (void *)1
>
> I think SEQ_OPEN_SINGLE also wants a comment documenting it.
> AFAICS the reason for it is that ->start needs to return something
> non-NULL for the seq_file code to make progress, and there is nothing
> better for the single_open case.

Ok.

     Arnd