linux-kernel - Re: [PATCH] RDMA/siw: Fix compiler warnings on 32-bit due to u64/pointer abuse

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Date:   Mon, 19 Aug 2019 19:14:59 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Bernard Metzler <bmt@...ich.ibm.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RDMA/siw: Fix compiler warnings on 32-bit due to
 u64/pointer abuse

Hi Joe,

On Mon, Aug 19, 2019 at 6:56 PM Joe Perches <joe@...ches.com> wrote:
> On Mon, 2019-08-19 at 12:05 +0200, Geert Uytterhoeven wrote:
> > When compiling on 32-bit:
> >
> >     drivers/infiniband/sw/siw/siw_cq.c:76:20: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
> >     drivers/infiniband/sw/siw/siw_qp.c:952:28: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
> []
> > Fix this by applying the following rules:
> >   1. When printing a u64, the %llx format specififer should be used,
> >      instead of casting to a pointer, and printing the latter.
> >   2. When assigning a pointer to a u64, the pointer should be cast to
> >      uintptr_t, not u64,
> >   3. When casting from u64 to pointer, an intermediate cast to uintptr_t
> >      should be added,
>
> I think a cast to unsigned long is rather more common.
>
> uintptr_t is used ~1300 times in the kernel.
> I believe a cast to unsigned long is much more common.

That is true, as uintptr_t was introduced in C99.
Similarly, unsigned long was used before size_t became common.

However, nowadays size_t and uintptr_t are preferred.

> It might be useful to add something to the Documentation
> for this style.  Documentation/process/coding-style.rst
>
> And trivia:
>
> > > diff --git a/drivers/infiniband/sw/siw/siw_verbs.c b/drivers/infiniband/sw/siw/siw_verbs.c
> []
> > @@ -842,8 +842,8 @@ int siw_post_send(struct ib_qp *base_qp, const struct ib_send_wr *wr,
> >                       rv = -EINVAL;
> >                       break;
> >               }
> > -             siw_dbg_qp(qp, "opcode %d, flags 0x%x, wr_id 0x%p\n",
> > -                        sqe->opcode, sqe->flags, (void *)sqe->id);
> > +             siw_dbg_qp(qp, "opcode %d, flags 0x%x, wr_id 0x%llx\n",
> > +                        sqe->opcode, sqe->flags, sqe->id);
>
> Printing possible pointers as %llx is generally not a good idea
> given the desire for %p obfuscation.

Are they pointers? Difficult to know with all the casting...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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