linux-kernel - Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

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Message-ID: <CAGETcx930eOFOLjf2zjY5RLvSNnyyESLRv+8M7s=cP8EnMDBLg@mail.gmail.com>
Date:   Tue, 10 Mar 2020 11:20:59 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

On Tue, Mar 10, 2020 at 9:16 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> + Saravana, who I spoke to briefly about this.
>
>
> On Tue, Mar 10, 2020 at 8:32 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Tue, Mar 10, 2020 at 8:31 PM David Laight <David.Laight@...lab.com> wrote:
> > >
> > > From: Nathan Chancellor
> > > > Sent: 10 March 2020 01:26
> > > ...
> > > > Sure, I can send v2 to do that but I think that sending 97 patches just
> > > > casting the small values (usually less than twenty) to unsigned long
> > > > then to the enum is rather frivolous. I audited at least ten to fifteen
> > > > of these call sites when creating the clang patch and they are all
> > > > basically false positives.
> > >
> > > Such casts just make the code hard to read.
> > > If misused casts can hide horrid bugs.
> > > IMHO sprinkling the code with casts just to remove
> > > compiler warnings will bite back one day.
> > >
> >
> > I agree that too much casts make the code hard to read,
> > but irrespective of this patch, there is no difference
> > in the fact that we need a cast to convert
> > (const void *) to a non-pointer value.
> >
> > The difference is whether we use
> > (uintptr_t) or (enum foo).
> >
> >
> >
> >
> > If we want to avoid casts completely,
> > we could use union in struct of_device_id
> > although this might be rejected.

The union like you suggested might fly. Maybe the new field data_ulong
or data_u32 might work and even help non-enum non-pointer values to be
stored in this directly too without needing the casting that's needed
today.

I still don't get why the compiler can't be smarter about this. If the
enum would fit inside the pointer, why not leave that alone and throw
a warning only when the enum really can overflow the pointer field?

> >
> >
> > FWIW:
> >
> > diff --git a/drivers/ata/ahci_brcm.c b/drivers/ata/ahci_brcm.c
> > index 6853dbb4131d..534170bea134 100644
> > --- a/drivers/ata/ahci_brcm.c
> > +++ b/drivers/ata/ahci_brcm.c
> > @@ -415,11 +415,11 @@ static struct scsi_host_template ahci_platform_sht = {
> >  };
> >
> >  static const struct of_device_id ahci_of_match[] = {
> > -       {.compatible = "brcm,bcm7425-ahci", .data = (void *)BRCM_SATA_BCM7425},
> > -       {.compatible = "brcm,bcm7445-ahci", .data = (void *)BRCM_SATA_BCM7445},
> > -       {.compatible = "brcm,bcm63138-ahci", .data = (void *)BRCM_SATA_BCM7445},
> > -       {.compatible = "brcm,bcm-nsp-ahci", .data = (void *)BRCM_SATA_NSP},
> > -       {.compatible = "brcm,bcm7216-ahci", .data = (void *)BRCM_SATA_BCM7216},
> > +       {.compatible = "brcm,bcm7425-ahci", .data2 = BRCM_SATA_BCM7425},
> > +       {.compatible = "brcm,bcm7445-ahci", .data2 = BRCM_SATA_BCM7445},
> > +       {.compatible = "brcm,bcm63138-ahci", .data2 = BRCM_SATA_BCM7445},
> > +       {.compatible = "brcm,bcm-nsp-ahci", .data2 = BRCM_SATA_NSP},
> > +       {.compatible = "brcm,bcm7216-ahci", .data2 = BRCM_SATA_BCM7216},
> >         {},
> >  };
> >  MODULE_DEVICE_TABLE(of, ahci_of_match);
> > @@ -442,7 +442,7 @@ static int brcm_ahci_probe(struct platform_device *pdev)
> >         if (!of_id)
> >                 return -ENODEV;
> >
> > -       priv->version = (enum brcm_ahci_version)of_id->data;
> > +       priv->version = of_id->data2;
> >         priv->dev = dev;
> >
> >         res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "top-ctrl");
> > diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> > index e3596db077dc..98d44ebf146a 100644
> > --- a/include/linux/mod_devicetable.h
> > +++ b/include/linux/mod_devicetable.h
> > @@ -261,7 +261,10 @@ struct of_device_id {
> >         char    name[32];
> >         char    type[32];
> >         char    compatible[128];
> > -       const void *data;
> > +       union {
> > +               const void *data;
> > +               unsigned long data2;
> > +       };
> >  };
> >

I've never (or long since forgotten) consciously declared a union
without a name and directly accessed it's fields. If this compiles,
this seems reasonable.

-Saravana