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From:   Eugenio Perez Martin <eperezma@redhat.com>
Date:   Thu, 26 May 2022 19:00:06 +0200
Message-ID: <CAJaqyWe4311B6SK997eijEJyhwnAxkBUGJ_0iuDNd=wZSt0DmQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/4] vhost-vdpa: introduce STOP backend feature bit
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     Stefano Garzarella <sgarzare@redhat.com>,
        "Dawar, Gautam" <gautam.dawar@amd.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Jason Wang <jasowang@redhat.com>,
        Zhu Lingshan <lingshan.zhu@intel.com>,
        "martinh@xilinx.com" <martinh@xilinx.com>,
        "ecree.xilinx@gmail.com" <ecree.xilinx@gmail.com>,
        Eli Cohen <elic@nvidia.com>, Parav Pandit <parav@nvidia.com>,
        Wu Zongyong <wuzongyong@linux.alibaba.com>,
        "dinang@xilinx.com" <dinang@xilinx.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Xie Yongji <xieyongji@bytedance.com>,
        "lulu@redhat.com" <lulu@redhat.com>,
        "martinpo@xilinx.com" <martinpo@xilinx.com>,
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        Longpeng <longpeng2@huawei.com>,
        "Piotr.Uminski@intel.com" <Piotr.Uminski@intel.com>,
        "Kamde, Tanuj" <tanuj.kamde@amd.com>,
        Si-Wei Liu <si-wei.liu@oracle.com>,
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        Zhang Min <zhang.min9@zte.com.cn>,
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On Thu, May 26, 2022 at 3:21 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> On Thu, May 26, 2022 at 02:44:02PM +0200, Eugenio Perez Martin wrote:
> > > >> +static bool vhost_vdpa_can_stop(const struct vhost_vdpa *v) {
> > > >> +       struct vdpa_device *vdpa = v->vdpa;
> > > >> +       const struct vdpa_config_ops *ops = vdpa->config;
> > > >> +
> > > >> +       return ops->stop;
> > > >> [GD>>] Would it be better to explicitly return a bool to match the return type?
> > > >
> > > >I'm not sure about the kernel code style regarding that casting. Maybe
> > > >it's better to return !!ops->stop here. The macros likely and unlikely
> > > >do that.
> > >
> > > IIUC `ops->stop` is a function pointer, so what about
> > >
> > >      return ops->stop != NULL;
> > >
> >
> > I'm ok with any method proposed. Both three ways can be found in the
> > kernel so I think they are all valid (although the double negation is
> > from bool to integer in (0,1) set actually).
> >
> > Maybe Jason or Michael (as maintainers) can state the preferred method here.
>
> Always just do whatever the person who responded feels like because
> they're likely the person who cares the most.  ;)
>

This is interesting and I think it's good advice :). I'm fine with
whatever we chose, I just wanted to "break the tie" between the three.

> I don't think there are any static analysis tools which will complain
> about this.  Smatch will complain if you return a negative literal.

Maybe a negative literal is a bad code signal, yes.

> It feels like returning any literal that isn't 1 or 0 should trigger a
> warning...  I've written that and will check it out tonight.
>

I'm not sure this should be so strict, or "literal" does not include pointers?

As an experiment, can Smatch be used to count how many times a
returned pointer is converted to int / bool before returning vs not
converted?

I find Smatch interesting, especially when switching between projects
frequently. Does it support changing the code like clang-format? To
offload cognitive load to tools is usually good :).

Thanks!

> Really anything negative should trigger a warning.  See new Smatch stuff
> below.
>
> regards,
> dan carpenter
>
> ================ TEST CASE =========================
>
> int x;
> _Bool one(int *p)
> {
>         if (p)
>                 x = -2;
>         return x;
> }
> _Bool two(int *p)
> {
>         return -4;  // returning 2 triggers a warning now
> }
>
> =============== OUTPUT =============================
>
> test.c:10 one() warn: potential negative cast to bool 'x'
> test.c:14 two() warn: signedness bug returning '(-4)'
> test.c:14 two() warn: '(-4)' is not bool
>
> =============== CODE ===============================
>
> #include "smatch.h"
> #include "smatch_extra.h"
> #include "smatch_slist.h"
>
> static int my_id;
>
> static void match_literals(struct expression *ret_value)
> {
>         struct symbol *type;
>         sval_t sval;
>
>         type = cur_func_return_type();
>         if (!type || sval_type_max(type).value != 1)
>                 return;
>
>         if (!get_implied_value(ret_value, &sval))
>                 return;
>
>         if (sval.value == 0 || sval.value == 1)
>                 return;
>
>         sm_warning("'%s' is not bool", sval_to_str(sval));
> }
>
> static void match_any_negative(struct expression *ret_value)
> {
>         struct symbol *type;
>         struct sm_state *extra, *sm;
>         sval_t sval;
>         char *name;
>
>         type = cur_func_return_type();
>         if (!type || sval_type_max(type).value != 1)
>                 return;
>
>         extra = get_extra_sm_state(ret_value);
>         if (!extra)
>                 return;
>         FOR_EACH_PTR(extra->possible, sm) {
>                 if (estate_get_single_value(sm->state, &sval) &&
>                     sval_is_negative(sval)) {
>                         name = expr_to_str(ret_value);
>                         sm_warning("potential negative cast to bool '%s'", name);
>                         free_string(name);
>                         return;
>                 }
>         } END_FOR_EACH_PTR(sm);
> }
>
> void check_bool_return(int id)
> {
>         my_id = id;
>
>         add_hook(&match_literals, RETURN_HOOK);
>         add_hook(&match_any_negative, RETURN_HOOK);
> }
>