linux-kernel - Re: [PATCH v2 5/9] ARM: oabi-compat: rework epoll_wait/epoll

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Date:   Sat, 26 Sep 2020 20:30:03 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Russell King <linux@...linux.org.uk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2 5/9] ARM: oabi-compat: rework epoll_wait/epoll_pwait emulation

On Sat, Sep 19, 2020 at 7:32 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> > index 855aa7cc9b8e..156880943c16 100644
> > --- a/arch/arm/include/asm/syscall.h
> > +++ b/arch/arm/include/asm/syscall.h
> > @@ -28,6 +28,17 @@ static inline int syscall_get_nr(struct task_struct *task,
> >       return task_thread_info(task)->syscall & ~__NR_OABI_SYSCALL_BASE;
> >  }
> >
> > +static inline bool __in_oabi_syscall(struct task_struct *task)
> > +{
> > +     return IS_ENABLED(CONFIG_OABI_COMPAT) &&
> > +             (task_thread_info(task)->syscall & __NR_OABI_SYSCALL_BASE);
> > +}
> > +
> > +static inline bool in_oabi_syscall(void)
> > +{
> > +     return __in_oabi_syscall(current);
> > +}
> > +
>
> Maybe split these infrastructure additions into a separate helper?

Sorry, I'm not following what you mean by this. Both of the above
are pretty minimal helpers already, in what way could they be split
further?

> So after you argued for this variant I still have minor nitpicks:
>
> I alway find positive ifdefs better where possible, e.g.
>
> #if defined(CONFIG_ARM) && defined(CONFIG_OABI_COMPAT)
> external declaration here
> #else
> the real thing
> #endif

Ok.

> but I still find the fact that the native case goes into the arch
> helper a little weird.

Would you prefer something like this:

static inline struct epoll_event __user *
epoll_put_uevent(__poll_t revents, __u64 data,
                 struct epoll_event __user *uevent)
{
#if defined(CONFIG_ARM) && defined(CONFIG_OABI_COMPAT)
        /* ARM OABI has an incompatible struct layout and needs a
special handler */
        extern struct epoll_event __user *
        epoll_oabi_put_uevent(__poll_t revents, __u64 data,
                              struct epoll_event __user *uevent);

        if (in_oabi_syscall())
                return epoll_oabi_put_uevent(revents, data, uevent);
#endif
        if (__put_user(revents, &uevent->events) ||
            __put_user(data, &uevent->data))
                return NULL;

        return uevent+1;
}

That would keep the native case in one place, but also mix in
more architecture specific stuff into the common source location,
which again seems worse to me.

     Arnd