linux-kernel - Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll

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Date:   Thu, 10 Dec 2020 17:59:07 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Soheil Hassas Yeganeh <soheil.kdev@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Shuo Chen <shuochen@...gle.com>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2

On Thu, Dec 10, 2020 at 3:34 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Thu, Dec 10, 2020 at 6:33 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> > On Sat, Nov 21, 2020 at 4:27 AM Arnd Bergmann <arnd@...nel.org> wrote:
> > > On Fri, Nov 20, 2020 at 11:28 PM Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> > > I would imagine this can be done like the way I proposed
> > > for get_bitmap() in sys_migrate_pages:
> > >
> > > https://lore.kernel.org/lkml/20201102123151.2860165-4-arnd@kernel.org/
> >
> > Coming back to this. Current patchset includes new select and poll
> > selftests to verify the changes. I need to send a small kselftest
> > patch for that first.
> >
> > Assuming there's no time pressure, I will finish up and send the main
> > changes after the merge window, for the next release then.
> >
> > Current state against linux-next at
> > https://github.com/wdebruij/linux-next-mirror/tree/select-compat-1
>
> Ok, sounds good to me. I've had a (very brief) look and have one
> suggestion: instead of open-coding the compat vs native mode
> in multiple places like
>
> if (!in_compat_syscall())
>      return copy_from_user(fdset, ufdset, FDS_BYTES(nr)) ? -EFAULT : 0;
> else
>      return compat_get_bitmap(fdset, ufdset, nr);
>
> maybe move this into a separate function and call that where needed.
>
> I've done this for the get_bitmap() function in my series at
>
> https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/commit/?h=compat-alloc-user-space-7&id=b1b23ebb12b635654a2060df49455167a142c5d2
>
> The definition is slightly differrent for cpumask, nodemask and fd_set,
> so we'd need to try out the best way to structure the code to end
> up with the most readable version, but it should be possible when
> there are only three callers (and duplicating the function would
> be the end of the world either)

For fd_set there is only a single caller for each direction. Do you
prefer helpers even so?

For sigmask, with three callers, something along the lines of this?

  @@ -1138,10 +1135,7 @@ static int do_ppoll(struct pollfd __user
*ufds, unsigned int nfds,
                          return -EINVAL;
          }

  -       if (!in_compat_syscall())
  -               ret = set_user_sigmask(sigmask, sigsetsize);
  -       else
  -               ret = set_compat_user_sigmask(sigmask, sigsetsize);
  +       ret = set_maybe_compat_user_sigmask(sigmask, sigsetsize);
          if (ret)
                  return ret;

  --- a/include/linux/compat.h
  +++ b/include/linux/compat.h
  @@ -942,6 +942,17 @@ static inline bool in_compat_syscall(void) {
return false; }

  +static inline int set_maybe_compat_user_sigmask(const void __user *sigmask,
  +                                               size_t sigsetsize)
  +{
  +#if defined CONFIG_COMPAT
  +       if (unlikely(in_compat_syscall()))
  +               return set_compat_user_sigmask(sigmask, sigsetsize);
  +#endif
  +
  +       return set_user_sigmask(sigmask, sigsetsize);
  +}