linux-ext4 - Re: [PATCH v3 RESEND] fcntl: Add 32bit filesystem mode

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Date:   Wed, 18 Nov 2020 10:00:19 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Dave Martin <Dave.Martin@....com>, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Florian Weimer <fw@...eb.enyo.de>,
        Peter Maydell <peter.maydell@...aro.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 RESEND] fcntl: Add 32bit filesystem mode

On Wed, Nov 18, 2020 at 12:38 AM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> On Tue, Oct 13, 2020 at 11:22 AM Dave Martin <Dave.Martin@....com> wrote:
>
> > >       case F_SETFD:
> > >               err = 0;
> > >               set_close_on_exec(fd, arg & FD_CLOEXEC);
> > > +             if (arg & FD_32BIT_MODE)
> > > +                     filp->f_mode |= FMODE_32BITHASH;
> > > +             else
> > > +                     filp->f_mode &= ~FMODE_32BITHASH;
> >
> > This seems inconsistent?  F_SETFD is for setting flags on a file
> > descriptor.  Won't setting a flag on filp here instead cause the
> > behaviour to change for all file descriptors across the system that are
> > open on this struct file?  Compare set_close_on_exec().
> >
> > I don't see any discussion on whether this should be an F_SETFL or an
> > F_SETFD, though I see F_SETFD was Ted's suggestion originally.
>
> I cannot honestly say I know the semantic difference.
>
> I would ask the QEMU people how a user program would expect
> the flag to behave.

I agree it should either use F_SETFD to set a bit in the fdtable structure
like set_close_on_exec() or it should use F_SETFL to set a bit in
filp->f_mode.

It appears the reason FMODE_32BITHASH is part of  filp->f_mode
is that the only user today is nfsd, which does not have a file
descriptor but only has a struct file. Similarly, the only code that
understands the difference (ext4_readdir()) has no reference to
the file descriptor.

If this becomes an O_DIR32BITHASH flag for F_SETFL,
I suppose it should also be supported by openat2().

       Arnd