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

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Date:   Mon, 15 Nov 2021 10:56:39 +0000
From:   Peter Maydell <peter.maydell@...aro.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>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 RESEND] fcntl: Add 32bit filesystem mode

On Tue, 17 Nov 2020 at 23:38, 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.

Apologies for the very late response -- I hadn't noticed that
this thread had stalled out waiting for an answer to this,
and was only reminded of it recently when another QEMU user
ran into the problem that this kernel patch is trying to resolve.

If I understand the distinction here correctly, I think
QEMU wouldn't care about it in practice. We want the "32 bit readdir
offsets" behaviour on all file descriptors that correspond
to where we're emulating "the guest opened this file descriptor".
We don't want (but probably won't notice if we get) that behaviour
on file descriptors that QEMU has opened for its own purposes.
But we'll never open a file descriptor for the guest and then
dup it into one for QEMU's purposes. (I guess there might be
some weird unlikely-to-happen edge cases where an emulated
guest binary opens an fd for a directory and then passes it
via exec to a host binary: but even there I expect the host
binary wouldn't notice it was getting 32-bit hashes.)

But overall I think that the more natural behaviour would be that
it is per-file-descriptor.

-- PMM