linux-ext4 - Re: [PATCH] ext4: Give 32bit personalities 32bit hashes

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Date:   Tue, 24 Mar 2020 09:29:58 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "Suzuki K. Poulose" <suzuki.poulose@....com>,
        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>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] ext4: Give 32bit personalities 32bit hashes

On Tue, 24 Mar 2020 at 02:34, Theodore Y. Ts'o <tytso@....edu> wrote:
> Another possibility, which would be messier for qemu, would be use a
> flag set via fcntl.  That would require qemu from noticing when the
> guest is calling open, openat, or openat2, and then inserting a fcntl
> system call to set the 32-bit readdir mode.  That's cleaner from the
> kernel interface complexity perspective, but it's messier for qemu.

On the contrary, that would be a much better interface for QEMU.
We always know when we're doing an open-syscall on behalf
of the guest, and it would be trivial to make the fcntl() call then.
That would ensure that we don't accidentally get the
'32-bit semantics' on file descriptors QEMU opens for its own
purposes, and wouldn't leave us open to the risk in future that
setting the PER_LINUX32 flag for all of QEMU causes
unexpected extra behaviour in future kernels that would be correct
for the guest binary but wrong/broken for QEMU's own internals.

thanks
-- PMM