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

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Date:   Tue, 17 Mar 2020 11:58:09 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     "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>, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: Give 32bit personalities 32bit hashes

On Tue, 17 Mar 2020 at 11:31, Linus Walleij <linus.walleij@...aro.org> wrote:
>
> It was brought to my attention that this bug from 2018 was
> still unresolved: 32 bit emulators like QEMU were given
> 64 bit hashes when running 32 bit emulation on 64 bit systems.
>
> The personality(2) system call supports to let processes
> indicate that they are 32 bit Linux to the kernel. This
> was suggested by Teo in the original thread, so I just wired
> it up and it solves the problem.

Thanks for having a look at this. I'm not sure this is what
QEMU needs, though. When QEMU runs, it is not a 32-bit
process, it's a 64-bit process. Some of the syscalls
it makes are on behalf of the guest and would need 32-bit
semantics (including this one of wanting 32-bit hash sizes
in directory reads). But some syscalls it makes for itself
(either directly, or via libraries it's linked against
including glibc and glib) -- those would still want the
usual 64-bit semantics, I would have thought.

> Programs that need the 32 bit hash only need to issue the
> personality(PER_LINUX32) call and things start working.

What in particular does this personality setting affect?
My copy of the personality(2) manpage just says:

       PER_LINUX32 (since Linux 2.2)
              [To be documented.]

which isn't very informative.

thanks
-- PMM