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Subject: Re: [PATCH v3 RESEND] fcntl: Add 32bit filesystem mode
To:     Linus Walleij <linus.walleij@linaro.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>
Cc:     Peter Maydell <peter.maydell@linaro.org>,
        linux-api@vger.kernel.org, qemu-devel@nongnu.org,
        Florian Weimer <fw@deneb.enyo.de>,
        Andy Lutomirski <luto@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org
References: <20201012220620.124408-1-linus.walleij@linaro.org>
From:   Eric Blake <eblake@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <e395753a-115c-57d1-4312-b28e5f0d6ebf@redhat.com>
Date:   Mon, 12 Oct 2020 19:08:55 -0500
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On 10/12/20 5:06 PM, Linus Walleij 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.
> 
> This adds a flag to the fcntl() F_GETFD and F_SETFD operations
> to set the underlying filesystem into 32bit mode even if the
> file handle was opened using 64bit mode without the compat
> syscalls.
> 
> Programs that need the 32 bit file system behavior need to
> issue a fcntl() system call such as in this example:
> 
>    #define FD_32BIT_MODE 2
> 
>    int main(int argc, char** argv) {
>      DIR* dir;
>      int err;
>      int fd;
> 
>      dir = opendir("/boot");
>      fd = dirfd(dir);
>      err = fcntl(fd, F_SETFD, FD_32BIT_MODE);

This is a blind set, and wipes out FD_CLOEXEC. Better would be to do a 
proper demonstration of the read-modify-write with F_GETFD that portable 
programs will have to use in practice.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org