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

[Eric Blake]

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