On Tue, 24 Mar 2020 at 02:34, Theodore Y. Ts'o [off-list ref] 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