On Sun, Sep 20, 2020 at 07:07:42PM +0100, Al Viro wrote:

	2) a few drivers are really fucked in head.  They use different
*DATA* layouts for reads/writes, depending upon the calling process.
IOW, if you fork/exec a 32bit binary and your stdin is one of those,
reads from stdin in parent and child will yield different data layouts.
On the same struct file.
	That's what Christoph worries about (/dev/sg he'd mentioned is
one of those).

	IMO we should simply have that dozen or so of pathological files
marked with FMODE_SHITTY_ABI; it's not about how they'd been opened -
it describes the userland ABI provided by those.  And it's cast in stone.

	Any in_compat_syscall() in ->read()/->write() instances is an ABI
bug, plain and simple.  Some are unfixable for compatibility reasons, but
any new caller like that should be a big red flag.

So an IOCB_COMPAT flag would let us know whether the caller is expecting
a 32-bit or 64-bit layout?  And io_uring could set it based on the
ctx->compat flag.

	Current list of those turds:
/dev/sg (pointer-chasing, generally insane)
/sys/firmware/efi/vars/*/raw_var (fucked binary structure)
/sys/firmware/efi/vars/new_var (fucked binary structure)
/sys/firmware/efi/vars/del_var (fucked binary structure)
/dev/uhid	(pointer-chasing for one obsolete command)
/dev/input/event* (timestamps)
/dev/uinput (timestamps)
/proc/bus/input/devices (fucked bitmap-to-text representation)
/sys/class/input/*/capabilities/* (fucked bitmap-to-text representation)