regression: zfs load-key on tty broken for keyformat=passphrase

[adamdmoss]

System information

Type	Version/Name
Distribution Name	ubuntu
Distribution Version	18.04.x
Linux Kernel	5.3.0-46-generic
Architecture	x64
ZFS Version	c14ca14
SPL Version	c14ca14

Describe the problem you're observing

At current git master HEAD - c14ca14 -

Attempts to enter keyphrase with
zfs load-key -a -r
are failing:

Enter passphrase for 'backups-on-6tbusb':
Key load error: Passphrase too short (min 8).

n.b. my passphrase isn't actually too short and this was working at 47c9299

At a guess, the regression is due to c14ca14

[adamdmoss]

Confirmed that 89a6610 is fine so I guess it probably is c14ca14 that's troublesome.

[adamdmoss]

Think I see the problem in c14ca14 .

	case ZFS_KEYLOCATION_PROMPT:
		if (isatty(fileno(stdin))) {
			can_retry = B_TRUE;
			ret = get_key_interactive(hdl, fsname, keyformat,
			    do_verify, newkey, km_out, kmlen_out);

...

	if ((ret = validate_key(hdl, keyformat, (const char *)km, kmlen)) != 0)
		goto error;

get_key_material() validates km/kmlen, but for the ZFS_KEYLOCATION_PROMPT isatty() case the output is put into km_out/kmlen_out instead.