Re: [PATCH net 1/2] netlink: add NLA_U8_BUGGY attribute type

From: David Ahern <redacted>
Date: Tue, 5 Dec 2017 09:41:21 -0700

On 12/5/17 9:34 AM, Johannes Berg wrote:

quoted

On Tue, 2017-12-05 at 11:31 -0500, David Miller wrote:

quoted

quoted

We could try to fix up the big endian problem here, but we
don't know *how* userspace misbehaved - if using nla_put_u32
then we could, but we also found a debug tool (which we'll
ignore for the purposes of this regression) that was putting
the padding into the length.

quoted

We're stuck with this thing forever... I'd like to consider other
options.

I've seen this problem at least one time before, therefore I
suggest when we see a U8 attribute with a U32's length:

1) We access it as a u32, this takes care of all endianness
   issues.

Possible, but as I said above, I've seen at least one tool (a debug
only script) now that will actually emit a U8 followed by 3 bytes of
padding to make it netlink-aligned, but set the length to 4. That would
be broken by making this change.

I'm not saying this is bad - but there are different levels of
compatibility and I'd probably go for "bug compatibility" here rather
than "fix-it-up compatibility".

Your call, ultimately - I've already fixed the tool I had found :-)

quoted

2) We emit a warning so that the app gets fixes.

The attached is my proposal. Basically, allow it the length mismatch but
print a warning. This restores previous behavior and tells users of bad
commands.

Where is the "access the U8 attribute as a U32 if length is 4" part
of my #1 above?

That's essential to handle this properly on all endianness.