Re: [PATCH v2 1/2][next] UAPI: ethtool: Use __struct_group() in struct ethtool_link_settings

[Kees Cook <kees@kernel.org>]

On Tue, Nov 12, 2024 at 07:08:32PM -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 11/11/24 16:22, Gustavo A. R. Silva wrote:
> > 
> > 
> > On 09/11/24 12:02, Jakub Kicinski wrote:
> > > On Tue, 29 Oct 2024 15:55:35 -0600 Gustavo A. R. Silva wrote:
> > > > Use the `__struct_group()` helper to create a new tagged
> > > > `struct ethtool_link_settings_hdr`. This structure groups together
> > > > all the members of the flexible `struct ethtool_link_settings`
> > > > except the flexible array. As a result, the array is effectively
> > > > separated from the rest of the members without modifying the memory
> > > > layout of the flexible structure.
> > > > 
> > > > This new tagged struct will be used to fix problematic declarations
> > > > of middle-flex-arrays in composite structs[1].
> > > 
> > > Possibly a very noob question, but I'm updating a C++ library with
> > > new headers and I think this makes it no longer compile.
> > > 
> > > $ cat > /tmp/t.cpp<<EOF
> > > extern "C" {
> > > #include "include/uapi/linux/ethtool.h"
> > > }
> > > int func() { return 0; }
> > > EOF
> > > 
> > > $ g++ /tmp/t.cpp -I../linux -o /dev/null -c -W -Wall -O2
> > > In file included from /usr/include/linux/posix_types.h:5,
> > >                   from /usr/include/linux/types.h:9,
> > >                   from ../linux/include/uapi/linux/ethtool.h:18,
> > >                   from /tmp/t.cpp:2:
> > > ../linux/include/uapi/linux/ethtool.h:2515:24: error: ‘struct
> > > ethtool_link_settings::<unnamed union>::ethtool_link_settings_hdr’
> > > invalid; an anonymous union may only have public non-static data
> > > members [-fpermissive]
> > >   2515 |         __struct_group(ethtool_link_settings_hdr, hdr, /* no attrs */,
> > >        |                        ^~~~~~~~~~~~~~~~~~~~~~~~~
> > > 
> > > 
> 
> This seems to work with Clang:
> 
> $ clang++-18 -fms-extensions /tmp/t.cpp -I../linux -o /dev/null -c -W -Wall -O2
> 
> However, `-fms-extensions` doesn't seem to work for this case with GCC:
> 
> https://godbolt.org/z/1shsPhz3s

Hm, we can't break UAPI even for C++, so even if we had compiler options
that would make it work, it's really unfriendly to userspace to make all
the projects there suddenly start needing to use it.

I think this means we just can't use tagged struct groups in UAPI. :(

I have what I think is a much simpler solution. Sending it now...

-Kees

-- 
Kees Cook