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].
[1] https://git.kernel.org/linus/d88cabfd9abc
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
- None.
v1:
- Link: https://lore.kernel.org/linux-hardening/e9ccb0cd7e490bfa270a7c20979e16ff84ac91e2.1729536776.git.gustavoars@kernel.org/
include/uapi/linux/ethtool.h | 33 ++++++++++++++++++---------------
1 file changed, 18 insertions(+), 15 deletions(-)
diff --git a/include/uapi/linux/ethtool.h b/include/uapi/linux/ethtool.h
index c405ed63acfa..fc1f54b065f9 100644
--- a/include/uapi/linux/ethtool.h
+++ b/include/uapi/linux/ethtool.h
@@ -2511,21 +2511,24 @@ enum ethtool_reset_flags {
* autonegotiation; 0 if unknown or not applicable. Read-only.
*/
struct ethtool_link_settings {
- __u32 cmd;
- __u32 speed;
- __u8 duplex;
- __u8 port;
- __u8 phy_address;
- __u8 autoneg;
- __u8 mdio_support;
- __u8 eth_tp_mdix;
- __u8 eth_tp_mdix_ctrl;
- __s8 link_mode_masks_nwords;
- __u8 transceiver;
- __u8 master_slave_cfg;
- __u8 master_slave_state;
- __u8 rate_matching;
- __u32 reserved[7];
+ /* New members MUST be added within the __struct_group() macro below. */
+ __struct_group(ethtool_link_settings_hdr, hdr, /* no attrs */,
+ __u32 cmd;
+ __u32 speed;
+ __u8 duplex;
+ __u8 port;
+ __u8 phy_address;
+ __u8 autoneg;
+ __u8 mdio_support;
+ __u8 eth_tp_mdix;
+ __u8 eth_tp_mdix_ctrl;
+ __s8 link_mode_masks_nwords;
+ __u8 transceiver;
+ __u8 master_slave_cfg;
+ __u8 master_slave_state;
+ __u8 rate_matching;
+ __u32 reserved[7];
+ );
__u32 link_mode_masks[];
/* layout of link_mode_masks fields:
* __u32 map_supported[link_mode_masks_nwords];
--
2.43.0On 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 */,
| ^~~~~~~~~~~~~~~~~~~~~~~~~
I don't know much about C++, tho, so quite possibly missing something
obvious.
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 */,
> | ^~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> I don't know much about C++, tho, so quite possibly missing something
> obvious.
We are in the same situation here.
It seems C++ considers it ambiguous to define a struct with a tag such
as `struct TAG { MEMBERS } ATTRS NAME;` within an anonymous union.
Let me look into this further...
--
Gustavo
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
-Gustavo
>> I don't know much about C++, tho, so quite possibly missing something
>> obvious.
>
> We are in the same situation here.
>
> It seems C++ considers it ambiguous to define a struct with a tag such
> as `struct TAG { MEMBERS } ATTRS NAME;` within an anonymous union.
>
> Let me look into this further...
> --
> Gustavo
>
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
On Sat, 9 Nov 2024 10:02:13 -0800 Jakub Kicinski wrote: > $ g++ /tmp/t.cpp -I../linux -o /dev/null -c -W -Wall -O2 gcc version 14.2.1 20240912 (Red Hat 14.2.1-3) (GCC)
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