[PATCH wireless-next] wifi: mac80211: add __packed to union members of struct ieee80211_rx_status

[PATCH wireless-next] wifi: mac80211: add __packed to union members of struct ieee80211_rx_status

[Ping-Ke Shih]

The arm-linux-gnueabi-gcc compiler, align the field followed by union
members, causing size of struct ieee80211_rx_status over skb->cb
(48 bytes). By investigation, the union member starts at offset 32,
and the offset of next field rate_idx is 36 instead of expected 33, and
the total size is (unexpected) 52.

When compiling rtw88 driver, it throws:

In file included from /work/linux-src/linux-stable/include/linux/string.h:386,
                 from /work/linux-src/linux-stable/include/linux/bitmap.h:13,
                 from /work/linux-src/linux-stable/include/linux/cpumask.h:11,
                 from /work/linux-src/linux-stable/include/linux/smp.h:13,
                 from /work/linux-src/linux-stable/include/linux/lockdep.h:14,
                 from /work/linux-src/linux-stable/include/linux/mutex.h:17,
                 from /work/linux-src/linux-stable/include/linux/kernfs.h:11,
                 from /work/linux-src/linux-stable/include/linux/sysfs.h:16,
                 from /work/linux-src/linux-stable/include/linux/kobject.h:20,
                 from /work/linux-src/linux-stable/include/linux/dmi.h:6,
                 from pci.c:5:
In function 'fortify_memcpy_chk',
    inlined from 'rtw_pci_rx_napi.constprop' at pci.c:1095:4:
/work/linux-src/linux-stable/include/linux/fortify-string.h:569:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  569 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After this patch, the size of struct ieee80211_rx_status is 48.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
---
Because of size assertion of rtw88's efuse map [1], I found
arm-linux-gnueabi-gcc compiler throws this warning, but x86 gcc is absolutely
silent and expected without this patch.

[1] https://lore.kernel.org/linux-wireless/7c65e315-5a2e-455e-87ee-8fc6d60ed807@gmail.com/T/#m43fdf8a1c2b8cff92c1ef50faab7993522647162
---
 include/net/mac80211.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 40cb20d9309c..02318a4be0e1 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -1747,18 +1747,18 @@ struct ieee80211_rx_status {
 			u8 he_ru:3;
 			u8 he_gi:2;
 			u8 he_dcm:1;
-		};
+		} __packed;
 		struct {
 			u8 ru:4;
 			u8 gi:2;
-		} eht;
+		} __packed eht;
 		struct {
 			u8 ru:4;
 			u8 gi:2;
 			u8 elr:1;
 			u8 im:1;
-		} uhr;
-	};
+		} __packed uhr;
+	} __packed;
 	u8 rate_idx;
 	u8 nss;
 	u8 rx_flags;
@@ -1771,6 +1771,8 @@ struct ieee80211_rx_status {
 	u8 link_valid:1, link_id:4;
 };
 
+static_assert(sizeof(struct ieee80211_rx_status) <= sizeof_field(struct sk_buff, cb));
+
 static inline u32
 ieee80211_rx_status_to_khz(struct ieee80211_rx_status *rx_status)
 {
-- 
2.25.1

RE: [PATCH wireless-next] wifi: mac80211: add __packed to union members of struct ieee80211_rx_status

[Ping-Ke Shih]

Ping-Ke Shih <pkshih@realtek.com> wrote:
> The arm-linux-gnueabi-gcc compiler, align the field followed by union
> members, causing size of struct ieee80211_rx_status over skb->cb
> (48 bytes). By investigation, the union member starts at offset 32,
> and the offset of next field rate_idx is 36 instead of expected 33, and
> the total size is (unexpected) 52.
> 
> When compiling rtw88 driver, it throws:
> 
> In file included from /work/linux-src/linux-stable/include/linux/string.h:386,
>                  from /work/linux-src/linux-stable/include/linux/bitmap.h:13,
>                  from /work/linux-src/linux-stable/include/linux/cpumask.h:11,
>                  from /work/linux-src/linux-stable/include/linux/smp.h:13,
>                  from /work/linux-src/linux-stable/include/linux/lockdep.h:14,
>                  from /work/linux-src/linux-stable/include/linux/mutex.h:17,
>                  from /work/linux-src/linux-stable/include/linux/kernfs.h:11,
>                  from /work/linux-src/linux-stable/include/linux/sysfs.h:16,
>                  from /work/linux-src/linux-stable/include/linux/kobject.h:20,
>                  from /work/linux-src/linux-stable/include/linux/dmi.h:6,
>                  from pci.c:5:
> In function 'fortify_memcpy_chk',
>     inlined from 'rtw_pci_rx_napi.constprop' at pci.c:1095:4:
> /work/linux-src/linux-stable/include/linux/fortify-string.h:569:25: warning: call to
> '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st
> parameter); maybe use struct_group()? [-Wattribute-warning]
>   569 |                         __write_overflow_field(p_size_field, size);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> After this patch, the size of struct ieee80211_rx_status is 48.
> 
> Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
> ---
> Because of size assertion of rtw88's efuse map [1], I found
> arm-linux-gnueabi-gcc compiler throws this warning, but x86 gcc is absolutely
> silent and expected without this patch.
> 
> [1]
> https://lore.kernel.org/linux-wireless/7c65e315-5a2e-455e-87ee-8fc6d60ed807@gmail.com/T/#m43fdf8a1
> c2b8cff92c1ef50faab7993522647162

I'd note that discussion thread [2] of original kernel test robot.
Arnd pointed out the cause is CONFIG_AEABI is not set, and he said
nobody should be using ARM OABI any more. 

Maybe, we can ignore the CPU and skip this patch.

[2] https://lore.kernel.org/lkml/6b6310b8-2b0d-4390-992e-5ccd81cef2e0@app.fastmail.com/

Re: [PATCH wireless-next] wifi: mac80211: add __packed to union members of struct ieee80211_rx_status

[Johannes Berg]

On Tue, 2026-04-14 at 00:55 +0000, Ping-Ke Shih wrote:
> 
> > Because of size assertion of rtw88's efuse map [1], I found
> > arm-linux-gnueabi-gcc compiler throws this warning, but x86 gcc is absolutely
> > silent and expected without this patch.

Yeah, depends on ABI padding rules.

> > [1]
> > https://lore.kernel.org/linux-wireless/7c65e315-5a2e-455e-87ee-8fc6d60ed807@gmail.com/T/#m43fdf8a1
> > c2b8cff92c1ef50faab7993522647162
> 
> I'd note that discussion thread [2] of original kernel test robot.
> Arnd pointed out the cause is CONFIG_AEABI is not set, and he said
> nobody should be using ARM OABI any more. 

Ah, right.

> Maybe, we can ignore the CPU and skip this patch.

Given that nobody complained in many years about not being able to use
wifi on those machines, I'd be inclined to just do that, yeah. But
thanks for digging into it!

johannes