linux-kernel - [PATCH 2/2] virt: vbox: uapi: Mark inner unions in packed structs as packed

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Message-Id: <20260115-kbuild-alignment-vbox-v1-2-076aed1623ff@linutronix.de>
Date: Thu, 15 Jan 2026 08:35:45 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: "K. Y. Srinivasan" <kys@...rosoft.com>, 
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
 Dexuan Cui <decui@...rosoft.com>, Long Li <longli@...rosoft.com>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Hans de Goede <hansg@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>, 
 kernel test robot <lkp@...el.com>
Subject: [PATCH 2/2] virt: vbox: uapi: Mark inner unions in packed structs
 as packed

The unpacked unions within a packed struct generates alignment warnings
on clang for 32-bit ARM:

./usr/include/linux/vbox_vmmdev_types.h:239:4: error: field u within 'struct vmmdev_hgcm_function_parameter32'
  is less aligned than 'union (unnamed union at ./usr/include/linux/vbox_vmmdev_types.h:223:2)'
  and is usually due to 'struct vmmdev_hgcm_function_parameter32' being packed,
  which can lead to unaligned accesses [-Werror,-Wunaligned-access]
     239 |         } u;
         |           ^

./usr/include/linux/vbox_vmmdev_types.h:254:6: error: field u within
  'struct vmmdev_hgcm_function_parameter64::(anonymous union)::(unnamed at ./usr/include/linux/vbox_vmmdev_types.h:249:3)'
  is less aligned than 'union (unnamed union at ./usr/include/linux/vbox_vmmdev_types.h:251:4)' and is usually due to
  'struct vmmdev_hgcm_function_parameter64::(anonymous union)::(unnamed at ./usr/include/linux/vbox_vmmdev_types.h:249:3)'
  being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]

With the recent changes to compile-test the UAPI headers in more cases,
these warning in combination with CONFIG_WERROR breaks the build.

Fix the warnings.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512140314.DzDxpIVn-lkp@intel.com/
Reported-by: Nathan Chancellor <nathan@...nel.org>
Closes: https://lore.kernel.org/linux-kbuild/20260110-uapi-test-disable-headers-arm-clang-unaligned-access-v1-1-b7b0fa541daa@kernel.org/
Suggested-by: Arnd Bergmann <arnd@...db.de>
Link: https://lore.kernel.org/linux-kbuild/29b2e736-d462-45b7-a0a9-85f8d8a3de56@app.fastmail.com/
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 include/uapi/linux/vbox_vmmdev_types.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/vbox_vmmdev_types.h b/include/uapi/linux/vbox_vmmdev_types.h
index 6073858d52a2..11f3627c3729 100644
--- a/include/uapi/linux/vbox_vmmdev_types.h
+++ b/include/uapi/linux/vbox_vmmdev_types.h
@@ -236,7 +236,7 @@ struct vmmdev_hgcm_function_parameter32 {
 			/** Relative to the request header. */
 			__u32 offset;
 		} page_list;
-	} u;
+	} __packed u;
 } __packed;
 VMMDEV_ASSERT_SIZE(vmmdev_hgcm_function_parameter32, 4 + 8);

@@ -251,7 +251,7 @@ struct vmmdev_hgcm_function_parameter64 {
 			union {
 				__u64 phys_addr;
 				__u64 linear_addr;
-			} u;
+			} __packed u;
 		} __packed pointer;
 		struct {
 			/** Size of the buffer described by the page list. */

-- 
2.52.0