linux-kernel - [PATCH] vboxsf: Use flexible arrays for trailing string member

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Message-Id: <20230720151458.never.673-kees@kernel.org>
Date:   Thu, 20 Jul 2023 08:15:06 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH] vboxsf: Use flexible arrays for trailing string member

The declaration of struct shfl_string used trailing fake flexible arrays
for the string member. This was tripping FORTIFY_SOURCE since commit
df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Replace the
utf8 and utf16 members with actual flexible arrays, drop the unused ucs2
member, and retriain a 2 byte padding to keep the structure size the same.

Reported-by: Larry Finger <Larry.Finger@...inger.net>
Closes: https://lore.kernel.org/lkml/ab3a70e9-60ed-0f13-e3d4-8866eaccc8c1@lwfinger.net/
Tested-by: Larry Finger <Larry.Finger@...inger.net>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/vboxsf/shfl_hostintf.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/vboxsf/shfl_hostintf.h b/fs/vboxsf/shfl_hostintf.h
index aca829062c12..069a019c9247 100644
--- a/fs/vboxsf/shfl_hostintf.h
+++ b/fs/vboxsf/shfl_hostintf.h
@@ -68,9 +68,9 @@ struct shfl_string {
 
 	/** UTF-8 or UTF-16 string. Nul terminated. */
 	union {
-		u8 utf8[2];
-		u16 utf16[1];
-		u16 ucs2[1]; /* misnomer, use utf16. */
+		u8 legacy_padding[2];
+		DECLARE_FLEX_ARRAY(u8, utf8);
+		DECLARE_FLEX_ARRAY(u16, utf16);
 	} string;
 };
 VMMDEV_ASSERT_SIZE(shfl_string, 6);
-- 
2.34.1