linux-kernel - [PATCH v5.10] ovl: Use "buf" flexible array for memcpy() destination

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Message-Id: <20251231100809.642262-1-shivani.agarwal@broadcom.com>
Date: Wed, 31 Dec 2025 02:08:09 -0800
From: Shivani Agarwal <shivani.agarwal@...adcom.com>
To: stable@...r.kernel.org,
	gregkh@...uxfoundation.org
Cc: miklos@...redi.hu,
	amir73il@...il.com,
	linux-unionfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	ajay.kaher@...adcom.com,
	alexey.makhalov@...adcom.com,
	vamsi-krishna.brahmajosyula@...adcom.com,
	yin.ding@...adcom.com,
	tapas.kundu@...adcom.com,
	Kees Cook <keescook@...omium.org>,
	syzbot+9d14351a171d0d1c7955@...kaller.appspotmail.com,
	"Gustavo A . R . Silva" <gustavoars@...nel.org>,
	Miklos Szeredi <mszeredi@...hat.com>,
	Shivani Agarwal <shivani.agarwal@...adcom.com>
Subject: [PATCH v5.10] ovl: Use "buf" flexible array for memcpy() destination

From: Kees Cook <keescook@...omium.org>

commit cf8aa9bf97cadf85745506c6a3e244b22c268d63 upstream.

The "buf" flexible array needs to be the memcpy() destination to avoid
false positive run-time warning from the recent FORTIFY_SOURCE
hardening:

  memcpy: detected field-spanning write (size 93) of single field "&fh->fb"
  at fs/overlayfs/export.c:799 (size 21)

Reported-by: syzbot+9d14351a171d0d1c7955@...kaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000763a6c05e95a5985@google.com/
Signed-off-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[Shivani: Modified to apply on 5.10.y]
Signed-off-by: Shivani Agarwal <shivani.agarwal@...adcom.com>
---
 fs/overlayfs/export.c    | 2 +-
 fs/overlayfs/overlayfs.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index f98128317..dd3e1969e 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -788,7 +788,7 @@ static struct ovl_fh *ovl_fid_to_fh(struct fid *fid, int buflen, int fh_type)
 		return ERR_PTR(-ENOMEM);
 
 	/* Copy unaligned inner fh into aligned buffer */
-	memcpy(&fh->fb, fid, buflen - OVL_FH_WIRE_OFFSET);
+	memcpy(fh->buf, fid, buflen - OVL_FH_WIRE_OFFSET);
 	return fh;
 }
 
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 87b7a4a74..5ac968f70 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -104,7 +104,7 @@ struct ovl_fh {
 	u8 padding[3];	/* make sure fb.fid is 32bit aligned */
 	union {
 		struct ovl_fb fb;
-		u8 buf[0];
+		DECLARE_FLEX_ARRAY(u8, buf);
 	};
 } __packed;
 
-- 
2.40.4