[PATCH] ovl: Use "buf" flexible array for memcpy() destination

[Kees Cook <keescook@chromium.org>]

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)

Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-unionfs@vger.kernel.org
Reported-by: syzbot+9d14351a171d0d1c7955@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000763a6c05e95a5985@google.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 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 e065a5b9a442..ac9c3ad04016 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -796,7 +796,7 @@ 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 87759165d32b..a0e450313ea4 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -108,7 +108,7 @@ 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;
 
-- 
2.34.1