linux-hardening - [PATCH] tty: mxser: Remove __counted_by from mxser

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Message-Id: <20240529-drop-counted-by-ports-mxser-board-v1-1-0ab217f4da6d@kernel.org>
Date: Wed, 29 May 2024 14:29:42 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Jiri Slaby <jirislaby@...nel.org>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Kees Cook <keescook@...omium.org>, 
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 linux-serial@...r.kernel.org, imx@...ts.linux.dev, 
 linux-arm-kernel@...ts.infradead.org, linux-hardening@...r.kernel.org, 
 llvm@...ts.linux.dev, patches@...ts.linux.dev, stable@...r.kernel.org, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] tty: mxser: Remove __counted_by from mxser_board.ports[]

Work for __counted_by on generic pointers in structures (not just
flexible array members) has started landing in Clang 19 (current tip of
tree). During the development of this feature, a restriction was added
to __counted_by to prevent the flexible array member's element type from
including a flexible array member itself such as:

  struct foo {
    int count;
    char buf[];
  };

  struct bar {
    int count;
    struct foo data[] __counted_by(count);
  };

because the size of data cannot be calculated with the standard array
size formula:

  sizeof(struct foo) * count

This restriction was downgraded to a warning but due to CONFIG_WERROR,
it can still break the build. The application of __counted_by on the
ports member of 'struct mxser_board' triggers this restriction,
resulting in:

  drivers/tty/mxser.c:291:2: error: 'counted_by' should not be applied to an array with element of unknown size because 'struct mxser_port' is a struct type with a flexible array member. This will be an error in a future compiler version [-Werror,-Wbounds-safety-counted-by-elt-type-unknown-size]
    291 |         struct mxser_port ports[] __counted_by(nports);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

Remove this use of __counted_by to fix the warning/error. However,
rather than remove it altogether, leave it commented, as it may be
possible to support this in future compiler releases.

Cc: stable@...r.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/2026
Fixes: f34907ecca71 ("mxser: Annotate struct mxser_board with __counted_by")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 drivers/tty/mxser.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/mxser.c b/drivers/tty/mxser.c
index 458bb1280ebf..5b97e420a95f 100644
--- a/drivers/tty/mxser.c
+++ b/drivers/tty/mxser.c
@@ -288,7 +288,7 @@ struct mxser_board {
 	enum mxser_must_hwid must_hwid;
 	speed_t max_baud;

-	struct mxser_port ports[] __counted_by(nports);
+	struct mxser_port ports[] /* __counted_by(nports) */;
 };

 static DECLARE_BITMAP(mxser_boards, MXSER_BOARDS);

---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20240529-drop-counted-by-ports-mxser-board-ed2a66f1a6e2

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>