Re: [PATCH] tty: mxser: Remove __counted_by from mxser_board.ports[]

Jiri Slaby
2024-05-30T06:22:06Z

On 29. 05. 24, 23:29, Nathan Chancellor wrote:
> 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.

Huh -- what am I missing:

struct mxser_port {
struct tty_port port;
struct mxser_board *board;

unsigned long ioaddr;
unsigned long opmode_ioaddr;

u8 rx_high_water;
u8 rx_low_water;
int type; /* UART type */

u8 x_char; /* xon/xoff character */
u8 IER; /* Interrupt Enable Register */
u8 MCR; /* Modem control register */
u8 FCR; /* FIFO control register */

struct async_icount icount;
unsigned int timeout;

u8 read_status_mask;
u8 ignore_status_mask;
u8 xmit_fifo_size;

spinlock_t slock;
};

?

> 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.

This looks like a compiler bug/deficiency.

What does gcc say BTW?

> Cc: stable@vger.kernel.org
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2026
> Fixes: f34907ecca71 ("mxser: Annotate struct mxser_board with __counted_by")

I would not say "Fixes" here. It only works around a broken compiler.

> Signed-off-by: Nathan Chancellor <nathan@kernel.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);

thanks,
--
js
suse labs