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

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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@xxxxxxxxxxxxxxx
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@xxxxxxxxxx>
---
  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





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