linux-kernel - Re: [PATCH] staging: gdm724x: gdm_tty: corrected macro by adding brackets

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Message-ID: <20200901133332.GY8299@kadam>
Date:   Tue, 1 Sep 2020 16:33:32 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     antoniprzybylik <antoni.przybylik@...pl>
Cc:     gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: gdm724x: gdm_tty: corrected macro by adding
 brackets

On Tue, Sep 01, 2020 at 12:43:11PM +0200, antoniprzybylik wrote:
                                          ^^^^^^^^^^^^^^^
Please, fix your From: header so that it says "Antoni Przybylik".

> Such macros are dangerous. Consider following example:
> 	#define GDM_TTY_READY(gdm) (gdm && gdm->tty_dev && gdm->port.count)
> 	GDM_TTY_READY(a + b)
> This macro will be expanded in such a way:
> 	(a + b && a + b->tty_dev && a + b->port.count)
> And it will lead to errors.
> 
> Signed-off-by: Antoni Przybylik <antoni.przybylik@...pl>
> ---
>  drivers/staging/gdm724x/gdm_tty.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/gdm724x/gdm_tty.c b/drivers/staging/gdm724x/gdm_tty.c
> index 6e813693a766..eab5c75ee5b1 100644
> --- a/drivers/staging/gdm724x/gdm_tty.c
> +++ b/drivers/staging/gdm724x/gdm_tty.c
> @@ -27,7 +27,7 @@
>  
>  #define MUX_TX_MAX_SIZE 2048
>  
> -#define GDM_TTY_READY(gdm) (gdm && gdm->tty_dev && gdm->port.count)
> +#define GDM_TTY_READY(gdm) ((gdm) && (gdm)->tty_dev && (gdm)->port.count)

I cannot imagine a real life example where adding these parentheses will
prevent a bug.  One idea it to silence this by making dereference ops
like this a special case where checkpatch.pl doesn't suggest adding
parentheses.

regards,
dan carpenter