linux-kernel - Re: [PATCH 3/3] floppy: suppress UBSAN warning in setup_rw

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Message-ID: <20200422082400.GA30239@infradead.org>
Date:   Wed, 22 Apr 2020 01:24:00 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Denis Efremov <efremov@...ux.com>
Cc:     Willy Tarreau <w@....eu>, Christoph Hellwig <hch@...radead.org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] floppy: suppress UBSAN warning in setup_rw_floppy()

On Wed, Apr 22, 2020 at 11:20:23AM +0300, Denis Efremov wrote:
> What do you think about changing it this way?
> 
> struct floppy_raw_cmd {
>  
>         unsigned char rate;
>  
> -#define FD_RAW_CMD_SIZE 16
> +#define FD_RAW_CMD_SIZE 33
>  #define FD_RAW_REPLY_SIZE 16
>  
>         unsigned char cmd_count;
> -       unsigned char cmd[FD_RAW_CMD_SIZE];
> -       unsigned char reply_count;
> -       unsigned char reply[FD_RAW_REPLY_SIZE];
> +       union {
> +               struct {
> +                       unsigned char reserved[16];
> +                       unsigned char reply_count;
> +                       unsigned char reply[FD_RAW_REPLY_SIZE];
> +               };
> +               unsigned char cmd[FD_RAW_CMD_SIZE];
> +       };

I don't think we can just change FD_RAW_CMD_SIZE or cmd as that could
break userspace. But otherwise, yes something very much like that:

>  #define FD_RAW_CMD_SIZE 16
>  #define FD_RAW_REPLY_SIZE 16
> +#define FD_RAW_FULL_CMD_SIZE (FD_RAW_CMD_SIZE + 1 + FD_RAW_CMD_SIZE)
>  
>         unsigned char cmd_count;
> -       unsigned char cmd[FD_RAW_CMD_SIZE];
> -       unsigned char reply_count;
> -       unsigned char reply[FD_RAW_REPLY_SIZE];
> +       union {
> +               struct {
> +                       unsigned char cmd[FD_RAW_CMD_SIZE];
> +                       unsigned char reply_count;
> +                       unsigned char reply[FD_RAW_REPLY_SIZE];
> +               };
> +               unsigned char full_cmd[FD_RAW_FULL_CMD_SIZE];
> +       };

>         int track;
> 
> Denis
---end quoted text---