linux-kernel - Re: [PATCH v3 1/3] scsi: 3w-9xxx: Use flexible array members to avoid struct padding

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Date:   Sat, 08 Aug 2020 19:37:57 -0700
From:   Joe Perches <joe@...ches.com>
To:     Samuel Holland <samuel@...lland.org>,
        Adam Radford <aradford@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] scsi: 3w-9xxx: Use flexible array members to
 avoid struct padding

On Sat, 2020-08-08 at 19:47 -0500, Samuel Holland wrote:
> In preparation for removing the "#pragma pack(1)" from the driver, fix
> all instances where a trailing array member could be replaced by a
> flexible array member. Since a flexible array member has zero size, it
> introduces no padding, whether or not the struct is packed.
[]
> diff --git a/drivers/scsi/3w-9xxx.c b/drivers/scsi/3w-9xxx.c
[]
> @@ -676,7 +676,7 @@ static long twa_chrdev_ioctl(struct file *file, unsigned int cmd, unsigned long
>  	data_buffer_length_adjusted = (driver_command.buffer_length + 511) & ~511;
>  
>  	/* Now allocate ioctl buf memory */
> -	cpu_addr = dma_alloc_coherent(&tw_dev->tw_pci_dev->dev, data_buffer_length_adjusted+sizeof(TW_Ioctl_Buf_Apache) - 1, &dma_handle, GFP_KERNEL);
> +	cpu_addr = dma_alloc_coherent(&tw_dev->tw_pci_dev->dev, data_buffer_length_adjusted + sizeof(TW_Ioctl_Buf_Apache), &dma_handle, GFP_KERNEL);

Trivia:

It's perhaps more sensible to order the arguments with
the size of the struct first then the data length so
the argument is in the order of the expected content.

	cpu_addr = dma_alloc_coherent(&tw_dev->tw_pci_dev->dev,
				      sizeof(TW_Ioctl_Buf_Apache) + data_buffer_length_adjusted,
				      &dma_handle, GFP_KERNEL);

> @@ -685,7 +685,7 @@ static long twa_chrdev_ioctl(struct file *file, unsigned int cmd, unsigned long
>  	tw_ioctl = (TW_Ioctl_Buf_Apache *)cpu_addr;
>  
>  	/* Now copy down the entire ioctl */
> -	if (copy_from_user(tw_ioctl, argp, driver_command.buffer_length + sizeof(TW_Ioctl_Buf_Apache) - 1))
> +	if (copy_from_user(tw_ioctl, argp, driver_command.buffer_length + sizeof(TW_Ioctl_Buf_Apache)))

	if (copy_from_user(tw_ioctl, argp, sizeof(TW_Ioctl_Buf_Apache) + driver_command.buffer_length))

etc...