Re: [PATCH] scsi: Replace zero-length array with flexible-array member

[Lee Duncan]

On 2/24/20 8:14 AM, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> ---
>  ...
> diff --git a/include/scsi/iscsi_if.h b/include/scsi/iscsi_if.h
> index 92b11c7e0b4f..b0e240b10bf9 100644
> --- a/include/scsi/iscsi_if.h
> +++ b/include/scsi/iscsi_if.h
> @@ -311,7 +311,7 @@ enum iscsi_param_type {
>  struct iscsi_param_info {
>  	uint32_t len;		/* Actual length of the param value */
>  	uint16_t param;		/* iscsi param */
> -	uint8_t value[0];	/* length sized value follows */
> +	uint8_t value[];	/* length sized value follows */
>  } __packed;
>  
>  struct iscsi_iface_param_info {
> @@ -320,7 +320,7 @@ struct iscsi_iface_param_info {
>  	uint16_t param;		/* iscsi param value */
>  	uint8_t iface_type;	/* IPv4 or IPv6 */
>  	uint8_t param_type;	/* iscsi_param_type */
> -	uint8_t value[0];	/* length sized value follows */
> +	uint8_t value[];	/* length sized value follows */
>  } __packed;
>  
>  /*
> @@ -697,7 +697,7 @@ enum iscsi_flashnode_param {
>  struct iscsi_flashnode_param_info {
>  	uint32_t len;		/* Actual length of the param */
>  	uint16_t param;		/* iscsi param value */
> -	uint8_t value[0];	/* length sized value follows */
> +	uint8_t value[];	/* length sized value follows */
>  } __packed;
>  
>  enum iscsi_discovery_parent_type {
> @@ -815,7 +815,7 @@ struct iscsi_stats {
>  	 * up to ISCSI_STATS_CUSTOM_MAX
>  	 */
>  	uint32_t custom_length;
> -	struct iscsi_stats_custom custom[0]
> +	struct iscsi_stats_custom custom[]
>  		__attribute__ ((aligned (sizeof(uint64_t))));
>  };
>  
> @@ -946,7 +946,7 @@ struct iscsi_offload_host_stats {
>  	 * up to ISCSI_HOST_STATS_CUSTOM_MAX
>  	 */
>  	uint32_t custom_length;
> -	struct iscsi_host_stats_custom custom[0]
> +	struct iscsi_host_stats_custom custom[]
>  		__aligned(sizeof(uint64_t));
>  };
>  
> diff --git a/include/scsi/scsi_bsg_iscsi.h b/include/scsi/scsi_bsg_iscsi.h
> index fa0c820a1663..6b8128005af8 100644
> --- a/include/scsi/scsi_bsg_iscsi.h
> +++ b/include/scsi/scsi_bsg_iscsi.h
> @@ -52,7 +52,7 @@ struct iscsi_bsg_host_vendor {
>  	uint64_t vendor_id;
>  
>  	/* start of vendor command area */
> -	uint32_t vendor_cmd[0];
> +	uint32_t vendor_cmd[];
>  };
>  
>  /* Response:
> diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
> index f8312a3e5b42..4dc158cf09b8 100644
> --- a/include/scsi/scsi_device.h
> +++ b/include/scsi/scsi_device.h
> @@ -231,7 +231,7 @@ struct scsi_device {
>  	struct mutex		state_mutex;
>  	enum scsi_device_state sdev_state;
>  	struct task_struct	*quiesced_by;
> -	unsigned long		sdev_data[0];
> +	unsigned long		sdev_data[];
>  } __attribute__((aligned(sizeof(unsigned long))));
>  
>  #define	to_scsi_device(d)	\
> @@ -315,7 +315,7 @@ struct scsi_target {
>  	char			scsi_level;
>  	enum scsi_target_state	state;
>  	void 			*hostdata; /* available to low-level driver */
> -	unsigned long		starget_data[0]; /* for the transport */
> +	unsigned long		starget_data[]; /* for the transport */
>  	/* starget_data must be the last element!!!! */
>  } __attribute__((aligned(sizeof(unsigned long))));
>  
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 7a97fb8104cf..e6811ea8f984 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -682,7 +682,7 @@ struct Scsi_Host {
>  	 * and also because some compilers (m68k) don't automatically force
>  	 * alignment to a long boundary.
>  	 */
> -	unsigned long hostdata[0]  /* Used for storage of host specific stuff */
> +	unsigned long hostdata[]  /* Used for storage of host specific stuff */
>  		__attribute__ ((aligned (sizeof(unsigned long))));
>  };
>  
> diff --git a/include/scsi/scsi_ioctl.h b/include/scsi/scsi_ioctl.h
> index 4fe69d863b5d..b465799f4d2d 100644
> --- a/include/scsi/scsi_ioctl.h
> +++ b/include/scsi/scsi_ioctl.h
> @@ -27,7 +27,7 @@ struct scsi_device;
>  typedef struct scsi_ioctl_command {
>  	unsigned int inlen;
>  	unsigned int outlen;
> -	unsigned char data[0];
> +	unsigned char data[];
>  } Scsi_Ioctl_Command;
>  
>  typedef struct scsi_idlun {
> diff --git a/include/scsi/srp.h b/include/scsi/srp.h
> index 9220758d5087..177d8026e96f 100644
> --- a/include/scsi/srp.h
> +++ b/include/scsi/srp.h
> @@ -109,7 +109,7 @@ struct srp_direct_buf {
>  struct srp_indirect_buf {
>  	struct srp_direct_buf	table_desc;
>  	__be32			len;
> -	struct srp_direct_buf	desc_list[0];
> +	struct srp_direct_buf	desc_list[];
>  } __attribute__((packed));
>  
>  /* Immediate data buffer descriptor as defined in SRP2. */
> @@ -244,7 +244,7 @@ struct srp_cmd {
>  	u8	reserved4;
>  	u8	add_cdb_len;
>  	u8	cdb[16];
> -	u8	add_data[0];
> +	u8	add_data[];
>  };
>  
>  enum {
> @@ -274,7 +274,7 @@ struct srp_rsp {
>  	__be32	data_in_res_cnt;
>  	__be32	sense_data_len;
>  	__be32	resp_data_len;
> -	u8	data[0];
> +	u8	data[];
>  } __attribute__((packed));
>  
>  struct srp_cred_req {
> @@ -306,7 +306,7 @@ struct srp_aer_req {
>  	struct scsi_lun	lun;
>  	__be32	sense_data_len;
>  	u32	reserved3;
> -	u8	sense_data[0];
> +	u8	sense_data[];
>  } __attribute__((packed));
>  
>  struct srp_aer_rsp {
> diff --git a/include/uapi/scsi/scsi_bsg_fc.h b/include/uapi/scsi/scsi_bsg_fc.h
> index 3ae65e93235c..7f5930801f72 100644
> --- a/include/uapi/scsi/scsi_bsg_fc.h
> +++ b/include/uapi/scsi/scsi_bsg_fc.h
> @@ -209,7 +209,7 @@ struct fc_bsg_host_vendor {
>  	__u64 vendor_id;
>  
>  	/* start of vendor command area */
> -	__u32 vendor_cmd[0];
> +	__u32 vendor_cmd[];
>  };
>  
>  /* Response:
> 

Reviewed-by: Lee Duncan <lduncan@xxxxxxxx>