linux-kernel - Re: [PATCH] scsi: libsas: Replace zero-length array with flexible-array

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Date:   Mon, 11 May 2020 09:14:31 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
CC:     <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] scsi: libsas: Replace zero-length array with
 flexible-array

On 07/05/2020 20:21, 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]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> 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 <gustavoars@...nel.org>

Reviewed-by: John Garry <john.garry@...wei.com>

> ---
>   drivers/scsi/aic94xx/aic94xx_sds.c |   14 +++++++-------
>   include/scsi/sas.h                 |    8 ++++----
>   2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
> index 3ddc8852bc32..105adba559a1 100644
> --- a/drivers/scsi/aic94xx/aic94xx_sds.c
> +++ b/drivers/scsi/aic94xx/aic94xx_sds.c
> @@ -406,7 +406,7 @@ struct asd_manuf_sec {
>   	u8    sas_addr[SAS_ADDR_SIZE];
>   	u8    pcba_sn[ASD_PCBA_SN_SIZE];
>   	/* Here start the other segments */
> -	u8    linked_list[0];
> +	u8    linked_list[];
>   } __attribute__ ((packed));
>   
>   struct asd_manuf_phy_desc {
> @@ -449,7 +449,7 @@ struct asd_ms_sb_desc {
>   	u8    type;
>   	u8    node_desc_index;
>   	u8    conn_desc_index;
> -	u8    _recvd[0];
> +	u8    _recvd[];
>   } __attribute__ ((packed));
>   
>   #if 0
> @@ -478,12 +478,12 @@ struct asd_ms_conn_desc {
>   	u8    size_sideband_desc;
>   	u32   _resvd;
>   	u8    name[16];
> -	struct asd_ms_sb_desc sb_desc[0];
> +	struct asd_ms_sb_desc sb_desc[];
>   } __attribute__ ((packed));
>   
>   struct asd_nd_phy_desc {
>   	u8    vp_attch_type;
> -	u8    attch_specific[0];
> +	u8    attch_specific[];
>   } __attribute__ ((packed));
>   
>   #if 0
> @@ -503,7 +503,7 @@ struct asd_ms_node_desc {
>   	u8    size_phy_desc;
>   	u8    _resvd;
>   	u8    name[16];
> -	struct asd_nd_phy_desc phy_desc[0];
> +	struct asd_nd_phy_desc phy_desc[];
>   } __attribute__ ((packed));
>   
>   struct asd_ms_conn_map {
> @@ -518,7 +518,7 @@ struct asd_ms_conn_map {
>   	u8    usage_model_id;
>   	u32   _resvd;
>   	struct asd_ms_conn_desc conn_desc[0];
> -	struct asd_ms_node_desc node_desc[0];
> +	struct asd_ms_node_desc node_desc[];
>   } __attribute__ ((packed));
>   
>   struct asd_ctrla_phy_entry {
> @@ -542,7 +542,7 @@ struct asd_ll_el {
>   	u8   id0;
>   	u8   id1;
>   	__le16  next;
> -	u8   something_here[0];
> +	u8   something_here[];
>   } __attribute__ ((packed));
>   
>   static int asd_poll_flash(struct asd_ha_struct *asd_ha)
> diff --git a/include/scsi/sas.h b/include/scsi/sas.h
> index a5d8ae49198c..4726c1bbec65 100644
> --- a/include/scsi/sas.h
> +++ b/include/scsi/sas.h
> @@ -324,7 +324,7 @@ struct ssp_response_iu {
>   	__be32 response_data_len;
>   
>   	u8     resp_data[0];

JFYI, We still have a zero-length array here, and some ideas to remove 
it are here:

https://lore.kernel.org/linux-scsi/CAK8P3a1=5dmgB+k9B_jk2qBhBPnMSFMWrByP4jRvyvaJwBo94A@mail.gmail.com/

> -	u8     sense_data[0];
> +	u8     sense_data[];
>   } __attribute__ ((packed));
>   
>   struct ssp_command_iu {
> @@ -346,7 +346,7 @@ struct ssp_command_iu {
>   	u8    add_cdb_len:6;
>   
>   	u8    cdb[16];
> -	u8    add_cdb[0];
> +	u8    add_cdb[];
>   } __attribute__ ((packed));
>   
>   struct xfer_rdy_iu {
> @@ -555,7 +555,7 @@ struct ssp_response_iu {
>   	__be32 response_data_len;
>   
>   	u8     resp_data[0];
> -	u8     sense_data[0];
> +	u8     sense_data[];
>   } __attribute__ ((packed));
>   
>   struct ssp_command_iu {
> @@ -577,7 +577,7 @@ struct ssp_command_iu {
>   	u8    _r_c:2;
>   
>   	u8    cdb[16];
> -	u8    add_cdb[0];
> +	u8    add_cdb[];
>   } __attribute__ ((packed));
>   
>   struct xfer_rdy_iu {
> 
> .
>