Re: [PATCH][next] ath11k: Replace one-element array with flexible-array member

"Gustavo A. R. Silva" [off-list ref] wrote:

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure. Kernel code
should always use "flexible array members"[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code a bit according to the use of a flexible-array member in
struct scan_chan_list_params instead of a one-element array, and use the
struct_size() helper.

Also, save 25 (too many) bytes that were being allocated:

$ pahole -C channel_param drivers/net/wireless/ath/ath11k/reg.o
struct channel_param {
        u8                         chan_id;              /*     0     1 */
        u8                         pwr;                  /*     1     1 */
        u32                        mhz;                  /*     2     4 */

        /* Bitfield combined with next fields */

        u32                        half_rate:1;          /*     4:16  4 */
        u32                        quarter_rate:1;       /*     4:17  4 */
        u32                        dfs_set:1;            /*     4:18  4 */
        u32                        dfs_set_cfreq2:1;     /*     4:19  4 */
        u32                        is_chan_passive:1;    /*     4:20  4 */
        u32                        allow_ht:1;           /*     4:21  4 */
        u32                        allow_vht:1;          /*     4:22  4 */
        u32                        allow_he:1;           /*     4:23  4 */
        u32                        set_agile:1;          /*     4:24  4 */
        u32                        psc_channel:1;        /*     4:25  4 */

        /* XXX 6 bits hole, try to pack */

        u32                        phy_mode;             /*     8     4 */
        u32                        cfreq1;               /*    12     4 */
        u32                        cfreq2;               /*    16     4 */
        char                       maxpower;             /*    20     1 */
        char                       minpower;             /*    21     1 */
        char                       maxregpower;          /*    22     1 */
        u8                         antennamax;           /*    23     1 */
        u8                         reg_class_id;         /*    24     1 */

        /* size: 25, cachelines: 1, members: 21 */
        /* sum members: 23 */
        /* sum bitfield members: 10 bits, bit holes: 1, sum bit holes: 6 bits */
        /* last cacheline: 25 bytes */
} __attribute__((__packed__));

as previously, sizeof(struct scan_chan_list_params) was 32 bytes:

$ pahole -C scan_chan_list_params drivers/net/wireless/ath/ath11k/reg.o
struct scan_chan_list_params {
        u32                        pdev_id;              /*     0     4 */
        u16                        nallchans;            /*     4     2 */
        struct channel_param       ch_param[1];          /*     6    25 */

        /* size: 32, cachelines: 1, members: 3 */
        /* padding: 1 */
        /* last cacheline: 32 bytes */
};

and now with the flexible array transformation it is just 8 bytes:

$ pahole -C scan_chan_list_params drivers/net/wireless/ath/ath11k/reg.o
struct scan_chan_list_params {
        u32                        pdev_id;              /*     0     4 */
        u16                        nallchans;            /*     4     2 */
        struct channel_param       ch_param[];           /*     6     0 */

        /* size: 8, cachelines: 1, members: 3 */
        /* padding: 2 */
        /* last cacheline: 8 bytes */
};

This helps with the ongoing efforts to globally enable -Warray-bounds and
get us closer to being able to tighten the FORTIFY_SOURCE routines on
memcpy().

This issue was found with the help of Coccinelle and audited and fixed,
manually.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <redacted>

Patch applied to ath-next branch of ath.git, thanks.

b2549465cdea ath11k: Replace one-element array with flexible-array member

-- 
https://patchwork.kernel.org/project/linux-wireless/patch/20210823172159.GA25800@embeddedor/

https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches