Re: [PATCH][next] wifi: cfg80211: Use __counted_by() in struct wmi_start_scan_cmd and avoid -Wfamnae warning

Jeff Johnson
2024-03-27T21:18:24Z

On 3/27/2024 11:50 AM, Gustavo A. R. Silva wrote:
> Prepare for the coming implementation by GCC and Clang of the
> __counted_by attribute. Flexible array members annotated with
> __counted_by can have their accesses bounds-checked at run-time
> via CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE
> (for strcpy/memcpy-family functions).
>
> Also, -Wflex-array-member-not-at-end is coming in GCC-14, and we are
> getting ready to enable it globally.
>
> So, use the `DEFINE_FLEX()` helper for an on-stack definition of
> a flexible structure where the size of the flexible-array member
> is known at compile-time, and refactor the rest of the code,
> accordingly.
>
> So, with these changes, fix the following warning:
> drivers/net/wireless/ath/wil6210/cfg80211.c:896:43: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> Link: https://github.com/KSPP/linux/issues/202
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> drivers/net/wireless/ath/wil6210/cfg80211.c | 22 ++++++++++-----------
> drivers/net/wireless/ath/wil6210/wmi.h | 2 +-
> 2 files changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/net/wireless/ath/wil6210/cfg80211.c b/drivers/net/wireless/ath/wil6210/cfg80211.c
> index dbe4b3478f03..836b49954171 100644
> --- a/drivers/net/wireless/ath/wil6210/cfg80211.c
> +++ b/drivers/net/wireless/ath/wil6210/cfg80211.c
> @@ -892,10 +892,8 @@ static int wil_cfg80211_scan(struct wiphy *wiphy,
> struct wil6210_priv *wil = wiphy_to_wil(wiphy);
> struct wireless_dev *wdev = request->wdev;
> struct wil6210_vif *vif = wdev_to_vif(wil, wdev);
> - struct {
> - struct wmi_start_scan_cmd cmd;
> - u16 chnl[4];
> - } __packed cmd;
> + DEFINE_FLEX(struct wmi_start_scan_cmd, cmd,
> + channel_list, num_channels, 4);
> uint i, n;
> int rc;
>
> @@ -977,9 +975,9 @@ static int wil_cfg80211_scan(struct wiphy *wiphy,
> vif->scan_request = request;
> mod_timer(&vif->scan_timer, jiffies + WIL6210_SCAN_TO);
>
> - memset(&cmd, 0, sizeof(cmd));
> - cmd.cmd.scan_type = WMI_ACTIVE_SCAN;
> - cmd.cmd.num_channels = 0;
> + memset(cmd, 0, sizeof(*cmd));

Isn't this unnecessary since DEFINE_FLEX() logic "{ .obj.COUNTER = COUNT, }"
should result in everything else being zeroed?

And if that isn't sufficient, DEFINE_FLEX() itself says we should "Use
__struct_size(@NAME) to get compile-time size of it afterwards"

Note the current memset won't zero the flex array and hence if the actual
number of channels is less than 4 then kernel stack contents could be exposed
to firmware.

> + cmd->scan_type = WMI_ACTIVE_SCAN;
> + cmd->num_channels = 0;
> n = min(request->n_channels, 4U);
> for (i = 0; i < n; i++) {
> int ch = request->channels[i]->hw_value;
> @@ -991,7 +989,8 @@ static int wil_cfg80211_scan(struct wiphy *wiphy,
> continue;
> }
> /* 0-based channel indexes */
> - cmd.cmd.channel_list[cmd.cmd.num_channels++].channel = ch - 1;
> + cmd->num_channels++;
> + cmd->channel_list[cmd->num_channels - 1].channel = ch - 1;
> wil_dbg_misc(wil, "Scan for ch %d : %d MHz\n", ch,
> request->channels[i]->center_freq);
> }
> @@ -1007,16 +1006,15 @@ static int wil_cfg80211_scan(struct wiphy *wiphy,
> if (rc)
> goto out_restore;
>
> - if (wil->discovery_mode && cmd.cmd.scan_type == WMI_ACTIVE_SCAN) {
> - cmd.cmd.discovery_mode = 1;
> + if (wil->discovery_mode && cmd->scan_type == WMI_ACTIVE_SCAN) {
> + cmd->discovery_mode = 1;
> wil_dbg_misc(wil, "active scan with discovery_mode=1\n");
> }
>
> if (vif->mid == 0)
> wil->radio_wdev = wdev;
> rc = wmi_send(wil, WMI_START_SCAN_CMDID, vif->mid,
> - &cmd, sizeof(cmd.cmd) +
> - cmd.cmd.num_channels * sizeof(cmd.cmd.channel_list[0]));
> + cmd, struct_size(cmd, channel_list, cmd->num_channels));
>
> out_restore:
> if (rc) {
> diff --git a/drivers/net/wireless/ath/wil6210/wmi.h b/drivers/net/wireless/ath/wil6210/wmi.h
> index 71bf2ae27a98..b47606d9068c 100644
> --- a/drivers/net/wireless/ath/wil6210/wmi.h
> +++ b/drivers/net/wireless/ath/wil6210/wmi.h
> @@ -474,7 +474,7 @@ struct wmi_start_scan_cmd {
> struct {
> u8 channel;
> u8 reserved;
> - } channel_list[];
> + } channel_list[] __counted_by(num_channels);
> } __packed;
>
> #define WMI_MAX_PNO_SSID_NUM (16)