wifi: cfg80211: Use __counted_by() in struct wmi_start_scan_cmd and avoid -Wfamnae warning

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 <[email protected]>

---
 drivers/net/wireless/ath/wil6210/cfg80211.c | 22 ++++++++++-----------
 drivers/net/wireless/ath/wil6210/wmi.h      |  2 +-
 2 files changed, 11 insertions(+), 13 deletions(-)

The subject should being with "wifi: wil6210:", I can fix that. (Didn't review the rest yet.)

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 <[email protected]>

---
 drivers/net/wireless/ath/wil6210/cfg80211.c | 22 ++++++++++-----------
 drivers/net/wireless/ath/wil6210/wmi.h      |  2 +-
 2 files changed, 11 insertions(+), 13 deletions(-)

drivers/net/wireless/ath/wil6210/cfg80211.c +2 -4

index dbe4b3478f03..836b49954171 100644
@@ -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;
 

drivers/net/wireless/ath/wil6210/cfg80211.c +7 -5

+	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) {

drivers/net/wireless/ath/wil6210/wmi.h +1 -1

index 71bf2ae27a98..b47606d9068c 100644
@@ -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)

drivers/net/wireless/ath/wil6210/cfg80211.c static int wil_cfg80211_scan(struct wiphy *wiphy, · line 978

Quoted 5 lines

drivers/net/wireless/ath/wil6210/cfg80211.c +1 -3

-	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.