wifi: cleanup ensure_hotspot_frequency()

wifi: choose a (stable) random channel for Wi-Fi hotspot

The channel depends on the SSID.

Based-on-patch-by: xiangnian <xiangnian@uniontech.com>

See-also: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1054

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1099

NEWS

@@ -8,6 +8,9 @@ subject to change and not guaranteed to be compatible with
 the later release.
 USE AT YOUR OWN RISK.  NOT RECOMMENDED FOR PRODUCTION USE!
 
+* Wi-Fi hotspots will use a (stable) random channel number unless one is
+  chosen manually.
+
 =============================================
 NetworkManager-1.36
 Overview of changes since NetworkManager-1.34

src/core/devices/wifi/nm-device-wifi.c

@@ -3103,28 +3103,75 @@ act_stage1_prepare(NMDevice *device, NMDeviceStateReason *out_failure_reason)
 static void
 ensure_hotspot_frequency(NMDeviceWifi *self, NMSettingWireless *s_wifi, NMWifiAP *ap)
 {
-    NMDevice     *device     = NM_DEVICE(self);
-    const char   *band       = nm_setting_wireless_get_band(s_wifi);
-    const guint32 a_freqs[]  = {5180, 5200, 5220, 5745, 5765, 5785, 5805, 0};
-    const guint32 bg_freqs[] = {2412, 2437, 2462, 2472, 0};
-    guint32       freq       = 0;
+    guint32     a_freqs[]  = {5180, 5200, 5220, 5745, 5765, 5785, 5805, 0};
+    guint32     bg_freqs[] = {2412, 2437, 2462, 2472, 0};
+    guint32    *rnd_freqs;
+    guint       rnd_freqs_len;
+    NMDevice   *device = NM_DEVICE(self);
+    const char *band   = nm_setting_wireless_get_band(s_wifi);
+    guint32     freq;
+    guint64     rnd;
+    guint       i;
+    guint       l;
 
-    g_assert(ap);
+    nm_assert(ap);
+    nm_assert(NM_IN_STRSET(band, NULL, "a", "bg"));
 
     if (nm_wifi_ap_get_freq(ap))
         return;
 
-    if (g_strcmp0(band, "a") == 0)
-        freq = nm_platform_wifi_find_frequency(nm_device_get_platform(device),
-                                               nm_device_get_ifindex(device),
-                                               a_freqs);
-    else
-        freq = nm_platform_wifi_find_frequency(nm_device_get_platform(device),
-                                               nm_device_get_ifindex(device),
-                                               bg_freqs);
+    {
+        GBytes       *ssid;
+        gsize         ssid_len;
+        const guint8 *ssid_data;
+        const guint8  random_seed[16] = {0x9a,
+                                        0xdc,
+                                        0x86,
+                                        0x9a,
+                                        0xa8,
+                                        0xa2,
+                                        0x07,
+                                        0x97,
+                                        0xbe,
+                                        0x6d,
+                                        0xe6,
+                                        0x99,
+                                        0x9f,
+                                        0xa8,
+                                        0x09,
+                                        0x2b};
 
-    if (!freq)
-        freq = (g_strcmp0(band, "a") == 0) ? 5180 : 2462;
+        /* Calculate a stable "random" number based on the SSID. */
+        ssid      = nm_setting_wireless_get_ssid(s_wifi);
+        ssid_data = g_bytes_get_data(ssid, &ssid_len);
+        rnd       = c_siphash_hash(random_seed, ssid_data, ssid_len);
+    }
+
+    if (nm_streq0(band, "a")) {
+        rnd_freqs     = a_freqs;
+        rnd_freqs_len = G_N_ELEMENTS(a_freqs) - 1;
+    } else {
+        rnd_freqs     = bg_freqs;
+        rnd_freqs_len = G_N_ELEMENTS(bg_freqs) - 1;
+    }
+
+    /* shuffle the frequencies (inplace). The idea is to choose
+     * a different frequency depending on the SSID. */
+    for (i = 0, l = rnd_freqs_len; l > 1; i++, l--) {
+        /* Add an arbitrary chosen (prime) number to rnd, to get more "random"
+         * numbers. Since we only shuffle a handful of elements, that's good
+         * enough (and stable). */
+        rnd += 5630246189u;
+        NM_SWAP(&rnd_freqs[i], &rnd_freqs[i + (rnd % l)]);
+    }
+
+    freq = nm_platform_wifi_find_frequency(nm_device_get_platform(device),
+                                           nm_device_get_ifindex(device),
+                                           rnd_freqs);
+    if (freq == 0)
+        freq = rnd_freqs[0];
+
+    _LOGD(LOGD_WIFI, "set frequency for hotspot AP to %u", freq);
 
     if (nm_wifi_ap_set_freq(ap, freq))
         _ap_dump(self, LOGL_DEBUG, ap, "updated", 0);

src/libnm-platform/wifi/nm-wifi-utils-nl80211.c

@@ -384,8 +384,10 @@ wifi_nl80211_find_freq(NMWifiUtils *data, const guint32 *freqs)
     int                 i;
     int                 j;
 
-    for (i = 0; i < self->num_freqs; i++) {
-        for (j = 0; freqs[j] != 0; j++) {
+    /* It's important to check the values in the order of @freqs, because
+     * that array might be sorted to contain preferred frequencies first. */
+    for (j = 0; freqs[j] != 0; j++) {
+        for (i = 0; i < self->num_freqs; i++) {
             if (self->freqs[i] == freqs[j])
                 return freqs[j];
         }

src/libnm-platform/wifi/nm-wifi-utils-wext.c

@@ -255,8 +255,10 @@ wifi_wext_find_freq(NMWifiUtils *data, const guint32 *freqs)
     guint            i;
     guint            j;
 
-    for (i = 0; i < wext->num_freqs; i++) {
-        for (j = 0; freqs[j] != 0; j++) {
+    /* It's important to check the values in the order of @freqs, because
+     * that array might be sorted to contain preferred frequencies first. */
+    for (j = 0; freqs[j] != 0; j++) {
+        for (i = 0; i < wext->num_freqs; i++) {
             if (wext->freqs[i] == freqs[j])
                 return freqs[j];
         }