Use stable random seed

native/src/base/misc.cpp

@@ -33,23 +33,29 @@ int fork_no_orphan() {
     return 0;
 }
 
-int gen_rand_str(char *buf, int len, bool varlen) {
-    constexpr char ALPHANUM[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
-    static std::mt19937 gen([]{
-        if (access("/dev/urandom", F_OK) != 0)
-            mknod("/dev/urandom", 0600 | S_IFCHR, makedev(1, 9));
+int gen_rand_str(char *buf, int len, const void *seed_buf, bool varlen) {
+    static mt19937_64 gen([&] {
+        mt19937_64::result_type seed;
+        if (seed_buf == nullptr) {
             int fd = xopen("/dev/urandom", O_RDONLY | O_CLOEXEC);
-        unsigned seed;
             xxread(fd, &seed, sizeof(seed));
+            close(fd);
+        } else {
+            memcpy(&seed, seed_buf, sizeof(seed));
+        }
         return seed;
     }());
-    std::uniform_int_distribution<int> dist(0, sizeof(ALPHANUM) - 2);
+
+    if (len == 0)
+        return 0;
     if (varlen) {
         std::uniform_int_distribution<int> len_dist(len / 2, len);
         len = len_dist(gen);
     }
-    for (int i = 0; i < len - 1; ++i)
-        buf[i] = ALPHANUM[dist(gen)];
+    std::uniform_int_distribution<int> alphabet('a', 'z');
+    for (int i = 0; i < len - 1; ++i) {
+        buf[i] = static_cast<char>(alphabet(gen));
+    }
     buf[len - 1] = '\0';
     return len - 1;
 }
@@ -134,19 +140,36 @@ void set_nice_name(const char *name) {
     prctl(PR_SET_NAME, name);
 }
 
+template<typename T>
+static T parse_num(string_view s, int base) {
+    T acc = 0;
+    for (char c: s) {
+        if (isdigit(c)) {
+            c -= '0';
+        } else if (base > 10 && isalpha(c)) {
+            c -= isupper(c) ? 'A' - 10 : 'a' - 10;
+        } else {
+            return -1;
+        }
+        if (c >= base) {
+            return -1;
+        }
+        acc *= base;
+        acc += c;
+    }
+    return acc;
+}
+
 /*
  * Bionic's atoi runs through strtol().
  * Use our own implementation for faster conversion.
  */
 int parse_int(string_view s) {
-    int val = 0;
-    for (char c : s) {
-        if (!c) break;
-        if (c > '9' || c < '0')
-            return -1;
-        val = val * 10 + c - '0';
-    }
-    return val;
+    return parse_num<int>(s, 10);
+}
+
+uint64_t parse_uint64_hex(string_view s) {
+    return parse_num<uint64_t>(s, 16);
 }
 
 uint32_t binary_gcd(uint32_t u, uint32_t v) {

native/src/base/misc.hpp

@@ -125,6 +125,7 @@ rust::Slice<uint8_t> byte_slice(T *buf, size_t sz) {
     return rust::Slice(reinterpret_cast<uint8_t *>(buf), sz);
 }
 
+uint64_t parse_uint64_hex(std::string_view s);
 int parse_int(std::string_view s);
 
 using thread_entry = void *(*)(void *);
@@ -158,7 +159,7 @@ void init_argv0(int argc, char **argv);
 void set_nice_name(const char *name);
 uint32_t binary_gcd(uint32_t u, uint32_t v);
 int switch_mnt_ns(int pid);
-int gen_rand_str(char *buf, int len, bool varlen = true);
+int gen_rand_str(char *buf, int len, const void *seed_buf = nullptr, bool varlen = true);
 std::string &replace_all(std::string &str, std::string_view from, std::string_view to);
 std::vector<std::string> split(const std::string &s, const std::string &delims);
 std::vector<std::string_view> split_ro(std::string_view, std::string_view delims);

native/src/init/rootdir.cpp

@@ -199,13 +199,17 @@ static void extract_files(bool sbin) {
 }
 
 void MagiskInit::parse_config_file() {
-    parse_prop_file("/data/.backup/.magisk", [this](auto key, auto value) -> bool {
+    uint64_t seed = 0;
+    parse_prop_file("/data/.backup/.magisk", [&](auto key, auto value) -> bool {
         if (key == "PREINITDEVICE") {
             preinit_dev = value;
-            return false;
+        } else if (key == "RANDOMSEED") {
+            value.remove_prefix(2); // 0x
+            seed = parse_uint64_hex(value);
         }
         return true;
     });
+    gen_rand_str(nullptr, 0, &seed);
 }
 
 #define ROOTMIR     MIRRDIR "/system_root"
@@ -220,7 +224,7 @@ void MagiskInit::patch_ro_root() {
     if (access("/sbin", F_OK) == 0) {
         tmp_dir = "/sbin";
     } else {
-        char buf[8];
+        char buf[16];
         gen_rand_str(buf, sizeof(buf));
         tmp_dir = "/dev/"s + buf;
         xmkdir(tmp_dir.data(), 0);

native/src/su/connect.cpp

@@ -194,7 +194,7 @@ int app_request(const su_context &ctx) {
     // Create FIFO
     char fifo[64];
     strcpy(fifo, "/dev/socket/");
-    gen_rand_str(fifo + 12, 32, true);
+    gen_rand_str(fifo + 12, 32);
     mkfifo(fifo, 0600);
     chown(fifo, ctx.info->mgr_uid, ctx.info->mgr_uid);
     setfilecon(fifo, "u:object_r:" SEPOL_FILE_TYPE ":s0");

scripts/avd_patch.sh

@@ -70,6 +70,8 @@ fi
 # For API 28, we also patch advancedFeatures.ini to disable SAR
 # Manually override skip_initramfs by setting RECOVERYMODE=true
 [ $API = "28" ] && echo 'RECOVERYMODE=true' >> config
+RANDOMSEED=$(tr -dc 'a-f0-9' < /dev/urandom | head -c 16)
+echo "RANDOMSEED=0x$RANDOMSEED" >> config
 
 ./magiskboot compress=xz magisk32 magisk32.xz
 ./magiskboot compress=xz magisk64 magisk64.xz

scripts/boot_patch.sh

@@ -173,6 +173,8 @@ if [ -n "$PREINITDEVICE" ]; then
   echo "PREINITDEVICE=$PREINITDEVICE" >> config
 fi
 [ -n "$SHA1" ] && echo "SHA1=$SHA1" >> config
+RANDOMSEED=$(tr -dc 'a-f0-9' < /dev/urandom | head -c 16)
+echo "RANDOMSEED=0x$RANDOMSEED" >> config
 
 ./magiskboot cpio ramdisk.cpio \
 "add 0750 $INIT magiskinit" \