Add preliminary zygote code injection support

Prototyping the injection setup and a clean "self unloading" mechanism.

native/jni/inject/entry.cpp

@@ -0,0 +1,124 @@
+#include <libgen.h>
+#include <dlfcn.h>
+#include <sys/mount.h>
+#include <sys/sendfile.h>
+#include <android/log.h>
+#include <atomic>
+
+#include <utils.hpp>
+
+#include "inject.hpp"
+
+using namespace std;
+
+static void *self_handle = nullptr;
+static atomic<int> active_threads = -1;
+
+#define alog(prio) [](auto fmt, auto ap){ \
+return __android_log_vprint(ANDROID_LOG_##prio, "Magisk", fmt, ap); }
+static void inject_logging() {
+    log_cb.d = alog(DEBUG);
+    log_cb.i = alog(INFO);
+    log_cb.w = alog(WARN);
+    log_cb.e = alog(ERROR);
+    log_cb.ex = nop_ex;
+}
+
+__attribute__((destructor))
+static void inject_cleanup() {
+    if (active_threads < 0)
+        return;
+
+    // Setup 1ms
+    timespec ts = { .tv_sec = 0, .tv_nsec = 1000000L };
+
+    // Check flag in busy loop
+    while (active_threads)
+        nanosleep(&ts, nullptr);
+
+    // Wait another 1ms to make sure all threads left our code
+    nanosleep(&ts, nullptr);
+}
+
+static inline void self_unload() {
+    new_daemon_thread(reinterpret_cast<void *(*)(void *)>(&dlclose), self_handle);
+    active_threads--;
+}
+
+static void *unload_first_stage(void *) {
+    // Setup 1ms
+    timespec ts = { .tv_sec = 0, .tv_nsec = 1000000L };
+
+    while (getenv(INJECT_ENV_2))
+        nanosleep(&ts, nullptr);
+
+    // Wait another 1ms to make sure all threads left our code
+    nanosleep(&ts, nullptr);
+
+    unmap_all(INJECT_LIB_1);
+    active_threads--;
+    return nullptr;
+}
+
+__attribute__((constructor))
+static void inject_init() {
+    inject_logging();
+    if (char *env = getenv(INJECT_ENV_1)) {
+        LOGD("zygote: inject 1st stage\n");
+
+        if (env[0] == '1')
+            unsetenv("LD_PRELOAD");
+        else
+            setenv("LD_PRELOAD", env, 1);  // Restore original LD_PRELOAD
+        unsetenv(INJECT_ENV_1);
+
+        // Setup second stage
+        setenv(INJECT_ENV_2, "1", 1);
+        cp_afc(INJECT_LIB_1, INJECT_LIB_2);
+        dlopen(INJECT_LIB_2, RTLD_LAZY);
+    } else if (getenv(INJECT_ENV_2)) {
+        LOGD("zygote: inject 2nd stage\n");
+
+        active_threads = 1;
+
+        // Get our own handle
+        self_handle = dlopen(INJECT_LIB_2, RTLD_LAZY);
+        dlclose(self_handle);
+
+        // Cleanup 1st stage maps
+        active_threads++;
+        new_daemon_thread(&unload_first_stage);
+        unsetenv(INJECT_ENV_2);
+
+        // TODO: actually inject stuffs, for now we demonstrate clean self unloading
+        self_unload();
+    }
+}
+
+int app_process_main(int argc, char *argv[]) {
+    inject_logging();
+    char buf[4096];
+    if (realpath("/proc/self/exe", buf) == nullptr)
+        return 1;
+
+    int in = xopen(buf, O_RDONLY);
+    int out = xopen(INJECT_LIB_1, O_CREAT | O_WRONLY | O_TRUNC, 0777);
+    sendfile(out, in, nullptr, INT_MAX);
+    close(in);
+    close(out);
+
+    if (char *ld = getenv("LD_PRELOAD")) {
+        char env[128];
+        sprintf(env, "%s:" INJECT_LIB_1, ld);
+        setenv("LD_PRELOAD", env, 1);
+        setenv(INJECT_ENV_1, ld, 1);  // Backup original LD_PRELOAD
+    } else {
+        setenv("LD_PRELOAD", INJECT_LIB_1, 1);
+        setenv(INJECT_ENV_1, "1", 1);
+    }
+
+    // Execute real app_process
+    xumount2(buf, MNT_DETACH);
+    execve(buf, argv, environ);
+    exit(1);
+}