Add b64xz to handle busybox decode/decompress in scripts

jni/Android.mk

@@ -74,9 +74,18 @@ LOCAL_SRC_FILES := \
 LOCAL_CFLAGS := -DZLIB_CONST
 include $(BUILD_EXECUTABLE)
 
-# busybox (32-bit only)
+# 32-bit static binaries
 ifneq ($(TARGET_ARCH_ABI), x86_64)
 ifneq ($(TARGET_ARCH_ABI), arm64-v8a)
+# b64xz
+include $(CLEAR_VARS)
+LOCAL_MODULE := b64xz
+LOCAL_STATIC_LIBRARIES := liblzma
+LOCAL_C_INCLUDES := $(LOCAL_PATH)/ndk-compression/xz/src/liblzma/api
+LOCAL_SRC_FILES := b64xz.c
+LOCAL_LDFLAGS := -static
+include $(BUILD_EXECUTABLE)
+# Busybox
 include jni/busybox/Android.mk
 endif
 endif

jni/b64xz.c

@@ -0,0 +1,83 @@
+/* b64xz.c - Base64-XZ Extractor
+ *
+ * This program expects data from stdin. The data should be compressed with xz and
+ * then encoded into base64 format. What b64xz does is basically the reverse of the
+ * mentioned process: decode base64 to uint8_ts, decompress xz, then dump to stdout
+ *
+ * The compiled binary will be hex-dumped into update-binary
+ * Busybox will be xz-compressed, base64 encoded and dumped into update-binary
+ * This program is to recover busybox for Magisk installation environment
+ *
+ * I intentionally removed stdio. This will result in a smaller binary size because
+ * all I/O are handled by system calls (read/write) instead of libc wrappers
+ */
+
+#include <unistd.h>
+#include <lzma.h>
+
+#define BUFSIZE 8192
+
+static const char trans_tbl[] =
+		"|$$$}rstuvwxyz{$$$$$$$>?@ABCDEFGHIJKLMNOPQRSTUVW$$$$$$XYZ[\\]^_`abcdefghijklmnopq";
+
+static void decodeblock(uint8_t* in, uint8_t* out) {
+	out[0] = (uint8_t)(in[0] << 2 | in[1] >> 4);
+	out[1] = (uint8_t)(in[1] << 4 | in[2] >> 2);
+	out[2] = (uint8_t)(((in[2] << 6) & 0xc0) | in[3]);
+}
+
+static int unxz(lzma_stream *strm, void *buf, size_t size) {
+	lzma_ret ret = 0;
+	uint8_t out[BUFSIZE];
+	strm->next_in = buf;
+	strm->avail_in = size;
+	do {
+		strm->next_out = out;
+		strm->avail_out = sizeof(out);
+		ret = lzma_code(strm, LZMA_RUN);
+		write(STDOUT_FILENO, out, sizeof(out) - strm->avail_out);
+	} while (strm->avail_out == 0 && ret == LZMA_OK);
+
+	if (ret != LZMA_OK && ret != LZMA_STREAM_END)
+		write(STDERR_FILENO, "LZMA error!\n", 13);
+	return ret;
+}
+
+int main(int argc, char const* argv[]) {
+	if (argc > 1)
+		return 0;
+
+	uint8_t in[4], buf[BUFSIZE];
+	int len = 0, pos = 0;
+	char c;
+
+	// Setup lzma stream
+	lzma_stream strm = LZMA_STREAM_INIT;
+	if (lzma_auto_decoder(&strm, UINT64_MAX, 0) != LZMA_OK) {
+		write(STDERR_FILENO, "Unable to init lzma stream\n", 28);
+		return 1;
+	}
+
+	while (read(STDIN_FILENO, &c, sizeof(c)) == 1) {
+		c = ((c < 43 || c > 122) ? -1 : (trans_tbl[c - 43] == '$' ? -1 : trans_tbl[c - 43] - 62));
+		if (c >= 0)
+			in[len++] = c;
+		if (len < 4)
+			continue;
+		len = 0;
+		decodeblock(in, buf + pos);
+		pos += 3;
+		if (pos > sizeof(buf) - 3) {
+			// Buffer is full, unxz
+			if (unxz(&strm, buf, pos))
+				return 1;
+			pos = 0;
+		}
+	}
+	if (pos) {
+		if (unxz(&strm, buf, pos))
+			return 1;
+	}
+	lzma_end(&strm);
+	return 0;
+}