fix(dalvikvm): bypass W^X restriction

termux/termux-packages#18557

Author: twaik

configure.ac

@@ -29,6 +29,7 @@ AM_INIT_AUTOMAKE([foreign])
 AC_PROG_MAKE_SET
 AC_PROG_CC
 AC_LANG(C)
+LT_INIT
 
 copyright="Copyright (C) 2022-2024 Termux."
 if test "${TERMUX_APP_PACKAGE+set}" = set; then

scripts/dalvikvm.in

@@ -11,4 +11,5 @@ else
 fi
 
 unset LD_LIBRARY_PATH LD_PRELOAD
+export LD_PRELOAD=@TERMUX_PREFIX@/lib/termux/libtermux-access.so
 exec "$DALVIKVM" -Xusejit:true -Xnoimage-dex2oat -Djava.io.tmpdir=@TERMUX_PREFIX@/tmp "$@"

src/Makefile.am

@@ -21,3 +21,13 @@ AM_CFLAGS = -Wall -Wextra -pedantic
 bin_PROGRAMS = cmd
 
 cmd_SOURCES = cmd.c
+
+libdir = $(prefix)/lib/termux
+
+lib_LTLIBRARIES = libtermux-access.la
+
+libtermux_access_la_SOURCES = termux_access.c
+libtermux_access_la_LDFLAGS = -shared -module -avoid-version -ldl
+
+clean-local:
+	rm -f libtermux-access.la

src/termux_access.c

@@ -0,0 +1,37 @@
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+#include <stdio.h>
+
+// This library is needed to bypass W^X restriction in Dalvik code.
+// https://cs.android.com/android/platform/superproject/+/dd6fde63193fda968440fc0a4cd9ccee5d282ab0:art/runtime/native/dalvik_system_DexFile.cc;l=381
+
+static int stub_access(const char* __unused path, int __unused mode) {
+    return -1;
+}
+
+// Function pointer for the original access function
+static int (*orig_access)(const char *path, int mode) = stub_access;
+
+// Constructor to load the original access function
+__attribute__((constructor, visibility("hidden")))
+static void load_orig_access(void) {
+    orig_access = (int (*)(const char *, int))dlsym(RTLD_NEXT, "access");
+    if (orig_access == NULL) {
+        dprintf(2, "Error in `dlsym`: %s\n", dlerror());
+        orig_access = stub_access;
+    }
+}
+
+int access(const char *path, int mode) {
+    // Checking if path ends with ".apk", if ".apk" is an extension and if mode is W_OK
+    const char *apk = path == NULL ? NULL : strstr(path, ".apk");
+    if (mode == W_OK && apk != NULL && !strcmp(apk, ".apk"))
+        return 1;
+
+    // Call the original access function for other cases
+    return orig_access(path, mode);
+}
+