fix(dalvikvm): bypass W^X restriction

[twaik]

Fixes termux/termux-packages#18557

[agnostic-apollo]

1. Assuming this is actually working, random paths outside of Termux rootfs MUST NOT be automatically assumed to be safe and returned as non-writable, especially ones on sd card.

   int access_syscall(const char *path, int mode);
   
   __attribute__((visibility("default")))
   int access(const char *path, int mode) {
       if (mode == W_OK && path != null) {
           char canonical_path[PATH_MAX];
           if (canonicalize((char *)path, canonical_path, PATH_MAX) < 0) {
               //strerror error out with "Bad pathname"
               return -1;
           }
   
           if (string_ends_with(canonical_path, ".apk") || string_ends_with(canonical_path, ".dex")) {
               int is_file_under_termux_rootfs_dir = is_path_in_dir_path("termux_rootfs_dir", canonical_path, TERMUX_BASE_DIR, true);
               if (is_file_under_termux_rootfs_dir < 0) {
                   return -1;
               }
   
               if (is_file_under_termux_rootfs_dir == 0) {
                   return 1;
               }
           }
       }
   
       // Call the original access function for other cases.
       return access_syscall(path, mode);
   }
   
   int access_syscall(const char *path, int mode) {
       return syscall(SYS_access, path, mode);
   }

   You can get called functions from following links. Can replace logger calls with manual print calls if required.

   • https://cs.android.com/android/platform/superproject/+/android-14.0.0_r1:libcore/ojluni/src/main/native/canonicalize_md.c
   • https://github.com/termux/termux-exec/blob/bb75f48d646aa0478527196f1878df53cd1a257c/src/data/data_utils.c#L24
   • https://github.com/termux/termux-exec/blob/bb75f48d646aa0478527196f1878df53cd1a257c/src/file/file_utils.c#L125

   Should likely be able to use syscall instead of dlsym as termux-exec does.

   • https://github.com/termux/termux-exec/blob/bb75f48d646aa0478527196f1878df53cd1a257c/src/exec/exec.c#L79

2. Compile library with -fvisibility=hidden and add __attribute__((visibility("default"))) manually to any functions that should be exported, like access().

3. Rename library to libtermux-dexfile.

4. Add -DTERMUX_BASE_DIR to Makefile, ideally should be read dynamically as termux-exec pull, but can be done later.