When EM_CACHE is overridden, Permission Denied crash occurs when installing system headers

[Technius]

When emscripten is installed to a location with read-only permissions (such as when installed with Nix) and the EM_CACHE environment variable is set to a folder with read-write-execute permissions, the emcc command still crashes when installing the system headers in the EM_CACHE.

---

Version of emscripten/emsdk:
Please include the output emcc -v here

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.8-git
clang version 20.1.4
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /nix/store/cf6j1f9f0z1syny3dgbickzc42zgcxhf-clang-20.1.4/bin

This bug does not occur with emcc 3.1.73

Failing command line in full:
If this is compile or link-time failure please include the full failing command
along with its entire output.

Log

Using an example.cpp with the contents:

#include <iostream>

int main(int argc, char** argv) {
    return 0;
}

Running emcc:

% EM_CACHE="$PWD"/emscriptencache emcc example.cpp
cache:INFO: generating system headers: sysroot_install.stamp... (this will be cached in "/private/tmp/example/emscriptencache/sysroot_install.stamp" for subsequent builds)
Traceback (most recent call last):
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/emcc.py", line 1597, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/nix/store/v0vy1c59f0vf71h64caw17hzymvg0zms-python3-3.12.10/lib/python3.12/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/emcc.py", line 1590, in main
    ret = run(args)
          ^^^^^^^^^
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/emcc.py", line 670, in run
    linker_args = phase_compile_inputs(options, state, newargs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/v0vy1c59f0vf71h64caw17hzymvg0zms-python3-3.12.10/lib/python3.12/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/emcc.py", line 919, in phase_compile_inputs
    system_libs.ensure_sysroot()
  File "/nix/store/v0vy1c59f0vf71h64caw17hzymvg0zms-python3-3.12.10/lib/python3.12/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/tools/system_libs.py", line 2519, in ensure_sysroot
    cache.get('sysroot_install.stamp', install_system_headers, what='system headers')
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/tools/cache.py", line 179, in get
    creator(str(cachename))
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/tools/system_libs.py", line 2486, in install_system_headers
    copytree_exist_ok(src, dest)
  File "/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/tools/system_libs.py", line 2463, in copytree_exist_ok
    shutil.copytree(src, dst, dirs_exist_ok=True)
  File "/nix/store/v0vy1c59f0vf71h64caw17hzymvg0zms-python3-3.12.10/lib/python3.12/shutil.py", line 600, in copytree
    return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/v0vy1c59f0vf71h64caw17hzymvg0zms-python3-3.12.10/lib/python3.12/shutil.py", line 554, in _copytree
    raise Error(errors)
shutil.Error: [('/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/system/lib/compiler-rt/include/sanitizer', '/private/tmp/example/emscriptencache/sysroot/include/sanitizer', "[Errno 13] Permission denied: '/private/tmp/example/emscriptencache/sysroot/include/sanitizer'")]

Example of permissions in system installation (/nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/system/lib/compiler-rt/include/sanitizer):

total 692
dr-xr-xr-x 18 root nixbld    576 Dec 31  1969 .
dr-xr-xr-x  3 root nixbld     96 Dec 31  1969 ..
-r--r--r--  1 root nixbld   4973 Dec 31  1969 allocator_interface.h
-r--r--r--  1 root nixbld  13654 Dec 31  1969 asan_interface.h
-r--r--r--  1 root nixbld  20763 Dec 31  1969 common_interface_defs.h
-r--r--r--  1 root nixbld   1133 Dec 31  1969 coverage_interface.h
-r--r--r--  1 root nixbld  10102 Dec 31  1969 dfsan_interface.h
-r--r--r--  1 root nixbld   5043 Dec 31  1969 hwasan_interface.h
-r--r--r--  1 root nixbld 219473 Dec 31  1969 linux_syscall_hooks.h
-r--r--r--  1 root nixbld   3993 Dec 31  1969 lsan_interface.h
-r--r--r--  1 root nixbld   2648 Dec 31  1969 memprof_interface.h
-r--r--r--  1 root nixbld   5992 Dec 31  1969 msan_interface.h
-r--r--r--  1 root nixbld 355693 Dec 31  1969 netbsd_syscall_hooks.h
-r--r--r--  1 root nixbld   2835 Dec 31  1969 nsan_interface.h
-r--r--r--  1 root nixbld   1513 Dec 31  1969 scudo_interface.h
-r--r--r--  1 root nixbld  14952 Dec 31  1969 tsan_interface.h
-r--r--r--  1 root nixbld  11232 Dec 31  1969 tsan_interface_atomic.h
-r--r--r--  1 root nixbld   1044 Dec 31  1969 ubsan_interface.h

Note that the corresponding folder in EM_CACHE -- /private/tmp/example/emscriptencache/sysroot/include/sanitizer -- is not actually created.

The files that do get installed have the permission bits copied from the system installation:

ls -la emscriptencache/sysroot/include/fakesdl | head -n 5
total 212
dr-xr-xr-x 55 bryantan wheel 1760 Dec 31  1969 .
dr-xr-xr-x 21 bryantan wheel  672 Dec 31  1969 ..
-r--r--r--  1 bryantan wheel   72 Dec 31  1969 SDL.h
-r--r--r--  1 bryantan wheel   72 Dec 31  1969 SDL_assert.h

---

A workaround for this bug is to copy the system installation of the cache manually and change the permissions bits before running emcc.

EM_CACHE="$PWD"/emscriptencache
cp -r /nix/store/bzq9l9651iqfk63glxdd3z7lxl10lilz-emscripten-4.0.8/share/emscripten/cache "$EM_CACHE"
chmod u+rwX -R "$EM_CACHE"
emcc example.cpp

[Technius]

The affected lines are:

emscripten/tools/system_libs.py

Lines 2476 to 2480 in ebccc05

	target_include_dir = cache.get_include_dir()
	for src, dest in install_dirs.items():
	src = utils.path_from_root('system', *src)
	dest = os.path.join(target_include_dir, dest)
	copytree_exist_ok(src, dest)

Based on some experimentation, shutill.copytree reports PermissionDenied when the parent folder of the destination path doesn't exist. So the real issue does not seem to be the permission bits; rather, it is because the parent folders are not created first.

[sbc100]

Hmm, I'm having trouble reproducing.

I tried EM_CACHE=/tmp/foo/bar/baz ./emcc ~/test/hello.c -c and the sysroot was successfully created in /tmp/foo/bar/baz/sysroot/. So there must be something else going on here other than the parent not existing.

[sbc100]

I was able to reproduce but maybe a read-only, root-owned copy of emscripten. Taking a look now.