Fix Windows launcher shebang bounds

launcher.c

@@ -247,7 +247,7 @@ char* join_executable_and_args(char *executable, char **args, int argc)
 
 int run(int argc, char **argv, int is_gui) {
 
-    char python[256];   /* python executable's filename*/
+    char python[257];   /* python executable's filename*/
     char *pyopt;        /* Python option */
     char script[256];   /* the script's filename */
 
@@ -256,7 +256,8 @@ int run(int argc, char **argv, int is_gui) {
     char **newargs, **newargsp, **parsedargs; /* argument array for exec */
     char *ptr, *end;    /* working pointers for string manipulation */
     char *cmdline;
-    int i, parsedargc;              /* loop counter */
+    int i, parsedargc;  /* loop counter */
+    int bytes_read;
 
     /* compute script name from our .exe name*/
     GetModuleFileNameA(NULL, script, sizeof(script));
@@ -282,13 +283,17 @@ int run(int argc, char **argv, int is_gui) {
     if (scriptf == -1) {
         return fail("Cannot open %s\n", script);
     }
-    end = python + read(scriptf, python, sizeof(python));
+    bytes_read = read(scriptf, python, sizeof(python) - 1);
     close(scriptf);
+    if (bytes_read < 0) {
+        return fail("Cannot read %s\n", script);
+    }
+    python[bytes_read] = '\0';
 
-    ptr = python-1;
-    while(++ptr < end && *ptr && *ptr!='\n' && *ptr!='\r') {;}
+    ptr = python;
+    while (*ptr && *ptr!='\n' && *ptr!='\r') ptr++;
 
-    *ptr-- = '\0';
+    *ptr = '\0';
 
     if (strncmp(python, "#!", 2)) {
         /* default to python.exe if no #! header */

newsfragments/+launcher-shebang-bounds.bugfix.rst

@@ -0,0 +1 @@
+Prevent the Windows launcher from writing past its shebang buffer when the first 256 bytes contain no newline.

setuptools/tests/test_windows_wrappers.py

@@ -12,8 +12,10 @@
 are to wrap.
 """
 
+import os
 import pathlib
 import platform
+import shutil
 import subprocess
 import sys
 import textwrap
@@ -256,3 +258,88 @@ def test_basic(self, tmpdir):
         with (tmpdir / 'test_output.txt').open('rb') as f_out:
             actual = f_out.read().decode('ascii')
         assert actual == repr('Test Argument')
+
+
+@pytest.fixture(scope='module')
+def source_launcher(tmp_path_factory):  # pragma: no cover
+    repo_root = pathlib.Path(__file__).parents[2]
+    build_dir = tmp_path_factory.mktemp('launcher-build')
+    if platform.machine() == 'ARM64':
+        cmake_architecture, runtime_architecture = 'ARM64', 'arm64'
+    elif sys.maxsize <= 2**32:
+        cmake_architecture, runtime_architecture = 'Win32', 'x86'
+    else:
+        cmake_architecture, runtime_architecture = 'x64', 'x64'
+    subprocess.run(
+        [
+            'cmake',
+            '-S',
+            str(repo_root / 'launcher'),
+            '-B',
+            str(build_dir),
+            '-A',
+            cmake_architecture,
+            '-DGUI=0',
+            '-DCMAKE_C_FLAGS_RELEASE=/O1 /fsanitize=address',
+        ],
+        check=True,
+    )
+    subprocess.run(
+        ['cmake', '--build', str(build_dir), '--config', 'Release'],
+        check=True,
+    )
+    vswhere = (
+        pathlib.Path(os.environ['ProgramFiles(x86)'])
+        / 'Microsoft Visual Studio'
+        / 'Installer'
+        / 'vswhere.exe'
+    )
+    installation = pathlib.Path(
+        subprocess.check_output(
+            [
+                vswhere,
+                '-latest',
+                '-products',
+                '*',
+                '-property',
+                'installationPath',
+            ],
+            text=True,
+            encoding='utf-8',
+        ).strip()
+    )
+    toolset = (
+        (
+            installation
+            / 'VC'
+            / 'Auxiliary'
+            / 'Build'
+            / 'Microsoft.VCToolsVersion.default.txt'
+        )
+        .read_text(encoding='utf-8')
+        .strip()
+    )
+    runtime_by_name = {
+        path.name: path
+        for path in (installation / 'VC' / 'Tools' / 'MSVC' / toolset / 'bin').glob(
+            f'Host*/{runtime_architecture}/clang_rt.asan_dynamic-*.dll'
+        )
+    }
+    assert len(runtime_by_name) == 1
+    runtime = next(iter(runtime_by_name.values()))
+    return build_dir / 'Release' / 'launcher.exe', runtime
+
+
+def test_maximum_shebang_stays_within_buffer(  # pragma: no cover
+    tmp_path, source_launcher
+):
+    launcher, sanitizer_runtime = source_launcher
+    wrapper = tmp_path / 'foo.exe'
+    shutil.copyfile(launcher, wrapper)
+    shutil.copyfile(sanitizer_runtime, tmp_path / sanitizer_runtime.name)
+
+    shebang = f'#!{subprocess.list2cmdline([sys.executable])}'.encode()
+    assert len(shebang) <= 256
+    (tmp_path / 'foo-script.py').write_bytes(shebang.ljust(256, b' '))
+
+    subprocess.run([wrapper], check=True, timeout=30)