diff --git a/docs/TROUBLESHOOTING.md b/docs/TROUBLESHOOTING.md
index f33f4f8..6cf198f 100644
--- a/docs/TROUBLESHOOTING.md
+++ b/docs/TROUBLESHOOTING.md
@@ -31,7 +31,7 @@ Use the [setup instructions](SETUP.md) from the repository root as your ordinary
 - **kernel** 5.14 or newer for `MADV_POPULATE_WRITE`.
 - **llvm** 16 or newer; 22 or 23 is what the patches are tested with. Iridium calls `LLVMGetVersion`, which LLVM 15 does not export, so an LLVM 15 runtime aborts as soon as Riot creates a Metal device ([#139](https://github.com/lucannez64/darling-lol/issues/139)). LLVM 16-21 builds with a warning.
 - **llvm-tools**: `llvm-objdump` and `llvm-nm`. They are now resolved as an unversioned name, as `llvm-<tool>-<N>`, or from `llvm-config --bindir`, so Ubuntu's `llvm-15-dev` no longer needs symlinks in `/usr/local/bin`.
-- **vulkan** API 1.3 and timeline semaphores. Indium rejects older devices, so this stops the setup with the version `vulkaninfo` reports. Without `vulkaninfo` it only warns, because the tool may be missing even when the driver is fine.
+- **vulkan** API 1.3 and timeline semaphores. Indium rejects older devices, so this stops the setup with the version `vulkaninfo` reports. An installed `vulkaninfo` that fails or times out is also required to be fixed, because the loader or the ICD files are then broken; without `vulkaninfo` at all it only warns, because the tool may be missing even when the driver is fine.
 - **x11** only warns: `DISPLAY` is probed with `xdpyinfo`, and the GL/EGL vendor is reported when a probe is installed. An inaccessible display is what made `iokitd` restart in a loop before [#140](https://github.com/lucannez64/darling-lol/issues/140); export `XAUTHORITY` from servers and SSH sessions. A build without a display is still valid.
 
 Run the same checks by hand, from the checkout:
diff --git a/scripts/preflight.py b/scripts/preflight.py
index b66f790..3dc72f3 100644
--- a/scripts/preflight.py
+++ b/scripts/preflight.py
@@ -27,7 +27,7 @@ MIN_LFS = (2, 13)      # Debian 11 ships 2.13, the oldest release the setup is t
 MIN_KERNEL = (5, 14)   # MADV_POPULATE_WRITE.
 MIN_VULKAN = (1, 3)    # Indium rejects older devices and needs timeline semaphores.
 
-SOURCE = ROOT / 'build/darling-src'
+SOURCE = Path(os.environ.get('RCLOL_PREFLIGHT_SOURCE') or ROOT / 'build/darling-src')
 POINTER = b'version https://git-lfs.github.com/spec/v1'
 LIBRARY_SUFFIXES = ('.dylib', '.a', '.so', '.o')
 
@@ -162,6 +162,12 @@ def vulkan_devices():
 
 def check_vulkan(report):
     devices = vulkan_devices()
+    if devices is None and shutil.which('vulkaninfo'):
+        report('vulkan', False, 'vulkaninfo did not finish',
+               'vulkaninfo is installed but exited with an error or timed out. Run '
+               '`vulkaninfo --summary` for the loader or ICD error, then fix or reinstall the '
+               'driver: the runtime needs Vulkan 1.3 and timeline semaphores.')
+        return
     if devices is None:
         report('vulkan', False, 'vulkaninfo not available',
                'Install vulkan-tools to check the device. The runtime needs Vulkan 1.3 and '
diff --git a/tests/test_preflight.py b/tests/test_preflight.py
index 3cfbc29..d999061 100644
--- a/tests/test_preflight.py
+++ b/tests/test_preflight.py
@@ -53,7 +53,9 @@ esac''')
         return path
 
     def preflight(self, *arguments, kernel=KERNEL, display=':0'):
-        env = dict(os.environ, PATH=str(self.bin), RCLOL_PREFLIGHT_KERNEL_RELEASE=kernel)
+        env = dict(os.environ, PATH=str(self.bin),
+                   RCLOL_PREFLIGHT_KERNEL_RELEASE=kernel,
+                   RCLOL_PREFLIGHT_SOURCE=str(Path(self.tmp.name) / 'not fetched'))
         if display is None:
             env.pop('DISPLAY', None)
         else:
@@ -157,6 +159,24 @@ esac''')
         self.assertIn('Vulkan 1.3 or newer is required', result.stdout)
 
 
+    def test_missing_vulkaninfo_only_warns(self):
+        (self.bin / 'vulkaninfo').unlink()
+        result = self.preflight()
+        self.assertEqual(result.returncode, 0, result.stdout + result.stderr)
+        self.assertIn('warn vulkan', result.stdout)
+        self.assertIn('vulkaninfo not available', result.stdout)
+        self.assertIn('Install vulkan-tools', result.stdout)
+
+
+    def test_broken_vulkaninfo_is_required(self):
+        self.stub('vulkaninfo', 'exit 1')
+        result = self.preflight()
+        self.assertEqual(result.returncode, 1)
+        self.assertIn('FAIL vulkan', result.stdout)
+        self.assertIn('vulkaninfo did not finish', result.stdout)
+        self.assertIn('timed out', result.stdout)
+
+
     def test_missing_timeline_semaphores_is_reported(self):
         self.stub('vulkaninfo', '''case "$*" in
  --summary) echo 'apiVersion = 1.3.289';;
@@ -287,6 +307,21 @@ class LlvmToolsFixture(unittest.TestCase):
         self.assertEqual(self.tools.find('llvm-nm'), str(expected))
 
 
+    def test_llvm_config_timeout_is_reported(self):
+        self.stub('llvm-config', 'exec /bin/sleep 5')
+        self.tools.TIMEOUT = 1
+        with self.assertRaises(SystemExit) as raised:
+            self.tools.config('--version')
+        self.assertIn('did not finish within 1 seconds', str(raised.exception))
+        self.assertIn('reinstall the LLVM development package', str(raised.exception))
+
+
+    def test_llvm_config_timeout_does_not_hang_the_bindir_probe(self):
+        self.stub('llvm-config', 'exec /bin/sleep 5')
+        self.tools.TIMEOUT = 1
+        self.assertIsNone(self.tools.find('llvm-objdump'))
+
+
     def test_require_explains_what_to_install(self):
         with self.assertRaises(SystemExit) as raised:
             self.tools.require('llvm-objdump')
diff --git a/tools/llvm_tools.py b/tools/llvm_tools.py
index e63f943..2de0b0e 100644
--- a/tools/llvm_tools.py
+++ b/tools/llvm_tools.py
@@ -9,6 +9,8 @@ reports. Nothing here runs a guest process.
 import os
 import subprocess
 
+TIMEOUT = 30  # A probe that hangs must not stall the preflight or the build.
+
 
 def _collect(directory, name, found):
     """Add every `name` and `name-<digits>` executable in the directory to found."""
@@ -41,13 +43,26 @@ def _preferred(candidates):
     return candidates[0 if 0 in candidates else max(candidates)] if candidates else None
 
 
+def _run(command):
+    """Run a short probe; return (completed process, problem), one of them None."""
+    try:
+        result = subprocess.run(command, capture_output=True, text=True, errors='replace',
+                                timeout=TIMEOUT)
+    except subprocess.TimeoutExpired:
+        return None, f'did not finish within {TIMEOUT} seconds'
+    except OSError as error:
+        return None, f'could not be started: {error}'
+    return result, None
+
+
 def _bindir():
     """`llvm-config --bindir` from the preferred llvm-config, or None."""
     configuration = _preferred(_executables('llvm-config'))
     if not configuration:
         return None
-    result = subprocess.run([configuration, '--bindir'],
-                            capture_output=True, text=True, errors='replace')
+    result, _ = _run([configuration, '--bindir'])
+    if result is None:
+        return None
     directory = result.stdout.strip()
     if result.returncode or not os.path.isdir(directory):
         return None
@@ -81,7 +96,10 @@ def require(name):
 def config(*arguments):
     """Run llvm-config with the arguments, or exit with an actionable message."""
     path = require('llvm-config')
-    result = subprocess.run([path, *arguments], capture_output=True, text=True, errors='replace')
+    result, problem = _run([path, *arguments])
+    called = f'{path} {" ".join(arguments)}'
+    if problem:
+        raise SystemExit(f'{called} {problem}; reinstall the LLVM development package.')
     if result.returncode:
-        raise SystemExit(f'{path} {" ".join(arguments)} failed: {result.stderr.strip()}')
+        raise SystemExit(f'{called} failed: {result.stderr.strip()}')
     return result.stdout.strip()