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[WasmFS] Allow reading from char devices in the Node backend #26599

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Merged
sbc100 merged 1 commit into from
Apr 2, 2026
Merged

[WasmFS] Allow reading from char devices in the Node backend #26599

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4 changes: 3 additions & 1 deletion system/lib/wasmfs/backends/node_backend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -191,7 +191,9 @@ class NodeDirectory : public Directory {
if (_wasmfs_node_get_mode(childPath.c_str(), &mode)) {
return nullptr;
}
if (S_ISREG(mode)) {
// Allow reading from character device files too (e.g. `/dev/random`,
// `/dev/urandom`)
if (S_ISREG(mode) || S_ISCHR(mode)) {
return std::make_shared<NodeFile>(mode, getBackend(), childPath);
} else if (S_ISDIR(mode)) {
return std::make_shared<NodeDirectory>(mode, getBackend(), childPath);
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6 changes: 4 additions & 2 deletions test/fs/test_fs_dev_random.c
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Original file line number Diff line number Diff line change
Expand Up @@ -13,12 +13,14 @@ int main() {
int nread;

fp = fopen("/dev/random", "r");
nread = fread(&data, 1, byte_count, fp);
assert(fp != NULL);
nread = fread(data, 1, byte_count, fp);
assert(nread == byte_count);
fclose(fp);

fp = fopen("/dev/urandom", "r");
nread = fread(&data, 1, byte_count, fp);
assert(fp != NULL);
nread = fread(data, 1, byte_count, fp);
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@sbc100 sbc100 Apr 1, 2026

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So you are adding an extra assert here? But it seems like if /dev/random was not openable/readable this test should already have been failing, no?

Or does this change not actually fix this test?

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@kleisauke kleisauke Apr 1, 2026

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Unfortunately, the CI log for this failure has expired. IIRC, it failed with RuntimeError: memory access out of bounds, which I can reproduce using:

--- a/test/fs/test_fs_dev_random.c
+++ b/test/fs/test_fs_dev_random.c
@@ -13,14 +13,12 @@ int main() {
   int nread;
 
   fp = fopen("/dev/random", "r");
-  assert(fp != NULL);
-  nread = fread(data, 1, byte_count, fp);
+  nread = fread(data, 1, byte_count, NULL);
   assert(nread == byte_count);
   fclose(fp);
 
   fp = fopen("/dev/urandom", "r");
-  assert(fp != NULL);
-  nread = fread(data, 1, byte_count, fp);
+  nread = fread(data, 1, byte_count, NULL);
   assert(nread == byte_count);
   fclose(fp);

In other words, previously fp (FILE *) was NULL without the change to node_backend.cpp. The added asserts are intended to provide clearer failure messages.

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@sbc100 sbc100 Apr 1, 2026

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Oh I see, but other.test_fs_dev_random_wasmfs_rawfs cannot actually be enabled as part of this PR for some reason?

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@sbc100 sbc100 Apr 1, 2026

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If so that sounds fair enough to me. Still nice to see fixes like this land as separate PRs.

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@kleisauke kleisauke Apr 1, 2026 •
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The other.test_fs_dev_random_wasmfs_rawfs test is enabled in this PR, but it passes because /dev and /tmp are virtualized. This virtualization is also the reason for issue #24836.

emscripten/system/lib/wasmfs/wasmfs.cpp

Lines 118 to 119 in 61b4b37

lockedDev.mountChild("random", SpecialFiles::getRandom());
lockedDev.mountChild("urandom", SpecialFiles::getURandom());

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@sbc100 sbc100 Apr 1, 2026

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But I don't see other.test_fs_dev_random_wasmfs_rawfs being enabled in this PR?

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@kleisauke kleisauke Apr 1, 2026

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Ah, sorry. I meant that it should have already been enabled by this decorator:

emscripten/test/test_other.py

Line 5551 in fcff6d8

@with_all_fs

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@kleisauke kleisauke Apr 2, 2026

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For reference, see the CircleCI log at https://circleci.com/gh/emscripten-core/emscripten/1164689 for the failure.

Details 
======================================================================
FAIL [0.001s]: test_fs_dev_random_wasmfs_rawfs (test_other.other)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/project/test/common.py", line 377, in resulting_test
    return func(self, *args)
  File "/root/project/test/decorators.py", line 324, in metafunc
    return func(self, *args, **kwargs)
  File "/root/project/test/test_other.py", line 5556, in test_fs_dev_random
    self.do_runf('fs/test_fs_dev_random.c', 'success')
  File "/root/project/test/common.py", line 1408, in do_runf
    return self._build_and_run(filename, expected_output, **kwargs)
  File "/root/project/test/common.py", line 1456, in _build_and_run
    js_output = self.run_js(js_file, engine, args,
  File "/root/project/test/common.py", line 1040, in run_js
    self.fail('JS subprocess failed (%s): %s (expected=%s).  Output:\n%s' % (error.cmd, error.returncode, assert_returncode, ret))
AssertionError: JS subprocess failed (/root/emsdk/node/22.16.0_64bit/bin/node --stack-trace-limit=50 --trace-uncaught /tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js): 1 (expected=0).  Output:
wasm://wasm/00058602:1


RuntimeError: memory access out of bounds
    at wasm://wasm/00058602:wasm-function[51]:0x1449
    at wasm://wasm/00058602:wasm-function[53]:0x1589
    at wasm://wasm/00058602:wasm-function[34]:0x913
    at wasm://wasm/00058602:wasm-function[35]:0x9f3
    at /tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:592:12
    at callMain (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2117:15)
    at doRun (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2170:24)
    at run (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2183:5)
    at removeRunDependency (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:823:11)
    at receiveInstance (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:696:5)
    at receiveInstantiationResult (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:713:12)
    at createWasm (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:739:17)
Thrown at:
    at $func51 (wasm://wasm/00058602:1:5194)
    at $func53 (wasm://wasm/00058602:1:5514)
    at $func34 (wasm://wasm/00058602:1:2324)
    at $main (wasm://wasm/00058602:1:2548)
    at /tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:592:12
    at callMain (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2117:15)
    at doRun (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2170:24)
    at run (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:2183:5)
    at removeRunDependency (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:823:11)
    at receiveInstance (/tmp/emtest_aitqg_pq/emscripten_test_other_piipb_5p/test_fs_dev_random.js:696:5)


Node.js v22.16.0


----------------------------------------------------------------------
Ran 1661 tests in 1227.050s

FAILED (failures=1, skipped=92)

assert(nread == byte_count);
fclose(fp);

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