Add differential fuzz tests comparing rshell builtins against GNU coreutils#62
Add differential fuzz tests comparing rshell builtins against GNU coreutils#62
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- Add .github/workflows/fuzz.yml: runs each Fuzz* target for 30s per push/PR across head, cat, wc, tail, and grep packages; caches corpus between runs; skips gracefully when no fuzz targets exist yet in a package. - Update .github/workflows/test.yml: add fuzz seed corpus regression step so any checked-in corpus entries that crash are caught on every PR. - Add testdata/fuzz/.gitkeep placeholders so corpus cache paths are consistent. - Document corpus retention policy in .gitignore. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…, tail) Each benchmark file adds both standard BenchmarkXxx functions (runnable with go test -bench) and TestXxxMemoryBounded functions that call testing.Benchmark internally and assert AllocedBytesPerOp stays below a documented ceiling: - head: < 512 KB for 10 MB input (truly O(1): ~17 KB observed) - wc: < 512 KB for 10 MB input (truly O(1): ~44 KB observed) - cat: < 6 MB for 1 MB input (O(n) output buffer: ~3 MB observed) - tail: < 24 MB for 10 MB input (O(n) allocs, O(1) live ring: ~11 MB observed) Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…eutils Implements FuzzHeadDifferentialLines, FuzzHeadDifferentialBytes, FuzzCatDifferential, FuzzWcDifferentialLines, FuzzWcDifferentialWords, FuzzWcDifferentialBytes, and FuzzTailDifferential. Each test is gated behind RSHELL_BASH_TEST=1, uses exec.LookPath to skip if the GNU tool is not available, and runs GNU coreutils in the same working directory as rshell so filenames in output match exactly. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Review Summary
This PR adds differential fuzz tests comparing rshell builtins against GNU coreutils, memory-bounded benchmarks, and a CI workflow (fuzz.yml). The intent is solid — differential fuzzing is an excellent technique for catching bash compatibility regressions — but the PR has a blocking build failure that prevents all four new fuzz packages from compiling, which will immediately break CI.
Overall assessment: needs fixes before merge.
Findings Summary
| # | Priority | File | Finding |
|---|---|---|---|
| 1 |  |
interp/builtins/tests/cat/cat_differential_fuzz_test.go:93, head/…:99,151, tail/…:100, wc/…:93,141,188 |
cmdRunCtx undefined in all four fuzz packages — build fails entirely |
| 2 |  |
.github/workflows/test.yml:28 |
go test -run '^Fuzz' ./interp/builtins/... will fail CI because the four broken packages are under ./interp/builtins/... |
| 3 |  |
interp/builtins/tests/tail/tail_differential_fuzz_test.go:107-108 |
Silent skip on "too large" / "exceeds" hides real differential failures where rshell errors and GNU tail succeeds |
| 4 | |
.github/workflows/fuzz.yml:27-28 |
grep matrix entry has no Go files — silently no-ops, misleads contributors |
| 5 | |
.github/workflows/fuzz.yml:66 |
` |
| 6 | |
.github/workflows/fuzz.yml:66 |
120s job timeout with 3 wc fuzz functions × 30s each plus overhead — may timeout before all functions run |
| 7 |  |
cat_differential_fuzz_test.go, head_…, tail_…, wc_… |
gnuCmd, runGNUInDir, and isSandboxError are copy-pasted identically into all four fuzz packages |
Findings
P0 — cmdRunCtx is undefined in all four fuzz packages (build failure)
Each differential fuzz file calls cmdRunCtx(ctx, t, script, dir) but no definition of that function exists anywhere in these packages. Each package (tests/cat, tests/head, tests/tail, tests/wc) contains only its single _differential_fuzz_test.go file — there is no companion helper file. This causes a hard compile error confirmed by go vet:
vet: interp/builtins/tests/cat/cat_differential_fuzz_test.go:93:39: undefined: cmdRunCtx
vet: interp/builtins/tests/head/head_differential_fuzz_test.go:99:39: undefined: cmdRunCtx
vet: interp/builtins/tests/tail/tail_differential_fuzz_test.go:100:39: undefined: cmdRunCtx
vet: interp/builtins/tests/wc/wc_differential_fuzz_test.go:93:39: undefined: cmdRunCtx
Remediation: Each package needs a helpers_test.go that defines cmdRunCtx. The pattern in interp/builtins/tests/cut/cut_test.go is a good reference. The function signature needed is:
func cmdRunCtx(ctx context.Context, t *testing.T, script, dir string) (string, string, int) {
t.Helper()
return runScriptCtx(ctx, t, script, dir, interp.AllowedPaths([]string{dir}))
}P1 — test.yml fuzz seed step breaks CI immediately
The new step go test -run '^Fuzz' -fuzztime=0s ./interp/builtins/... includes all packages under interp/builtins/, which includes the four broken fuzz packages. This step will fail in CI for every push until finding #1 is fixed.
Remediation: Fix the undefined cmdRunCtx first.
P2 — Tail fuzz test silently skips real differential failures
The tail differential fuzz test skips when rshell's stderr contains "too large" or "exceeds". These match tail's actual error messages ("input too large: read limit exceeded", "input too large: ring buffer memory limit exceeded", etc.). If the fuzzer finds an input where rshell errors out but GNU tail succeeds and produces output, the test skips instead of failing.
The existing n > 10000 and len(input) > 64*1024 guards already ensure inputs are well within rshell's operational limits. This skip should never be needed and masks real divergences.
Remediation: Remove the too large/exceeds skip block.
P2 — grep matrix entry has no Go test files
The fuzz workflow matrix includes grep but interp/builtins/tests/grep/ contains only a .gitkeep. The step silently no-ops for grep.
Remediation: Remove grep from the matrix until grep_differential_fuzz_test.go is written.
P2 — || true silences fuzz crash reports in CI
Go's fuzzer exits non-zero when it finds a crasher. || true means a real fuzzer-found bug makes CI pass silently.
Remediation: Remove || true.
P2 — 120s timeout may be insufficient for wc (3 functions × 30s)
3 fuzz functions × 30s = 90s minimum, plus CI overhead → likely to timeout before all three run.
Remediation: Increase to -timeout 300s or reduce -fuzztime to 20s.
P3 — Helper functions copy-pasted across four packages
gnuCmd, runGNUInDir, and isSandboxError are identically copy-pasted into all four differential fuzz test files.
Remediation: Extract into a shared test helper package, e.g. interp/builtins/tests/testutil/.
Positive Observations
- Placing differential fuzz tests under
interp/builtins/tests/<cmd>/(separate from builtins) is correct — it allowsos/execimports that are banned in production code. - Gating differential tests behind
RSHELL_BASH_TEST=1is consistent with existing infrastructure. - The input size cap and N-value clamp in all fuzz targets are good practice.
TestCatMemoryBounded,TestHeadMemoryBoundedLines, etc. are well-structured and the ceiling values are sensibly calibrated.- Pre-creating fuzz corpus directories ensures CI cache paths are stable.
actions/cache/save@v4withif: always()correctly saves corpus even on fuzz step failure.
|
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Review Summary
This PR adds differential fuzz tests and benchmark tests for cat, head, tail, and wc commands, plus a new fuzz.yml CI workflow. The approach is solid — differential fuzzing against GNU coreutils is excellent for catching compatibility regressions. However, the PR has a critical build failure that causes all 4 new test packages to fail to compile, which blocks CI. This must be fixed before merging.
Overall assessment: needs fixes
| # | Priority | File | Finding |
|---|---|---|---|
| 1 | |
interp/builtins/tests/cat/cat_differential_fuzz_test.go:93 |
cmdRunCtx undefined — package fails to compile |
| 2 | |
interp/builtins/tests/head/head_differential_fuzz_test.go:99 |
cmdRunCtx undefined — package fails to compile |
| 3 | |
interp/builtins/tests/tail/tail_differential_fuzz_test.go:100 |
cmdRunCtx undefined — package fails to compile |
| 4 | |
interp/builtins/tests/wc/wc_differential_fuzz_test.go:93 |
cmdRunCtx undefined — package fails to compile |
| 5 | |
.github/workflows/test.yml:28 |
fuzz seed corpus CI step runs ./interp/builtins/... which includes broken packages |
| 6 | |
interp/builtins/tests/grep/ |
Matrix entry for grep in fuzz.yml but the package has no Go files or fuzz functions |
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| ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second) | ||
| defer cancel() | ||
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||
| rshellOut, rshellErr, rshellCode := cmdRunCtx(ctx, t, "cat input.txt", dir) |
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The new differential fuzz test packages call cmdRunCtx but do not define or import it, so these tests do not compile (undefined: cmdRunCtx) and the new seed-corpus CI step cannot execute them. I reproduced this with go test ./interp/builtins/tests/... -run '^$', and the same missing symbol appears in the head, tail, and wc differential files too.
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| fi | ||
| for FUNC in $FUZZ_FUNCS; do | ||
| echo "Fuzzing $FUNC..." | ||
| go test -fuzz="^${FUNC}$" -fuzztime=30s ${{ matrix.pkg }} -timeout 120s || true |
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This command masks every go test -fuzz failure with || true, so crashes/regressions found by fuzzing still produce a green workflow. The testing docs describe fuzz target failures as normal test failures, so swallowing the exit status here removes the workflow’s ability to block bad changes.
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| # Run actual fuzzing for a short duration | ||
| - name: Fuzz (${{ matrix.name }}) | ||
| run: | | ||
| FUZZ_FUNCS=$(grep -r '^func Fuzz' ${{ matrix.pkg }} 2>/dev/null | grep -v 'Differential' | sed 's/.*func \(Fuzz[^(]*\).*/\1/' | sort -u) |
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The workflow excludes any fuzz function containing Differential, but every fuzz test added in this commit is named Fuzz*Differential*, so the head/cat/wc/tail matrix entries never execute those tests and always skip with "No fuzz targets found." This leaves the new differential coverage effectively unused in CI.
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- Add helpers_test.go to tests/cat, tests/head, tests/tail, tests/wc defining cmdRunCtx and runScriptCtx, fixing the undefined-symbol build failures that blocked all four differential fuzz packages from compiling - Remove silent skip on "too large"/"exceeds" in tail fuzz test; the existing n<=10000 and len(input)<=64KB guards already keep inputs within rshell limits, so these skips mask real differential failures - Remove grep from fuzz.yml matrix (no Go test files exist yet) - Remove "|| true" from fuzz step so fuzzer-found crashes surface as CI failures - Increase fuzz job timeout from 120s to 300s to accommodate wc's 3 targets Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds testing.F fuzz tests for head, cat, wc, tail, and grep builtins. Each command gets seed corpus covering empty input, no trailing newline, NUL bytes, buffer boundaries (4097 bytes), very long single lines, and all-newlines input. Fuzz functions use context.WithTimeout to catch hangs, assert exit codes are 0 or 1, and verify output invariants (e.g. head -n K produces at most K lines). Both file-based and stdin-via-redirection variants are included for each command. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
- Remove unused cmdRun function from all 5 fuzz test files (cat, grep, head, tail, wc); only cmdRunCtx (with context for timeout) is needed - Remove dead fixedPatterns() function and stale comment from grep fuzz tests (artifact of an earlier design approach) - Add utf8.ValidString guard in FuzzGrepFileContent to skip non-UTF-8 patterns that would be rejected by the shell parser before reaching the grep builtin, ensuring the fuzz corpus exercises grep logic - Add FuzzTailLinesOffset and FuzzTailBytesOffset to cover the +N offset code paths in tail (skip-first-N-lines/bytes mode), which were not previously fuzzed Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Remove duplicate cmdRunCtx declarations by making helpers_test.go cross-platform (drop !windows build tag) and reusing it from native fuzz test files. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add fuzz tests for echo, cut, uniq, strings, testcmd, and ls. Update fuzz.yml workflow matrix to include all new packages. Tests cover: - echo: basic args and -e escape sequence parsing - cut: field/byte selection with specs, custom delimiters, stdin - uniq: basic, count, flag combinations, field/char skipping, stdin - strings: file content, min length, radix offset, stdin - test: string ops, integer ops, file ops, unary string tests - ls: flag combinations with random filenames, recursive traversal Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
…e cases and CVE-class inputs Added targeted seed corpus entries to all 11 fuzz test packages based on: - Implementation constants and boundary conditions (MaxLineBytes=1MiB, MaxCount=1<<31-1, maxRecursionDepth=256, maxStringLen=1MiB, maxParenDepth=128, countFieldWidth=7) - Security-relevant edge cases: CRLF, null bytes, invalid UTF-8, high bytes, integer overflow, ReDoS-class regex patterns, printable byte boundaries - Existing unit test scenarios (CRLF in cut, complement, suppress, output-delimiter) - New fuzz functions: FuzzCatDisplayFlags, FuzzWcChars, FuzzGrepPatterns, FuzzEchoFlagInteraction, FuzzStringsRadix, FuzzTestNesting, FuzzLsHumanReadable, FuzzLsMultipleFiles, FuzzCutComplement Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
… magic bytes Incorporates findings from GNU coreutils/binutils CVE research: - cat: ANSI/terminal escape injection sequences (OSC 2/50, ANSI color, DCS, cursor movement, ELF magic bytes) - grep: new FuzzGrepFixedStrings function targeting CVE-2015-1345 (-F mode Boyer-Moore-Horspool code path) and CVE-2012-5667 (integer overflow at line length boundary); regex metacharacters tested as fixed-string literals - uniq: CVE-2013-0222 pattern (long line with embedded null bytes near MaxLineBytes; distro alloca() stack overflow was triggered at 50MB — our fixed buffers prevent this class entirely); CRLF vs LF duplicate comparison edge case - strings: ELF/PE/ZIP/PDF binary format magic bytes (CVE-2014-8485 used crafted ELF headers to exploit libbfd; our raw-byte scanner is unaffected but confirms graceful handling) - testcmd: int64 max/min boundary seeds for integer comparison overflow behavior Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Documents the three sources for fuzz seed corpus construction: a) Implementation edge cases — named constants, boundary checks, clamps, buffer sizes b) CVE and security history — integer overflow, long lines, null bytes, CRLF, invalid UTF-8, binary magic bytes, terminal escape injection, ReDoS patterns c) Existing test coverage — every input from unit tests and YAML scenarios Adds cmdRunCtxFuzz helper pattern (avoids redeclaration with existing test helpers), per-mode function guidance, seed verification commands, and CI fuzz.yml integration. Renumbers documentation update to Step 10. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Memory benchmark tests belong in a separate PR per user request. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Fuzzer found two real bugs in seed filtering: - FuzzGrepFixedStrings: missing utf8.ValidString(pattern) check; non-UTF-8 byte (e.g. 0x85) embedded in shell script caused parse error - FuzzTestStringOps/FuzzTestStringUnary: C1 control chars (U+0080–U+009F) passed the utf8.Valid check but caused "EOF without closing quote" in the shell script parser Add corpus entries for fuzzer-discovered inputs. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Document that failing fuzzer-discovered inputs must be committed as corpus files (testdata/fuzz/<FuzzFuncName>/<hash>) to serve as permanent regression tests. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Go's -fuzztime flag does not accept 0s as a valid duration, causing the "Run fuzz seed corpus (regression test)" step to fail on all platforms. Remove the flag entirely — running go test -run '^Fuzz' without -fuzz is the correct way to execute seed corpus entries. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
… bash job - fuzz.yml seed corpus step: remove -fuzztime=0s (invalid duration) - test.yml bash job: add 30s differential fuzz runs for cat/head/tail/wc against GNU tools under RSHELL_BASH_TEST=1 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
GNU wc in POSIX locale (Debian/Ubuntu) treats non-whitespace control characters (C0 0x00-0x1f, DEL 0x7f, C1 0x80-0x9f) as transparent: they neither start nor end words. Only printable chars form words. Previously rshell wc counted any non-whitespace byte as a word character, so a file containing just \x1a produced 1 word instead of 0. - wc.go: add unicode.IsControl branch in countReader word loop - wc_test.go, wc_gnu_compat_test.go: update expectations to 0 words - allowed_symbols_test.go: add unicode.IsControl to allowlist - add fuzz corpus entry ee500f173c25a234 (\x1a) as regression test Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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there are some docker bash tests failing
it's working 😄 |
- Change build tag from !windows to linux: differential tests only make sense on Linux where GNU coreutils behaviour is well-defined and C.UTF-8 locale is available - Set LC_ALL=C.UTF-8 on every GNU tool invocation in runGNUInDir: pins character handling to UTF-8 while keeping POSIX whitespace rules, making results reproducible across all Linux environments - Remove gnuCmd / darwin "g" prefix logic: no longer needed Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
In C.UTF-8 locale, bytes that are not valid UTF-8 (e.g. \xff) are not characters — they contribute 0 to -m (char count) and are transparent to -w (word count), the same behaviour as C0 control chars. Previously, utf8.DecodeRune returned RuneError for invalid bytes and they fell through to the printable-char branch: counted as 1 word and 1 char. Also, incomplete UTF-8 sequences left in carry at EOF were being counted as chars. - Move char counting into the per-rune loop; skip RuneError/size==1 - Remove carry-at-EOF char count (incomplete sequence = not a char) - Remove utf8.RuneCount from allowlist (no longer called); add RuneError Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…ll functions on failure - Treat Unicode space separators (Zs: NBSP, thin space, etc.) as word delimiters, matching GNU wc under LC_ALL=C.UTF-8 - Treat unassigned codepoints (Cn) as transparent like C0/C1 controls, matching GNU wc under LC_ALL=C.UTF-8 (U+89249 was counting as 1 word) - Add unicode.Co, unicode.IsGraphic, unicode.Zs to the import allowlist - Add regression corpus entry FuzzWcDifferentialWords/1c6e2e9cd7371f3e - CI: continue fuzzing all functions even if one fails (OVERALL_STATUS pattern), so a single CI run surfaces all outstanding bugs Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
head,cat,wc, andtailthat compare rshell builtin output byte-for-byte against GNU coreutilsRSHELL_BASH_TEST=1and skipped (not failed) when the GNU tool is unavailable (exec.LookPath)Tests added
interp/builtins/tests/head/head_differential_fuzz_test.goFuzzHeadDifferentialLines(-n),FuzzHeadDifferentialBytes(-c)interp/builtins/tests/cat/cat_differential_fuzz_test.goFuzzCatDifferentialinterp/builtins/tests/wc/wc_differential_fuzz_test.goFuzzWcDifferentialLines(-l),FuzzWcDifferentialWords(-w),FuzzWcDifferentialBytes(-c)interp/builtins/tests/tail/tail_differential_fuzz_test.goFuzzTailDifferential(-n)Design notes
cmd.Dirset to the same temp directory as rshell so relative filenames in command output match exactly (important forwcwhich includes the filename)g-prefixed tools are used (e.g.ghead,gwc)//go:build !windowssince GNU tools are not available on WindowsTest plan
go test ./interp/builtins/tests/... -run ^$— verify compilationgo test ./interp/builtins/tests/... -run 'FuzzHeadDifferential|FuzzCatDifferential|FuzzWcDifferential|FuzzTailDifferential'— verify seeds skip without env varRSHELL_BASH_TEST=1 go test ./interp/builtins/tests/... -run 'FuzzHeadDifferential|FuzzCatDifferential|FuzzWcDifferential|FuzzTailDifferential'— verify all seeds pass against GNU toolsRSHELL_BASH_TEST=1 go test -fuzz=FuzzHeadDifferentialLines -fuzztime=30s ./interp/builtins/tests/head/Agent context (for resuming work)
Original prompt: Research formal testing approaches for rshell builtin implementations. Implement Option 4: differential/oracle fuzz testing against GNU coreutils.
Research findings that shaped this PR:
RSHELL_BASH_TEST=1gate (used by Docker bash comparison tests) was reused to avoid adding a new env var; differential fuzz tests require the GNU tool to be present locally.exec.LookPathgates each test: if the GNU tool is missing, tests skip rather than fail. This means the tests compile and run cleanly on all CI platforms (Windows included, via build tag exclusion).g(e.g.ghead,gwc,gtail). ThegnuCmd()helper handles this automatically.cmd.Dirmust be set to the same temp directory as rshell's sandbox root so thatwc file.txtproduces matching filenames in output from both implementations.Key design decisions:
helpers_test.goin each package providescmdRunCtx— the shared test infrastructure required by differential tests alongside unit testsMerge note: This PR and PR #63 (native fuzz) both write to
interp/builtins/tests/<cmd>/. Merge one first to avoid conflicts on the helper files.🤖 Generated with Claude Code