bunfig: stop panicking when the config path does not fit in a path buffer - #38370
bunfig: stop panicking when the config path does not fit in a path buffer#38370robobun wants to merge 5 commits into
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load_config joined the working directory and "bunfig.toml" (or the --config value) into a stack PathBuffer with the unchecked join, and get_home_config_path did the same with $XDG_CONFIG_HOME / $HOME and ".bunfig.toml". A cwd within 12 bytes of PATH_MAX, a --config value of PATH_MAX bytes or more, or an over-long config home panicked at startup with a slice index out of bounds. Build the paths with join_abs_string_buf_checked, leaving room for the NUL, and length-check the absolute --config arm. A path that does not fit cannot be opened anyway, so it is handled like any other unreadable config: auto-loaded configs (bunfig.toml in the cwd, the global .bunfig.toml) are skipped, and an explicit --config fails with the same ENAMETOOLONG message open() would produce.
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WalkthroughBunfig path resolution now uses bounded path construction. Automatically loaded unreadable configs are skipped, while explicit config errors remain fatal. Tests cover path-buffer boundaries for discovered, explicit, normalized, and global configs. ChangesBunfig path hardening
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Suggested reviewers: 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Status: ready for review (head 1a40f6d).
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Updated 4:05 AM PT - Aug 14th, 2026
❌ @robobun, your commit 1a40f6d has 1 failures in
🧪 To try this PR locally: bunx bun-pr 38370That installs a local version of the PR into your bun-38370 --bun |
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LGTM — focused fix that routes over-long config paths through the existing unreadable-config handling instead of panicking in the path-buffer join.
What was reviewed:
join_config_pathreserves the NUL slot viabuf[..len-1]before callingjoin_abs_string_buf_checked; verified the checked helper's fast/slow paths and that a 4095-byte result still fits while 4096 returnsNone.- Absolute
--configarm'slen < config_buf.len()guard matchesresolve_path::z's own>= MAX_PATH_BYTESprecondition. get_home_config_pathrefactor preserves the XDG-then-HOME precedence (no fallback to HOME when XDG is set but too long);unreadable_configextraction is byte-identical to the old inline error path.- Tests cover the exact-fit boundary, the NUL-write boundary, and the longer-than-buffer case at each of the four sites, plus a normalization control.
Extended reasoning...
Overview
This PR fixes a class of startup panics in src/bunfig/arguments.rs where config paths built into a fixed-size PathBuffer (PATH_MAX bytes) could overflow: joining cwd + bunfig.toml, joining $XDG_CONFIG_HOME/$HOME + .bunfig.toml, and copying an absolute --config value. The fix introduces join_config_path (uses the existing join_abs_string_buf_checked into a slice that reserves the NUL slot), length-guards the absolute arm before resolve_path::z, and factors the existing "auto-loaded → skip, explicit → exit 1" error handling into unreadable_config so the too-long case reuses it with a synthesized ENAMETOOLONG error. 15 new tests in test/config/bunfig/bunfig-errors.test.ts cover each site at the exact-fit, NUL-overflow, and over-buffer boundaries, plus a control showing long inputs that normalize short still load.
Security risks
None. The change strictly tightens handling of adversarial-length inputs (cwd, argv, env vars) by replacing an unchecked buffer write (which panicked in Rust rather than corrupting memory) with a checked one that takes the existing error path. No new file opens, no new trust boundaries.
Level of scrutiny
Medium. This is CLI startup path-building code, not memory-unsafe (Rust bounds checks were catching it as panics), and the fix reuses existing checked helpers rather than hand-rolling arithmetic. I verified join_abs_string_buf_checked at src/paths/resolve_path.rs:1640 returns Option<&[u8]> with the semantics the PR relies on (normalize-then-check, so long-but-normalizes-short inputs still fit), and that resolve_path::z at line 39 requires the exact len < MAX_PATH_BYTES guard the caller now provides. The get_home_config_path refactor to .or_else() preserves the original precedence: XDG_CONFIG_HOME wins when set, and a too-long XDG value returns None rather than silently falling back to HOME.
Other factors
- The removal of
awdfrom thepartsarray is correct:join_abs_string_bufalready uses the first argument as the base, and passing it again inpartswas redundant (and would have inflated the checked variant's size estimate). unreadable_configtakes&bun_sys::Errorand is called from both the new too-long path and the existingto_sourcefailure — behavior for the latter is unchanged.- Tests follow harness conventions:
tempDir+using,bunEnvspread,Buffer.alloc(n, fill), concurrent subprocess drain,skipIf(isWindows)with a stated reason, and boundary controls that would pass on the unfixed build (so failures are attributable). - No CODEOWNERS entry for
src/bunfig/; no outstanding human review comments.
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Two notes for whoever picks this up, prompted by the review above:
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bun pm cache creates the directory named by BUN_INSTALL_CACHE_DIR and falls back to node_modules/.cache when it cannot, so a marker under / only worked when the tests ran as root.
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Inline comments:
In `@test/config/bunfig/bunfig-errors.test.ts`:
- Around line 50-56: Update pathOfLength to measure and construct paths using
UTF-8 byte lengths via Buffer.byteLength rather than string .length, including
its loop and padding calculation. Adjust dependent config boundary assertions
and process.cwd() output checks to compare byte lengths consistently, preserving
the intended filesystem boundary values for non-ASCII temporary directories.
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📒 Files selected for processing (2)
src/bunfig/arguments.rstest/config/bunfig/bunfig-errors.test.ts
The path buffer holds bytes, so a temporary directory with non-ASCII characters shifted every boundary case when lengths were measured in UTF-16 code units.
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Follow-ups pushed since the first revision (the fix itself is unchanged):
All review threads are resolved. |
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LGTM — the .repeat() nit from the last pass is addressed in 1a40f6d, and no new issues surfaced.
What was reviewed:
join_config_pathreserves the NUL slot (buf[..len-1]) before callingjoin_abs_string_buf_checked, sobuf[len] = 0is always in bounds; the absolute-path guardlen < config_buf.len()matchesresolve_path::z's own precondition.get_home_config_path's XDG-then-HOME precedence is unchanged (a too-long XDG skips rather than falling through), andunreadable_configis a straight extraction of the existing auto-loaded/explicit split inload_bunfig.- Dropping the cwd from the
partsarray is correct —join_abs_string_bufalready takes it as the base, and keeping it would have double-counted in the checked variant's size estimate. - Tests hit the exact boundary (MAX-1 loads, MAX and longer fail/skip) at all four join sites, measure in UTF-8 bytes, drain pipes concurrently, and skip on Windows with a stated reason.
Extended reasoning...
Overview
The PR fixes three buffer-overflow panics in src/bunfig/arguments.rs where config paths were joined into a fixed-size PathBuffer without a length check: the cwd + bunfig.toml join in load_config, the absolute --config copy, and the $XDG_CONFIG_HOME/$HOME + .bunfig.toml join in get_home_config_path. The fix introduces join_config_path (built on the existing join_abs_string_buf_checked, reserving one byte for the NUL) and factors the existing "auto-loaded → skip, explicit → exit 1" branch out of load_bunfig into unreadable_config so the new too-long case can reuse it with a synthesized ENAMETOOLONG. About 50 lines of Rust net, plus 15 new tests in the existing bunfig-errors.test.ts.
Security risks
None. The panic being fixed was a bounds-checked slice index (a DoS on the user's own process from a deep cwd), not memory unsafety. The replacement uses the existing checked-join helper and the existing bun_sys::Error machinery. No new inputs are trusted; over-long paths that would have been rejected by open() anyway now take the same error route the kernel's ENAMETOOLONG already did.
Level of scrutiny
Medium. This is CLI-startup path handling — every command hits it, but the change is mechanical (unchecked join → checked join) with no happy-path behavior change. I traced join_abs_string_buf_checked at src/paths/resolve_path.rs:1640 and resolve_path::z at :39 to confirm the guards line up: the checked join returns None when the normalized result exceeds the passed slice (here buf[..MAX_PATH_BYTES-1]), so buf[len] = 0 writes at most index MAX_PATH_BYTES-1; z requires len < MAX_PATH_BYTES, which the absolute arm's guard enforces exactly.
Other factors
Since my previous review the only change is 1a40f6d, which swaps the last .repeat() for Buffer.alloc() per the harness convention nit — that was the sole open item. All bot threads (comment-cop on the doc comments, CodeRabbit on byte-length measurement) are resolved with pushed fixes. The test matrix is thorough: exact-boundary controls that still load (proving the fix isn't over-eager), the two overflow shapes (NUL-write-at-len and join-overflow) at each of the four sites, a normalization case showing long input that shrinks still loads, and both env-var routes. The PR description documents 10 of the 15 new tests failing on the unfixed build and all passing with the fix, and the boundary tests assert the resolved absolute path in the parse error so they can't pass vacuously. The remaining same-class sites (.npmrc join, --cwd, process.chdir) are named as out of scope with the constraint that keeps the global-config tests below the .npmrc threshold, which is the right call for a focused fix.
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Problem
bun x.cjs,bun -e 0and every command inALWAYS_LOADS_CONFIG(bun test,bun build,bun install,bun pm, ...) crash at startup when the working directory is 4084..4095 bytes long on Linux (1012..1023 on macOS, whereMAX_PATH_BYTESis 1024):MAX_PATH_BYTES - 12bytes:panic: index out of bounds: the len is 4096 but the index is 4096panic: range end index 4101 out of range for slice of length 4095(end index = cwd length + 11), top framebun_paths::resolve_path::normalize_string_generic_tz(src/paths/resolve_path.rs:1071), called frombun_bunfig::arguments::load_config--config=<value>when the value (absolute) or cwd + value (relative) isMAX_PATH_BYTESbytes or longer, and forbun install/bun pm ...when$XDG_CONFIG_HOME(or$HOME) isMAX_PATH_BYTES - 13bytes or longer.src/bunfig/arguments.rs:load_configjoined cwd + config name into a stackPathBufferwith the uncheckedjoin_abs_string_buf(L213) and wrote the NUL atconfig_buf[len](L217). A result of exactlyMAX_PATH_BYTESbytes fits the join but the NUL write is out of bounds; anything longer overflows inside the join.--configarm copied the argument into the buffer without a length check (L192).get_home_config_pathjoined$XDG_CONFIG_HOME/$HOME+.bunfig.tomlwith the uncheckedjoin_abs_string_buf_z(L24, L30), which writes the NUL itself.bun -e 0from a directory of that length: the path is built before anything is opened, so no bunfig.toml has to exist.Fix
join_config_pathbuildsdir/namewithjoin_abs_string_buf_checkedintobuf[..len - 1](reserving the NUL slot) and returnsNonewhen the result does not fit; the cwd join andget_home_config_pathboth use it. The absolute--configarm length-checks and usesresolve_path::z, the guard-then-zshape its other callers use.load_bunfig, factored intounreadable_config: auto-loaded configs (bunfig.tomlin the cwd, the global.bunfig.toml) are skipped, as they already are on any read error; an explicit--configexits 1 withENAMETOOLONG: <path>: File name too long (open())/while reading config "<path>", the message a kernel ENAMETOOLONG on the same flag already produces.open()rejects any path ofMAX_PATH_BYTESbytes or more (bun_sys::openat_areturns ENAMETOOLONG for the same condition before the syscall), so no file at such a path could have been loaded; the failure now takes the existing route instead of overflowing the buffer.join_abs_string_buf_checkednormalizes before deciding, so a long--configvalue that normalizes to a path that fits still loads. Forbun <file>/bun -e,run_commandadditionally loadsbunfig.tomlrelative to the cwd whenload_configloaded nothing, so a bunfig.toml in such a deep directory is still picked up there.partsentry:join_abs_string_bufalready uses it as the base, so the result is identical, and the copy would have made the checked join count the cwd twice in its size estimate.test/config/bunfig/bunfig-errors.test.ts(15 new tests): on the unfixed build the 10 overflow cases fail with the panics above and the 5 boundary/normalization controls pass; with the fix all 20 tests in the file pass. Per site they cover the longest path that still loads (exactlyMAX_PATH_BYTES - 1bytes, asserted through the absolute path in the parse error), theMAX_PATH_BYTEScase that failed on the NUL write, and a longer one, forbun -eandbun file.jsin a deep cwd, relative and absolute--config, and$XDG_CONFIG_HOME/$HOMEthroughbun pm cache. Skipped on Windows, where the buffer (~96 KiB) is longer than any path, argument or environment value the OS accepts.test/config/bunfig/preload.test.ts,test/cli/install/bun-run-bunfig.test.ts,test/cli/bunfig-test-options.test.ts,test/cli/install/npmrc.test.ts, the global-bunfig tests intest/cli/install/minimum-release-age.test.ts;cargo clippy -p bun_bunfigandcargo fmtare clean.bun install/bun pmwith$XDG_CONFIG_HOME/$HOMEofMAX_PATH_BYTES - 7bytes or longer still panic one step later in the.npmrclookup (src/install/PackageManager.rs, same unchecked join), so the global-config tests here stay below that length;--cwd <over-long value>(src/runtime/cli/Arguments.rs) andprocess.chdir()into a directory of exactlyMAX_PATH_BYTES - 1bytes are the same bug class at other sites. Working directories ofMAX_PATH_BYTESbytes or more fail ingetcwdbefore this code runs; cli: refuse to start when the cwd is longer than PATH_MAX instead of using the executable's directory #38363 covers those.Background
PathBufferis bun's fixed stack buffer for path syscalls,MAX_PATH_BYTESlong (the platform'sPATH_MAX: 4096 on Linux, 1024 on macOS). Paths in it are NUL-terminated, so the longest path it holds isMAX_PATH_BYTES - 1bytes.resolve_path::join_abs_string_buf(cwd, buf, parts)ispath.resolveinto a caller buffer: it concatenates, normalizes and writes the result, assuming it fits.join_abs_string_buf_checkedis the variant for unbounded input and returnsNoneinstead of writing when the normalized result is longer thanbuf.resolve_path::zcopies a slice into aPathBufferwith a NUL and expects the caller to have checked the length.load_configloadsbunfig.tomlfrom the cwd for the commands inALWAYS_LOADS_CONFIGand forbun <file>/bun -e(auto_loaded), or the--configvalue (not auto-loaded); install-family commands first load$XDG_CONFIG_HOME/.bunfig.toml, else$HOME/.bunfig.toml.load_bunfigignores read errors for auto-loaded configs and exits with the error for explicit ones.