kernel: handle AT_EMPTY_PATH in fstatat so File#stat on an fd works#865
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brandonpayton wants to merge 1 commit into
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kernel: handle AT_EMPTY_PATH in fstatat so File#stat on an fd works#865brandonpayton wants to merge 1 commit into
brandonpayton wants to merge 1 commit into
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`fstatat(fd, "", AT_EMPTY_PATH)` must stat the open fd itself, equivalent to `fstat(fd)`. glibc/musl's `fstat` and Ruby's `File#stat`/`IO#stat` route through this form. Previously the empty path fell through to `resolve_at_path`, which returns ENOTDIR when dirfd is a non-directory fd — so `File#stat` on any regular-file fd failed with Errno::ENOTDIR. This broke Homebrew inside Kandelo: `brew`'s download-lock code opens the lock file (RDWR|CREAT) and calls `File#stat` on the fd, which raised ENOTDIR and aborted every `brew install`/download before fetching. Detect AT_EMPTY_PATH with an empty path and a real dirfd up front and delegate to sys_fstat. `sys_statx` already delegates to `sys_fstatat`, so statx-based callers are fixed too. Pure kernel behavior; no ABI change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Purpose
fstatat(fd, "", AT_EMPTY_PATH)must stat the open fd itself — equivalent tofstat(fd). glibc/musl'sfstatand Ruby'sFile#stat/IO#statroute through this form. The kernel did not recognizeAT_EMPTY_PATH, so the empty path fell through toresolve_at_path, which returnsENOTDIRwhendirfdis a non-directory fd. As a result,File#staton any regular-file fd failed withErrno::ENOTDIR.This is a real POSIX gap, surfaced by running upstream Homebrew inside Kandelo:
brew's download-lock code opens the lock file (RDWR|CREAT) and callsFile#staton the resulting fd. That raisedENOTDIRand aborted everybrew install/ download before fetching anything.Fix
Detect
AT_EMPTY_PATHwith an empty path and a realdirfdup front insys_fstatatand delegate tosys_fstat.sys_statxalready delegates tosys_fstatat, sostatx-based callers (glibc's modernfstat) are fixed by the same change.Linux semantics preserved: without
AT_EMPTY_PATH, an empty path against a file fd still returnsENOTDIR.Validation
test_fstatat_empty_path_on_file: empty-path stat on a regular-file fd and on a pipe fd both succeed and match plainfstat; without the flag, empty path still yieldsENOTDIR.cargo test -p kandelo --target aarch64-apple-darwin --lib→ all pass (964 on main + this test).scripts/check-abi-version.sh: snapshot in sync — no ABI change (pure kernel behavior).brew installinside Kandelo advances past the download-lockFile#statand reaches the fetch stage (previously it aborted at lock acquisition).🤖 Generated with Claude Code