test(chown): add end-to-end coverage for chown/fchown/lchown/fchownat family

chown/lchown/fchownat/fchown previously had no integration coverage: no
scenario in cmd/ioworkload/ and no test in integrationtests/. All four are
correctly classified (chown/lchown KindPathname args[0]; fchownat KindPathname
args[1]; fchown KindFd args[0]; all FamilyFS, all UNCLASSIFIED ret) but nothing
exercised them end-to-end.

Add a chown-basic scenario that, on a temp file and a symlink the caller owns,
issues raw chown(path,-1,-1), lchown(symlink,-1,-1), fchownat(AT_FDCWD,path,
-1,-1,0) and fchown(fd,-1,-1). Owner/group -1/-1 means "leave both ids
unchanged", which the kernel accepts without CAP_CHOWN, so the scenario is
fully UNPRIVILEGED and nothing is actually modified. Raw syscalls are used so
each distinct tracepoint fires rather than glibc redirecting chown to fchownat.

TestChownBasic asserts enter_chown and enter_fchownat capture the regular file
path (chownfile.txt), enter_lchown captures the symlink (chownlink), and
enter_fchown fires (KindFd, no path). Mirrors the chmod coverage
(scenario_chmod.go / chmod_test.go).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2 files changed, 151 insertions, 0 deletions

diff --git a/cmd/ioworkload/scenario_chown.go b/cmd/ioworkload/scenario_chown.go
new file mode 100644
index 0000000..e171284
--- /dev/null
+++ b/cmd/ioworkload/scenario_chown.go
@@ -0,0 +1,150 @@
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+	"syscall"
+	"unsafe"
+)
+
+// chownBasic drives the chown ownership-change family end-to-end on a file and
+// a symlink the caller owns. Every call passes owner/group -1/-1 ("don't
+// change either id"), which the kernel accepts without CAP_CHOWN, so the whole
+// scenario is UNPRIVILEGED and nothing is actually modified. It exercises, in
+// order:
+//
+//   - chown(path, -1, -1)               — path at args[0], KindPathname, FamilyFS
+//   - lchown(symlink, -1, -1)           — path at args[0], KindPathname, FamilyFS
+//   - fchownat(AT_FDCWD, path, -1,-1,0) — path at args[1], KindPathname, FamilyFS
+//   - fchown(fd, -1, -1)                — fd   at args[0], KindFd,       FamilyFS
+//
+// We use raw syscalls (rather than os.Chown / syscall.Chown etc.) so each
+// distinct tracepoint actually fires; glibc/Go wrappers can redirect chown to
+// fchownat and hide the syscall under test. The temp file and symlink are
+// cleaned up afterwards.
+func chownBasic() error {
+	dir, cleanup, err := makeTempDir("chown-basic")
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+
+	path := filepath.Join(dir, "chownfile.txt")
+	fd, err := syscall.Open(path, syscall.O_RDWR|syscall.O_CREAT, 0o644)
+	if err != nil {
+		return fmt.Errorf("open: %w", err)
+	}
+	defer syscall.Close(fd)
+
+	symlink := filepath.Join(dir, "chownlink")
+	if err := syscall.Symlink(path, symlink); err != nil {
+		return fmt.Errorf("symlink: %w", err)
+	}
+
+	pathBytes, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return fmt.Errorf("path bytes: %w", err)
+	}
+	symlinkBytes, err := syscall.BytePtrFromString(symlink)
+	if err != nil {
+		return fmt.Errorf("symlink bytes: %w", err)
+	}
+
+	if err := callChown(pathBytes); err != nil {
+		return err
+	}
+	if err := callLchown(symlinkBytes); err != nil {
+		return err
+	}
+	if err := callFchownat(pathBytes); err != nil {
+		return err
+	}
+	if err := callFchown(fd); err != nil {
+		return err
+	}
+	return nil
+}
+
+// callChown issues raw chown(path, -1, -1). chown takes the filesystem path at
+// args[0], so ior captures it as a KindPathname event under enter_chown. Owner
+// and group -1 mean "leave both unchanged", so no CAP_CHOWN is required.
+func callChown(pathBytes *byte) error {
+	_, _, errno := syscall.Syscall(
+		syscall.SYS_CHOWN,
+		uintptr(unsafe.Pointer(pathBytes)),
+		uintptr(_UID_NOCHANGE),
+		uintptr(_GID_NOCHANGE),
+	)
+	runtime.KeepAlive(pathBytes)
+	if errno != 0 {
+		return fmt.Errorf("chown: %w", errno)
+	}
+	return nil
+}
+
+// callLchown issues raw lchown(symlink, -1, -1). Like chown the path is at
+// args[0] (KindPathname), but lchown acts on the symlink itself rather than its
+// target. The -1/-1 owner/group keep it unprivileged.
+func callLchown(symlinkBytes *byte) error {
+	_, _, errno := syscall.Syscall(
+		syscall.SYS_LCHOWN,
+		uintptr(unsafe.Pointer(symlinkBytes)),
+		uintptr(_UID_NOCHANGE),
+		uintptr(_GID_NOCHANGE),
+	)
+	runtime.KeepAlive(symlinkBytes)
+	if errno != 0 {
+		return fmt.Errorf("lchown: %w", errno)
+	}
+	return nil
+}
+
+// callFchownat issues raw fchownat(AT_FDCWD, path, -1, -1, 0). The path is at
+// args[1] (after the dirfd), so ior captures it as a KindPathname event under
+// enter_fchownat. A runtime int holds AT_FDCWD so the negative value survives
+// the uintptr conversion instead of overflowing.
+func callFchownat(pathBytes *byte) error {
+	dirfd := _AT_FDCWD
+	_, _, errno := syscall.Syscall6(
+		syscall.SYS_FCHOWNAT,
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(pathBytes)),
+		uintptr(_UID_NOCHANGE),
+		uintptr(_GID_NOCHANGE),
+		0, // flags
+		0,
+	)
+	runtime.KeepAlive(pathBytes)
+	if errno != 0 {
+		return fmt.Errorf("fchownat: %w", errno)
+	}
+	return nil
+}
+
+// callFchown issues raw fchown(fd, -1, -1). fchown operates on an open fd at
+// args[0] (KindFd) and carries no path, so ior records enter_fchown keyed by
+// the descriptor rather than a filename.
+func callFchown(fd int) error {
+	_, _, errno := syscall.Syscall(
+		syscall.SYS_FCHOWN,
+		uintptr(fd),
+		uintptr(_UID_NOCHANGE),
+		uintptr(_GID_NOCHANGE),
+	)
+	if errno != 0 {
+		return fmt.Errorf("fchown: %w", errno)
+	}
+	return nil
+}
+
+// _UID_NOCHANGE and _GID_NOCHANGE are the (uid_t)-1 / (gid_t)-1 sentinels that
+// tell the chown family "leave this id unchanged". Using them for both owner
+// and group means the call performs no ownership change and therefore needs no
+// CAP_CHOWN, keeping the scenario fully unprivileged. They are -1 cast to the
+// unsigned 32-bit id types, i.e. 0xFFFFFFFF, which we form directly so the
+// value passed through uintptr is the kernel's expected (uid_t)-1.
+const (
+	_UID_NOCHANGE uint32 = 0xFFFFFFFF
+	_GID_NOCHANGE uint32 = 0xFFFFFFFF
+)
diff --git a/cmd/ioworkload/scenarios.go b/cmd/ioworkload/scenarios.go
index c234f78..131e5f0 100644
--- a/cmd/ioworkload/scenarios.go
+++ b/cmd/ioworkload/scenarios.go
@@ -111,6 +111,7 @@ var scenarios = map[string]func() error{
 	"xattr-listxattrat":             xattrListxattrat,
 	"xattr-removexattrat":           xattrRemovexattrat,
 	"chmod-basic":                   chmodBasic,
+	"chown-basic":                   chownBasic,
 	"utime-basic":                   utimeBasic,
 	"utime-utimes":                  utimeUtimes,
 	"utime-enoent":                  utimeEnoent,