os: add Root.Link

For #67002

Change-Id: I223f3f2dbc8b02726f4ce5a017c628c4a20f109a
Reviewed-on: https://go-review.googlesource.com/c/go/+/659757
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

15 files changed, 183 insertions, 1 deletions

diff --git a/src/internal/syscall/unix/asm_darwin.s b/src/internal/syscall/unix/asm_darwin.s
index a72240f512..79d384c941 100644
--- a/src/internal/syscall/unix/asm_darwin.s
+++ b/src/internal/syscall/unix/asm_darwin.s
@@ -28,3 +28,4 @@ TEXT ·libc_mkdirat_trampoline(SB),NOSPLIT,$0-0; JMP libc_mkdirat(SB)
 TEXT ·libc_fchmodat_trampoline(SB),NOSPLIT,$0-0; JMP libc_fchmodat(SB)
 TEXT ·libc_fchownat_trampoline(SB),NOSPLIT,$0-0; JMP libc_fchownat(SB)
 TEXT ·libc_renameat_trampoline(SB),NOSPLIT,$0-0; JMP libc_renameat(SB)
+TEXT ·libc_linkat_trampoline(SB),NOSPLIT,$0-0; JMP libc_linkat(SB)
diff --git a/src/internal/syscall/unix/asm_openbsd.s b/src/internal/syscall/unix/asm_openbsd.s
index 2b88b6988c..481dd7d700 100644
--- a/src/internal/syscall/unix/asm_openbsd.s
+++ b/src/internal/syscall/unix/asm_openbsd.s
@@ -20,3 +20,5 @@ TEXT ·libc_fchownat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_fchownat(SB)
 TEXT ·libc_renameat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_renameat(SB)
+TEXT ·libc_linkat_trampoline(SB),NOSPLIT,$0-0
+        JMP	libc_linkat(SB)
diff --git a/src/internal/syscall/unix/at.go b/src/internal/syscall/unix/at.go
index be7920c115..4549a07f8c 100644
--- a/src/internal/syscall/unix/at.go
+++ b/src/internal/syscall/unix/at.go
@@ -136,3 +136,25 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	}
 	return nil
 }
+
+func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall.Syscall6(linkatTrap,
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		uintptr(flag),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/at_aix.go b/src/internal/syscall/unix/at_aix.go
index d277cd332f..573554927e 100644
--- a/src/internal/syscall/unix/at_aix.go
+++ b/src/internal/syscall/unix/at_aix.go
@@ -7,6 +7,7 @@ package unix
 //go:cgo_import_dynamic libc_fchmodat fchmodat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_fchownat fchownat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_fstatat fstatat "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_linkat linkat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_openat openat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_renameat renameat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_unlinkat unlinkat "libc.a/shr_64.o"
diff --git a/src/internal/syscall/unix/at_darwin.go b/src/internal/syscall/unix/at_darwin.go
index 4f39b76ad1..61437672ee 100644
--- a/src/internal/syscall/unix/at_darwin.go
+++ b/src/internal/syscall/unix/at_darwin.go
@@ -128,3 +128,29 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	}
 	return nil
 }
+
+func libc_linkat_trampoline()
+
+//go:cgo_import_dynamic libc_linkat linkat "/usr/lib/libSystem.B.dylib"
+
+func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_linkat_trampoline),
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		uintptr(flag),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/at_libc.go b/src/internal/syscall/unix/at_libc.go
index 36a9f22b2a..b32d3bba39 100644
--- a/src/internal/syscall/unix/at_libc.go
+++ b/src/internal/syscall/unix/at_libc.go
@@ -19,6 +19,7 @@ import (
 //go:linkname procFchmodat libc_fchmodat
 //go:linkname procFchownat libc_fchownat
 //go:linkname procRenameat libc_renameat
+//go:linkname procLinkat libc_linkat
 
 var (
 	procFstatat,
@@ -28,7 +29,8 @@ var (
 	procMkdirat,
 	procFchmodat,
 	procFchownat,
-	procRenameat uintptr
+	procRenameat,
+	procLinkat uintptr
 )
 
 func Unlinkat(dirfd int, path string, flags int) error {
@@ -184,3 +186,25 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	}
 	return nil
 }
+
+func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall6(uintptr(unsafe.Pointer(&procLinkat)), 5,
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		uintptr(flag),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/at_openbsd.go b/src/internal/syscall/unix/at_openbsd.go
index bd56aac70d..2a433930f3 100644
--- a/src/internal/syscall/unix/at_openbsd.go
+++ b/src/internal/syscall/unix/at_openbsd.go
@@ -119,3 +119,29 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	}
 	return nil
 }
+
+func libc_linkat_trampoline()
+
+//go:cgo_import_dynamic libc_linkat linkat "libc.so"
+
+func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_linkat_trampoline),
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		uintptr(flag),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/at_solaris.go b/src/internal/syscall/unix/at_solaris.go
index 1241827cff..abfda15688 100644
--- a/src/internal/syscall/unix/at_solaris.go
+++ b/src/internal/syscall/unix/at_solaris.go
@@ -16,6 +16,7 @@ func rawSyscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, e
 //go:cgo_import_dynamic libc_fchmodat fchmodat "libc.so"
 //go:cgo_import_dynamic libc_fchownat fchownat "libc.so"
 //go:cgo_import_dynamic libc_fstatat fstatat "libc.so"
+//go:cgo_import_dynamic libc_linkat linkat "libc.so"
 //go:cgo_import_dynamic libc_openat openat "libc.so"
 //go:cgo_import_dynamic libc_renameat renameat "libc.so"
 //go:cgo_import_dynamic libc_unlinkat unlinkat "libc.so"
diff --git a/src/internal/syscall/unix/at_sysnum_dragonfly.go b/src/internal/syscall/unix/at_sysnum_dragonfly.go
index f4418776cd..3ba2c54152 100644
--- a/src/internal/syscall/unix/at_sysnum_dragonfly.go
+++ b/src/internal/syscall/unix/at_sysnum_dragonfly.go
@@ -15,6 +15,7 @@ const (
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap   uintptr = syscall.SYS_RENAMEAT
+	linkatTrap     uintptr = syscall.SYS_LINKAT
 
 	AT_EACCESS          = 0x4
 	AT_FDCWD            = 0xfffafdcd
diff --git a/src/internal/syscall/unix/at_sysnum_freebsd.go b/src/internal/syscall/unix/at_sysnum_freebsd.go
index d1bec15343..032b8b5276 100644
--- a/src/internal/syscall/unix/at_sysnum_freebsd.go
+++ b/src/internal/syscall/unix/at_sysnum_freebsd.go
@@ -22,4 +22,5 @@ const (
 	fchmodatTrap       uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap       uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap       uintptr = syscall.SYS_RENAMEAT
+	linkatTrap         uintptr = syscall.SYS_LINKAT
 )
diff --git a/src/internal/syscall/unix/at_sysnum_linux.go b/src/internal/syscall/unix/at_sysnum_linux.go
index 35cc4307e9..6b8bebff2a 100644
--- a/src/internal/syscall/unix/at_sysnum_linux.go
+++ b/src/internal/syscall/unix/at_sysnum_linux.go
@@ -13,6 +13,7 @@ const (
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
+	linkatTrap     uintptr = syscall.SYS_LINKAT
 )
 
 const (
diff --git a/src/internal/syscall/unix/at_sysnum_netbsd.go b/src/internal/syscall/unix/at_sysnum_netbsd.go
index db42be58b7..01e10ddd59 100644
--- a/src/internal/syscall/unix/at_sysnum_netbsd.go
+++ b/src/internal/syscall/unix/at_sysnum_netbsd.go
@@ -15,6 +15,7 @@ const (
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap   uintptr = syscall.SYS_RENAMEAT
+	linkatTrap     uintptr = syscall.SYS_LINKAT
 )
 
 const (
diff --git a/src/internal/syscall/unix/at_wasip1.go b/src/internal/syscall/unix/at_wasip1.go
index 2bd55ca0e7..72537caf1e 100644
--- a/src/internal/syscall/unix/at_wasip1.go
+++ b/src/internal/syscall/unix/at_wasip1.go
@@ -129,6 +129,25 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	))
 }
 
+//go:wasmimport wasi_snapshot_preview1 path_link
+//go:noescape
+func path_link(oldFd int32, oldFlags uint32, oldPath *byte, oldPathLen size, newFd int32, newPath *byte, newPathLen size) syscall.Errno
+
+func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int) error {
+	if oldpath == "" || newpath == "" {
+		return syscall.EINVAL
+	}
+	return errnoErr(path_link(
+		int32(olddirfd),
+		0,
+		unsafe.StringData(oldpath),
+		size(len(oldpath)),
+		int32(newdirfd),
+		unsafe.StringData(newpath),
+		size(len(newpath)),
+	))
+}
+
 //go:wasmimport wasi_snapshot_preview1 path_create_directory
 //go:noescape
 func path_create_directory(fd int32, path *byte, pathLen size) syscall.Errno
diff --git a/src/internal/syscall/windows/at_windows.go b/src/internal/syscall/windows/at_windows.go
index 311e143b9b..4b939d46ab 100644
--- a/src/internal/syscall/windows/at_windows.go
+++ b/src/internal/syscall/windows/at_windows.go
@@ -328,3 +328,51 @@ func Renameat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle,
 	}
 	return err
 }
+
+func Linkat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle, newpath string) error {
+	objAttrs := &OBJECT_ATTRIBUTES{}
+	if err := objAttrs.init(olddirfd, oldpath); err != nil {
+		return err
+	}
+	var h syscall.Handle
+	err := NtOpenFile(
+		&h,
+		SYNCHRONIZE|FILE_WRITE_ATTRIBUTES,
+		objAttrs,
+		&IO_STATUS_BLOCK{},
+		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
+		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT,
+	)
+	if err != nil {
+		return ntCreateFileError(err, 0)
+	}
+	defer syscall.CloseHandle(h)
+
+	linkInfo := FILE_LINK_INFORMATION{
+		RootDirectory: newdirfd,
+	}
+	p16, err := syscall.UTF16FromString(newpath)
+	if err != nil {
+		return err
+	}
+	if len(p16) > len(linkInfo.FileName) {
+		return syscall.EINVAL
+	}
+	copy(linkInfo.FileName[:], p16)
+	linkInfo.FileNameLength = uint32((len(p16) - 1) * 2)
+
+	const (
+		FileLinkInformation = 11
+	)
+	err = NtSetInformationFile(
+		h,
+		&IO_STATUS_BLOCK{},
+		uintptr(unsafe.Pointer(&linkInfo)),
+		uint32(unsafe.Sizeof(FILE_LINK_INFORMATION{})),
+		FileLinkInformation,
+	)
+	if st, ok := err.(NTStatus); ok {
+		return st.Errno()
+	}
+	return err
+}
diff --git a/src/internal/syscall/windows/types_windows.go b/src/internal/syscall/windows/types_windows.go
index 718a4b863a..adc8b00bd8 100644
--- a/src/internal/syscall/windows/types_windows.go
+++ b/src/internal/syscall/windows/types_windows.go
@@ -238,3 +238,11 @@ type FILE_RENAME_INFORMATION_EX struct {
 	FileNameLength uint32
 	FileName       [syscall.MAX_PATH]uint16
 }
+
+// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_file_link_information
+type FILE_LINK_INFORMATION struct {
+	ReplaceIfExists bool
+	RootDirectory   syscall.Handle
+	FileNameLength  uint32
+	FileName        [syscall.MAX_PATH]uint16
+}