os: add Root.Symlink

For #67002

Change-Id: Ia1637b61eae49e97e1d07f058ad2390e74cd3403
Reviewed-on: https://go-review.googlesource.com/c/go/+/660635
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Auto-Submit: Damien Neil <dneil@google.com>

14 files changed, 303 insertions, 9 deletions

diff --git a/src/internal/syscall/unix/asm_darwin.s b/src/internal/syscall/unix/asm_darwin.s
index 79d384c941..9803c7260f 100644
--- a/src/internal/syscall/unix/asm_darwin.s
+++ b/src/internal/syscall/unix/asm_darwin.s
@@ -29,3 +29,4 @@ 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)
+TEXT ·libc_symlinkat_trampoline(SB),NOSPLIT,$0-0; JMP libc_symlinkat(SB)
diff --git a/src/internal/syscall/unix/asm_openbsd.s b/src/internal/syscall/unix/asm_openbsd.s
index 481dd7d700..d7c230555c 100644
--- a/src/internal/syscall/unix/asm_openbsd.s
+++ b/src/internal/syscall/unix/asm_openbsd.s
@@ -22,3 +22,5 @@ TEXT ·libc_renameat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_renameat(SB)
 TEXT ·libc_linkat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_linkat(SB)
+TEXT ·libc_symlinkat_trampoline(SB),NOSPLIT,$0-0
+        JMP	libc_symlinkat(SB)
diff --git a/src/internal/syscall/unix/at.go b/src/internal/syscall/unix/at.go
index 4549a07f8c..96272afc7b 100644
--- a/src/internal/syscall/unix/at.go
+++ b/src/internal/syscall/unix/at.go
@@ -158,3 +158,22 @@ func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int
 	}
 	return nil
 }
+
+func Symlinkat(oldpath string, newdirfd int, newpath string) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall.Syscall(symlinkatTrap,
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)))
+	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 573554927e..8bf7b4dd81 100644
--- a/src/internal/syscall/unix/at_aix.go
+++ b/src/internal/syscall/unix/at_aix.go
@@ -10,6 +10,7 @@ package unix
 //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_symlinkat symlinkat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_unlinkat unlinkat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_readlinkat readlinkat "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_mkdirat mkdirat "libc.a/shr_64.o"
diff --git a/src/internal/syscall/unix/at_darwin.go b/src/internal/syscall/unix/at_darwin.go
index 61437672ee..c74c827626 100644
--- a/src/internal/syscall/unix/at_darwin.go
+++ b/src/internal/syscall/unix/at_darwin.go
@@ -154,3 +154,29 @@ func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int
 	}
 	return nil
 }
+
+func libc_symlinkat_trampoline()
+
+//go:cgo_import_dynamic libc_symlinkat symlinkat "/usr/lib/libSystem.B.dylib"
+
+func Symlinkat(oldpath string, newdirfd int, newpath string) 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_symlinkat_trampoline),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0,
+		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 b32d3bba39..5c64b34d48 100644
--- a/src/internal/syscall/unix/at_libc.go
+++ b/src/internal/syscall/unix/at_libc.go
@@ -20,6 +20,7 @@ import (
 //go:linkname procFchownat libc_fchownat
 //go:linkname procRenameat libc_renameat
 //go:linkname procLinkat libc_linkat
+//go:linkname procSymlinkat libc_symlinkat
 
 var (
 	procFstatat,
@@ -30,7 +31,8 @@ var (
 	procFchmodat,
 	procFchownat,
 	procRenameat,
-	procLinkat uintptr
+	procLinkat,
+	procSymlinkat uintptr
 )
 
 func Unlinkat(dirfd int, path string, flags int) error {
@@ -208,3 +210,23 @@ func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int
 	}
 	return nil
 }
+
+func Symlinkat(oldpath string, newdirfd int, newpath string) 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(&procSymlinkat)), 3,
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0, 0, 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 2a433930f3..0fd5e90e5c 100644
--- a/src/internal/syscall/unix/at_openbsd.go
+++ b/src/internal/syscall/unix/at_openbsd.go
@@ -145,3 +145,29 @@ func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int
 	}
 	return nil
 }
+
+func libc_symlinkat_trampoline()
+
+//go:cgo_import_dynamic libc_symlinkat symlinkat "libc.so"
+
+func Symlinkat(oldpath string, newdirfd int, newpath string) 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_symlinkat_trampoline),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0,
+		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 abfda15688..5d69ae5bee 100644
--- a/src/internal/syscall/unix/at_solaris.go
+++ b/src/internal/syscall/unix/at_solaris.go
@@ -19,6 +19,7 @@ func rawSyscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, e
 //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_symlinkat symlinkat "libc.so"
 //go:cgo_import_dynamic libc_unlinkat unlinkat "libc.so"
 //go:cgo_import_dynamic libc_readlinkat readlinkat "libc.so"
 //go:cgo_import_dynamic libc_mkdirat mkdirat "libc.so"
diff --git a/src/internal/syscall/unix/at_sysnum_dragonfly.go b/src/internal/syscall/unix/at_sysnum_dragonfly.go
index 3ba2c54152..9728b969c4 100644
--- a/src/internal/syscall/unix/at_sysnum_dragonfly.go
+++ b/src/internal/syscall/unix/at_sysnum_dragonfly.go
@@ -16,6 +16,7 @@ const (
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap   uintptr = syscall.SYS_RENAMEAT
 	linkatTrap     uintptr = syscall.SYS_LINKAT
+	symlinkatTrap  uintptr = syscall.SYS_SYMLINKAT
 
 	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 032b8b5276..c1fdcabf41 100644
--- a/src/internal/syscall/unix/at_sysnum_freebsd.go
+++ b/src/internal/syscall/unix/at_sysnum_freebsd.go
@@ -23,4 +23,5 @@ const (
 	fchownatTrap       uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap       uintptr = syscall.SYS_RENAMEAT
 	linkatTrap         uintptr = syscall.SYS_LINKAT
+	symlinkatTrap      uintptr = syscall.SYS_SYMLINKAT
 )
diff --git a/src/internal/syscall/unix/at_sysnum_linux.go b/src/internal/syscall/unix/at_sysnum_linux.go
index 6b8bebff2a..bb7f244fe2 100644
--- a/src/internal/syscall/unix/at_sysnum_linux.go
+++ b/src/internal/syscall/unix/at_sysnum_linux.go
@@ -14,6 +14,7 @@ const (
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
 	linkatTrap     uintptr = syscall.SYS_LINKAT
+	symlinkatTrap  uintptr = syscall.SYS_SYMLINKAT
 )
 
 const (
diff --git a/src/internal/syscall/unix/at_sysnum_netbsd.go b/src/internal/syscall/unix/at_sysnum_netbsd.go
index 01e10ddd59..b59b5e0cf9 100644
--- a/src/internal/syscall/unix/at_sysnum_netbsd.go
+++ b/src/internal/syscall/unix/at_sysnum_netbsd.go
@@ -16,6 +16,7 @@ const (
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
 	renameatTrap   uintptr = syscall.SYS_RENAMEAT
 	linkatTrap     uintptr = syscall.SYS_LINKAT
+	symlinkatTrap  uintptr = syscall.SYS_SYMLINKAT
 )
 
 const (
diff --git a/src/internal/syscall/unix/at_wasip1.go b/src/internal/syscall/unix/at_wasip1.go
index 72537caf1e..dfbb365f2a 100644
--- a/src/internal/syscall/unix/at_wasip1.go
+++ b/src/internal/syscall/unix/at_wasip1.go
@@ -101,6 +101,10 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	))
 }
 
+//go:wasmimport wasi_snapshot_preview1 path_create_directory
+//go:noescape
+func path_create_directory(fd int32, path *byte, pathLen size) syscall.Errno
+
 func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
 	// WASI preview 1 doesn't support changing file modes.
 	return syscall.ENOSYS
@@ -111,10 +115,6 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	return syscall.ENOSYS
 }
 
-//go:wasmimport wasi_snapshot_preview1 path_rename
-//go:noescape
-func path_rename(oldFd int32, oldPath *byte, oldPathLen size, newFd int32, newPath *byte, newPathLen size) syscall.Errno
-
 func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
 	if oldpath == "" || newpath == "" {
 		return syscall.EINVAL
@@ -129,9 +129,9 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error
 	))
 }
 
-//go:wasmimport wasi_snapshot_preview1 path_link
+//go:wasmimport wasi_snapshot_preview1 path_rename
 //go:noescape
-func path_link(oldFd int32, oldFlags uint32, oldPath *byte, oldPathLen size, newFd int32, newPath *byte, newPathLen size) syscall.Errno
+func path_rename(oldFd int32, 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 == "" {
@@ -148,9 +148,26 @@ func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flag int
 	))
 }
 
-//go:wasmimport wasi_snapshot_preview1 path_create_directory
+//go:wasmimport wasi_snapshot_preview1 path_link
 //go:noescape
-func path_create_directory(fd int32, path *byte, pathLen size) syscall.Errno
+func path_link(oldFd int32, oldFlags uint32, oldPath *byte, oldPathLen size, newFd int32, newPath *byte, newPathLen size) syscall.Errno
+
+func Symlinkat(oldpath string, newdirfd int, newpath string) error {
+	if oldpath == "" || newpath == "" {
+		return syscall.EINVAL
+	}
+	return errnoErr(path_symlink(
+		unsafe.StringData(oldpath),
+		size(len(oldpath)),
+		int32(newdirfd),
+		unsafe.StringData(newpath),
+		size(len(newpath)),
+	))
+}
+
+//go:wasmimport wasi_snapshot_preview1 path_symlink
+//go:noescape
+func path_symlink(oldPath *byte, oldPathLen size, fd int32, newPath *byte, newPathLen size) syscall.Errno
 
 func errnoErr(errno syscall.Errno) error {
 	if errno == 0 {
diff --git a/src/internal/syscall/windows/at_windows.go b/src/internal/syscall/windows/at_windows.go
index 4b939d46ab..f04de276b9 100644
--- a/src/internal/syscall/windows/at_windows.go
+++ b/src/internal/syscall/windows/at_windows.go
@@ -5,6 +5,8 @@
 package windows
 
 import (
+	"runtime"
+	"structs"
 	"syscall"
 	"unsafe"
 )
@@ -376,3 +378,176 @@ func Linkat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle, ne
 	}
 	return err
 }
+
+// SymlinkatFlags configure Symlinkat.
+//
+// Symbolic links have two properties: They may be directory or file links,
+// and they may be absolute or relative.
+//
+// The Windows API defines flags describing these properties
+// (SYMBOLIC_LINK_FLAG_DIRECTORY and SYMLINK_FLAG_RELATIVE),
+// but the flags are passed to different system calls and
+// do not have distinct values, so we define our own enumeration
+// that permits expressing both.
+type SymlinkatFlags uint
+
+const (
+	SYMLINKAT_DIRECTORY = SymlinkatFlags(1 << iota)
+	SYMLINKAT_RELATIVE
+)
+
+func Symlinkat(oldname string, newdirfd syscall.Handle, newname string, flags SymlinkatFlags) error {
+	// Temporarily acquire symlink-creating privileges if possible.
+	// This is the behavior of CreateSymbolicLinkW.
+	//
+	// (When passed the SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag,
+	// CreateSymbolicLinkW ignores errors in acquiring privileges, as we do here.)
+	return withPrivilege("SeCreateSymbolicLinkPrivilege", func() error {
+		return symlinkat(oldname, newdirfd, newname, flags)
+	})
+}
+
+func symlinkat(oldname string, newdirfd syscall.Handle, newname string, flags SymlinkatFlags) error {
+	oldnameu16, err := syscall.UTF16FromString(oldname)
+	if err != nil {
+		return err
+	}
+	oldnameu16 = oldnameu16[:len(oldnameu16)-1] // trim off terminal NUL
+
+	var options uint32
+	if flags&SYMLINKAT_DIRECTORY != 0 {
+		options |= FILE_DIRECTORY_FILE
+	} else {
+		options |= FILE_NON_DIRECTORY_FILE
+	}
+
+	objAttrs := &OBJECT_ATTRIBUTES{}
+	if err := objAttrs.init(newdirfd, newname); err != nil {
+		return err
+	}
+	var h syscall.Handle
+	err = NtCreateFile(
+		&h,
+		SYNCHRONIZE|FILE_WRITE_ATTRIBUTES|DELETE,
+		objAttrs,
+		&IO_STATUS_BLOCK{},
+		nil,
+		syscall.FILE_ATTRIBUTE_NORMAL,
+		0,
+		FILE_CREATE,
+		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT|options,
+		0,
+		0,
+	)
+	if err != nil {
+		return ntCreateFileError(err, 0)
+	}
+	defer syscall.CloseHandle(h)
+
+	// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer
+	type reparseDataBufferT struct {
+		_ structs.HostLayout
+
+		ReparseTag        uint32
+		ReparseDataLength uint16
+		Reserved          uint16
+
+		SubstituteNameOffset uint16
+		SubstituteNameLength uint16
+		PrintNameOffset      uint16
+		PrintNameLength      uint16
+		Flags                uint32
+	}
+
+	const (
+		headerSize = uint16(unsafe.Offsetof(reparseDataBufferT{}.SubstituteNameOffset))
+		bufferSize = uint16(unsafe.Sizeof(reparseDataBufferT{}))
+	)
+
+	// Data buffer containing a SymbolicLinkReparseBuffer followed by the link target.
+	rdbbuf := make([]byte, bufferSize+uint16(2*len(oldnameu16)))
+
+	rdb := (*reparseDataBufferT)(unsafe.Pointer(&rdbbuf[0]))
+	rdb.ReparseTag = syscall.IO_REPARSE_TAG_SYMLINK
+	rdb.ReparseDataLength = uint16(len(rdbbuf)) - uint16(headerSize)
+	rdb.SubstituteNameOffset = 0
+	rdb.SubstituteNameLength = uint16(2 * len(oldnameu16))
+	rdb.PrintNameOffset = 0
+	rdb.PrintNameLength = rdb.SubstituteNameLength
+	if flags&SYMLINKAT_RELATIVE != 0 {
+		rdb.Flags = SYMLINK_FLAG_RELATIVE
+	}
+
+	namebuf := rdbbuf[bufferSize:]
+	copy(namebuf, unsafe.String((*byte)(unsafe.Pointer(&oldnameu16[0])), 2*len(oldnameu16)))
+
+	err = syscall.DeviceIoControl(
+		h,
+		FSCTL_SET_REPARSE_POINT,
+		&rdbbuf[0],
+		uint32(len(rdbbuf)),
+		nil,
+		0,
+		nil,
+		nil)
+	if err != nil {
+		// Creating the symlink has failed, so try to remove the file.
+		const FileDispositionInformation = 13
+		NtSetInformationFile(
+			h,
+			&IO_STATUS_BLOCK{},
+			uintptr(unsafe.Pointer(&FILE_DISPOSITION_INFORMATION{
+				DeleteFile: true,
+			})),
+			uint32(unsafe.Sizeof(FILE_DISPOSITION_INFORMATION{})),
+			FileDispositionInformation,
+		)
+		return err
+	}
+
+	return nil
+}
+
+// withPrivilege temporariliy acquires the named privilege and runs f.
+// If the privilege cannot be acquired it runs f anyway,
+// which should fail with an appropriate error.
+func withPrivilege(privilege string, f func() error) error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	err := ImpersonateSelf(SecurityImpersonation)
+	if err != nil {
+		return f()
+	}
+	defer RevertToSelf()
+
+	curThread, err := GetCurrentThread()
+	if err != nil {
+		return f()
+	}
+	var token syscall.Token
+	err = OpenThreadToken(curThread, syscall.TOKEN_QUERY|TOKEN_ADJUST_PRIVILEGES, false, &token)
+	if err != nil {
+		return f()
+	}
+	defer syscall.CloseHandle(syscall.Handle(token))
+
+	privStr, err := syscall.UTF16PtrFromString(privilege)
+	if err != nil {
+		return f()
+	}
+	var tokenPriv TOKEN_PRIVILEGES
+	err = LookupPrivilegeValue(nil, privStr, &tokenPriv.Privileges[0].Luid)
+	if err != nil {
+		return f()
+	}
+
+	tokenPriv.PrivilegeCount = 1
+	tokenPriv.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED
+	err = AdjustTokenPrivileges(token, false, &tokenPriv, 0, nil, nil)
+	if err != nil {
+		return f()
+	}
+
+	return f()
+}