syscall: use fchmodat2 in Fchmodat

The fchmodat2 syscall was added in Linux kernel 6.6.  Mirror the
implementation in golang.org/x/sys/unix.Fchmodat (CL 539635) and use
fchmodat2 in Fchmodat if flags are given. It will return ENOSYS on older
kernels (or EINVAL or any other bogus error in some container
implementations).

Also update ztypes_linux_$GOARCH.go for all linux platforms to add
_AT_EMPTY_PATH. It was added to linux/types in CL 407694 but was only
updated for linux/loong64 at that time.

Updates #61636

Change-Id: I863d06e35cd366f1cf99052e9f77c22ab8168b3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/540435
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>

Author: tklauser

src/syscall/syscall_linux.go

@@ -241,15 +241,23 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 }
 
 //sys	fchmodat(dirfd int, path string, mode uint32) (err error)
-
-func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
-	// Linux fchmodat doesn't support the flags parameter. Mimic glibc's behavior
-	// and check the flags. Otherwise the mode would be applied to the symlink
-	// destination which is not what the user expects.
-	if flags&^_AT_SYMLINK_NOFOLLOW != 0 {
-		return EINVAL
-	} else if flags&_AT_SYMLINK_NOFOLLOW != 0 {
-		return EOPNOTSUPP
+//sys	fchmodat2(dirfd int, path string, mode uint32, flags int) (err error) = _SYS_fchmodat2
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	// Linux fchmodat doesn't support the flags parameter, but fchmodat2 does.
+	// Try fchmodat2 if flags are specified.
+	if flags != 0 {
+		err := fchmodat2(dirfd, path, mode, flags)
+		if err == ENOSYS {
+			// fchmodat2 isn't available. If the flags are known to be valid,
+			// return EOPNOTSUPP to indicate that fchmodat doesn't support them.
+			if flags&^(_AT_SYMLINK_NOFOLLOW|_AT_EMPTY_PATH) != 0 {
+				return EINVAL
+			} else if flags&(_AT_SYMLINK_NOFOLLOW|_AT_EMPTY_PATH) != 0 {
+				return EOPNOTSUPP
+			}
+		}
+		return err
 	}
 	return fchmodat(dirfd, path, mode)
 }

src/syscall/syscall_linux_386.go

@@ -11,6 +11,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 func setTimespec(sec, nsec int64) Timespec {

src/syscall/syscall_linux_amd64.go

@@ -13,6 +13,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 //sys	Dup2(oldfd int, newfd int) (err error)

src/syscall/syscall_linux_arm.go

@@ -11,6 +11,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 func setTimespec(sec, nsec int64) Timespec {

src/syscall/syscall_linux_arm64.go

@@ -11,6 +11,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error) = SYS_EPOLL_PWAIT

src/syscall/syscall_linux_loong64.go

@@ -11,6 +11,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error) = SYS_EPOLL_PWAIT

src/syscall/syscall_linux_mips64x.go

@@ -15,6 +15,7 @@ const (
 	_SYS_clone3            = 5435
 	_SYS_faccessat2        = 5439
 	_SYS_pidfd_send_signal = 5424
+	_SYS_fchmodat2         = 5452
 )
 
 //sys	Dup2(oldfd int, newfd int) (err error)

src/syscall/syscall_linux_mipsx.go

@@ -13,6 +13,7 @@ const (
 	_SYS_clone3            = 4435
 	_SYS_faccessat2        = 4439
 	_SYS_pidfd_send_signal = 4424
+	_SYS_fchmodat2         = 4452
 )
 
 func Syscall9(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err Errno)

src/syscall/syscall_linux_ppc64x.go

@@ -15,6 +15,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 //sys	Dup2(oldfd int, newfd int) (err error)

src/syscall/syscall_linux_riscv64.go

@@ -11,6 +11,7 @@ const (
 	_SYS_clone3            = 435
 	_SYS_faccessat2        = 439
 	_SYS_pidfd_send_signal = 424
+	_SYS_fchmodat2         = 452
 )
 
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error) = SYS_EPOLL_PWAIT