1 files changed, 51 insertions, 0 deletions
diff --git a/src/internal/syscall/unix/fchmodat_linux.go b/src/internal/syscall/unix/fchmodat_linux.go
new file mode 100644
index 0000000000..786ec29df2
--- /dev/null
+++ b/src/internal/syscall/unix/fchmodat_linux.go
@@ -0,0 +1,51 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+
+package unix
+
+import (
+	"internal/strconv"
+	"syscall"
+)
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	// On Linux, the fchmodat syscall silently ignores the AT_SYMLINK_NOFOLLOW flag.
+	// We need to use fchmodat2 instead.
+	// syscall.Fchmodat handles this.
+	if err := syscall.Fchmodat(dirfd, path, mode, flags); err != syscall.EOPNOTSUPP {
+		return err
+	}
+
+	// This kernel doesn't appear to support fchmodat2 (added in Linux 6.6).
+	// We can't fall back to Fchmod, because it requires write permissions on the file.
+	// Instead, use the same workaround as GNU libc and musl, which is to open the file
+	// and then fchmodat the FD in /proc/self/fd.
+	// See: https://lwn.net/Articles/939217/
+	fd, err := Openat(dirfd, path, O_PATH|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(fd)
+	procPath := "/proc/self/fd/" + strconv.Itoa(fd)
+
+	// Check to see if this file is a symlink.
+	// (We passed O_NOFOLLOW above, but O_PATH|O_NOFOLLOW will open a symlink.)
+	var st syscall.Stat_t
+	if err := syscall.Stat(procPath, &st); err != nil {
+		if err == syscall.ENOENT {
+			// /proc has probably not been mounted. Give up.
+			return syscall.EOPNOTSUPP
+		}
+		return err
+	}
+	if st.Mode&syscall.S_IFMT == syscall.S_IFLNK {
+		// fchmodat on the proc FD for a symlink apparently gives inconsistent
+		// results, so just refuse to try.
+		return syscall.EOPNOTSUPP
+	}
+
+	return syscall.Fchmodat(AT_FDCWD, procPath, mode, flags&^AT_SYMLINK_NOFOLLOW)
+}