Shares: allow any folder, not only ZFS datasets

SMB shares and NFS exports may now point to any existing directory
(USB disks under /mnt or /media, /srv, home folders...) instead of only
ZFS dataset mountpoints. System trees (/etc, /usr, /var, /boot, /proc,
/root, ...) are refused, also after resolving symlinks. The path
suggestions list mounted data filesystems from /proc/self/mounts in
addition to ZFS mountpoints.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/shares/validate.go

@@ -54,8 +54,23 @@ func ValidateComment(s string) error {
 	return nil
 }
 
-// ValidatePath accepts an absolute, clean path without control characters.
-// Whether it lies under a pool is checked by the caller (needs live state).
+// systemRoots are OS trees that must never be shared.
+var systemRoots = []string{"/etc", "/proc", "/sys", "/dev", "/boot", "/run", "/usr", "/bin", "/sbin",
+	"/lib", "/lib32", "/lib64", "/libx32", "/var", "/root", "/snap", "/lost+found"}
+
+// ValidateNotSystem refuses the root filesystem and system directories.
+func ValidateNotSystem(p string) error {
+	if p == "/" {
+		return errors.New("refusing to share the root filesystem")
+	}
+	if UnderRoots(p, systemRoots) {
+		return fmt.Errorf("%s is a system directory and cannot be shared", p)
+	}
+	return nil
+}
+
+// ValidatePath accepts an absolute, clean, non-system path without control
+// characters. Existence and symlinks are checked by the caller.
 func ValidatePath(p string) error {
 	if p == "" {
 		return errors.New("path is required")
@@ -63,10 +78,7 @@ func ValidatePath(p string) error {
 	if !strings.HasPrefix(p, "/") || path.Clean(p) != p || strings.ContainsAny(p, "\"\\\r\n\t") {
 		return errors.New("path must be absolute and normalised (no .., trailing / or quotes)")
 	}
-	if p == "/" {
-		return errors.New("refusing to share the root filesystem")
-	}
-	return nil
+	return ValidateNotSystem(p)
 }
 
 // UnderRoots reports whether p equals or lies below one of the roots.