Neither the dentry nor the argv[0] seems a good solution

There is a reason the user cannot access the comm when a process has been executed with fexecve(). That is because the kernel has no guarantees that the file being executed is the same as the one on the disk. A process could open an executable file, tamper with it, then execute it. The real safe behavior is the existing one, neither the dentry nor the argv[0] seems a good solution.