The three triples are checked exclusively

One triple, not three

A mode is nine bits in three groups: what the owner may do, what the group may do, and what everybody else may do. That much everybody knows.

What people get wrong is that the kernel consults exactly one of them. It asks, in order: are you the owner? then only the owner bits apply. Are you in the group? then only the group bits apply. Otherwise, only the other bits.

So the permissions are not additive. chmod 077 gives the owner nothing and everybody else everything, and the owner really is locked out of their own file while a stranger reads it. That is not a bug or an edge case — it is the rule, and it is the shape of every permission surprise there is.

The owner can always chmod it back, because changing a mode is a question about ownership rather than about the mode. That is the only reason this is survivable.

#include <fcntl.h>
#include <unistd.h>

int main(void) {
  chmod("/work/mine", 0077);
  int fd = open("/work/mine", O_RDONLY);
  return fd;
}

Before you run it

Directories use the same three bits and they mean something different. Read is permission to list the entries. Execute is permission to traverse — to reach something inside by name.