bpf: force BPF_F_RDONLY_PROG on insn array creation

The original implementation added a hack to check_mem_access()
to prevent programs from writing into insn arrays. To get rid
of this hack, enforce BPF_F_RDONLY_PROG on map creation.

Also fix the corresponding selftest, as the error message changes
with this patch.

Suggested-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Anton Protopopov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/bpf_insn_array.c

···
 
 	bpf_map_init_from_attr(&insn_array->map, attr);
 
+	/* BPF programs aren't allowed to write to the map */
+	insn_array->map.map_flags |= BPF_F_RDONLY_PROG;
+
 	return &insn_array->map;
 }
 

kernel/bpf/verifier.c

···
 			verbose(env, "R%d leaks addr into map\n", value_regno);
 			return -EACCES;
 		}
-		if (t == BPF_WRITE && insn_array) {
-			verbose(env, "writes into insn_array not allowed\n");
-			return -EACCES;
-		}
-
 		err = check_map_access_type(env, regno, off, size, t);
 		if (err)
 			return err;
···
 		} else if (t == BPF_READ && value_regno >= 0) {
 			struct bpf_map *map = reg->map_ptr;
 
-			/* if map is read-only, track its contents as scalars */
+			/*
+			 * If map is read-only, track its contents as scalars,
+			 * unless it is an insn array (see the special case below)
+			 */
 			if (tnum_is_const(reg->var_off) &&
 			    bpf_map_is_rdonly(map) &&
-			    map->ops->map_direct_value_addr) {
+			    map->ops->map_direct_value_addr &&
+			    map->map_type != BPF_MAP_TYPE_INSN_ARRAY) {
 				int map_off = off + reg->var_off.value;
 				u64 val = 0;
 

tools/testing/selftests/bpf/progs/verifier_gotox.c

···
 }
 
 SEC("socket")
-__failure __msg("writes into insn_array not allowed")
+__failure __msg("write into map forbidden, value_size=16 off=8 size=8")
 __naked void jump_table_no_writes(void)
 {
 	asm volatile ("						\