Re: bpf: allow bpf programs to tail-call other bpf programs

[Daniel Borkmann <daniel@iogearbox.net>]

On 05/26/2015 06:01 PM, Alexei Starovoitov wrote:
> On 5/23/15 10:33 AM, Dan Carpenter wrote:
>> Hello Alexei Starovoitov,
>>
>> This is a semi-automatic email about new static checker warnings.
>>
>> The patch 04fd61ab36ec: "bpf: allow bpf programs to tail-call other
>> bpf programs" from May 19, 2015, leads to the following Smatch
>> complaint:
>>
>> kernel/bpf/verifier.c:921 check_call()
>>      error: we previously assumed 'map' could be null (see line 911)
>>
>> kernel/bpf/verifier.c
>>     910
>>     911        if (map && map->map_type = BPF_MAP_TYPE_PROG_ARRAY &&
>>                      ^^^
>> Patch introduces a check for NULL.
>>
>>     912            func_id != BPF_FUNC_tail_call)
>>     913            /* prog_array map type needs extra care:
>>     914             * only allow to pass it into bpf_tail_call() for now.
>>     915             * bpf_map_delete_elem() can be allowed in the future,
>>     916             * while bpf_map_update_elem() must only be done via syscall
>>     917             */
>>     918            return -EINVAL;
>>     919
>>     920        if (func_id = BPF_FUNC_tail_call &&
>>     921            map->map_type != BPF_MAP_TYPE_PROG_ARRAY)
>>                      ^^^^^^^^^^^^^
>> New unchecked dereference.
>
> that is false positive.
> See that 'if (func_id = BPF_FUNC_tail_call ...)' is done before
> map->map_type dereference.
> tail_call_proto has:
> .arg2_type = ARG_CONST_MAP_PTR
> so call at line 869:
> check_func_arg(env, BPF_REG_2, fn->arg2_type, &map);
> will check that arg2 is map_ptr and will assign valid map pointer
> into map variable.
> So if line 920 is true, then line 869 was ok as well and map
> pointer is valid. No need for extra check at line 921.

A bit hard to parse, perhaps. The control flow might have been easier
to understand, if it would have looked like:

if (!map)
	/* Nothing to check further. */
	return 0;

if (map->map_type = BPF_MAP_TYPE_PROG_ARRAY &&
     func_id != BPF_FUNC_tail_call)
	return -EINVAL;

if (map->map_type != BPF_MAP_TYPE_PROG_ARRAY &&
     func_id = BPF_FUNC_tail_call)
	return -EINVAL;

return 0;

Cheers,
Daniel