Re: Re: [PATCH 1/5] bpf: Allow non struct type for btf ctx access

[Yonghong Song <yhs@fb.com>]

On 1/7/20 9:55 AM, Yonghong Song wrote:
> 
> 
> On 1/7/20 7:50 AM, Jiri Olsa wrote:
>> On Tue, Jan 07, 2020 at 01:13:23PM +0100, Jiri Olsa wrote:
>>> On Mon, Jan 06, 2020 at 09:36:17PM +0000, Yonghong Song wrote:
>>>>
>>>>
>>>> On 12/29/19 6:37 AM, Jiri Olsa wrote:
>>>>> I'm not sure why the restriction was added,
>>>>> but I can't access pointers to POD types like
>>>>> const char * when probing vfs_read function.
>>>>>
>>>>> Removing the check and allow non struct type
>>>>> access in context.
>>>>>
>>>>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>>>>> ---
>>>>>     kernel/bpf/btf.c | 6 ------
>>>>>     1 file changed, 6 deletions(-)
>>>>>
>>>>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>>>>> index ed2075884724..ae90f60ac1b8 100644
>>>>> --- a/kernel/bpf/btf.c
>>>>> +++ b/kernel/bpf/btf.c
>>>>> @@ -3712,12 +3712,6 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>>>>     	/* skip modifiers */
>>>>>     	while (btf_type_is_modifier(t))
>>>>>     		t = btf_type_by_id(btf, t->type);
>>>>> -	if (!btf_type_is_struct(t)) {
>>>>> -		bpf_log(log,
>>>>> -			"func '%s' arg%d type %s is not a struct\n",
>>>>> -			tname, arg, btf_kind_str[BTF_INFO_KIND(t->info)]);
>>>>> -		return false;
>>>>> -	}
>>>>
>>>> Hi, Jiri, the RFC looks great! Especially, you also referenced this will
>>>> give great performance boost for bcc scripts.
>>>>
>>>> Could you provide more context on why the above change is needed?
>>>> The function btf_ctx_access is used to check validity of accessing
>>>> function parameters which are wrapped inside a structure, I am wondering
>>>> what kinds of accesses you tried to address here.
>>>
>>> when I was transforming opensnoop.py to use this I got fail in
>>> there when I tried to access filename arg in do_sys_open
>>>
>>> but actualy it seems this should get recognized earlier by:
>>>
>>>             if (btf_type_is_int(t))
>>>                   /* accessing a scalar */
>>>                   return true;
>>>
>>> I'm not sure why it did not pass for const char*, I'll check
>>
>> it seems we don't check for pointer to scalar (just void),
>> which is the case in my example 'const char *filename'
> 
> Thanks for clarification. See some comments below.
> 
>>
>> I'll post this in v2 with other changes
>>
>> jirka
>>
>>
>> ---
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index ed2075884724..650df4ed346e 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -3633,7 +3633,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>    		    const struct bpf_prog *prog,
>>    		    struct bpf_insn_access_aux *info)
>>    {
>> -	const struct btf_type *t = prog->aux->attach_func_proto;
>> +	const struct btf_type *tp, *t = prog->aux->attach_func_proto;
>>    	struct bpf_prog *tgt_prog = prog->aux->linked_prog;
>>    	struct btf *btf = bpf_prog_get_target_btf(prog);
>>    	const char *tname = prog->aux->attach_func_name;
>> @@ -3695,6 +3695,17 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>    		 */
>>    		return true;
>>    
>> +	tp = btf_type_by_id(btf, t->type);
>> +	/* skip modifiers */
>> +	while (btf_type_is_modifier(tp))
>> +		tp = btf_type_by_id(btf, tp->type);
>> +
>> +	if (btf_type_is_int(tp))
>> +		/* This is a pointer scalar.
>> +		 * It is the same as scalar from the verifier safety pov.
>> +		 */
>> +		return true;
> 
> This should work since:
>      - the int pointer will be treated as a scalar later on
>      - bpf_probe_read() will be used to read the contents
> 
> I am wondering whether we should add proper verifier support
> to allow pointer to int ctx access. There, users do not need
> to use bpf_probe_read() to dereference the pointer.
> 
> Discussed with Martin, maybe somewhere in check_ptr_to_btf_access(),
> before btf_struct_access(), checking if it is a pointer to int/enum,
> it should just allow and return SCALAR_VALUE.

double checked check_ptr_to_btf_access() and btf_struct_access().
btf_struct_access() already returns SCALAR_VALUE for pointer to 
int/enum. So verifier change is probably not needed.

In your above code, could you do
    btf_type_is_int(t) || btf_type_is_enum(t)
which will cover pointer to enum as well?

> 
> If you do verifier changes, please ensure bpf_probe_read() is not
> needed any more. In bcc, you need to hack to prevent rewriter to
> re-introduce bpf_probe_read() :-).
> 
>> +
>>    	/* this is a pointer to another type */
>>    	info->reg_type = PTR_TO_BTF_ID;
>>    	info->btf_id = t->type;
>>