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

[Yonghong Song <yhs@fb.com>]

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.

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;
>