Re: [PATCH v3 bpf-next 1/5] bpf: Add support for forcing kfunc args to be referenced

[Yonghong Song <yhs@fb.com>]

On 5/18/22 11:06 AM, Kumar Kartikeya Dwivedi wrote:
> On Wed, May 18, 2022 at 11:28:12PM IST, Yonghong Song wrote:
>>
>>
>> On 5/18/22 3:43 AM, Lorenzo Bianconi wrote:
>>> From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>>>
>>> Similar to how we detect mem, size pairs in kfunc, teach verifier to
>>> treat __ref suffix on argument name to imply that it must be a
>>> referenced pointer when passed to kfunc. This is required to ensure that
>>> kfunc that operate on some object only work on acquired pointers and not
>>> normal PTR_TO_BTF_ID with same type which can be obtained by pointer
>>> walking. Release functions need not specify such suffix on release
>>> arguments as they are already expected to receive one referenced
>>> argument.
>>>
>>> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>>> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
>>> ---
>>>    kernel/bpf/btf.c   | 40 ++++++++++++++++++++++++++++++----------
>>>    net/bpf/test_run.c |  5 +++++
>>>    2 files changed, 35 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>>> index 2f0b0440131c..83a354732d96 100644
>>> --- a/kernel/bpf/btf.c
>>> +++ b/kernel/bpf/btf.c
>>> @@ -6021,18 +6021,13 @@ static bool __btf_type_is_scalar_struct(struct bpf_verifier_log *log,
>>>    	return true;
>>>    }
>>> -static bool is_kfunc_arg_mem_size(const struct btf *btf,
>>> -				  const struct btf_param *arg,
>>> -				  const struct bpf_reg_state *reg)
>>> +static bool btf_param_match_suffix(const struct btf *btf,
>>> +				   const struct btf_param *arg,
>>> +				   const char *suffix)
>>>    {
>>> -	int len, sfx_len = sizeof("__sz") - 1;
>>> -	const struct btf_type *t;
>>> +	int len, sfx_len = strlen(suffix);
>>>    	const char *param_name;
>>> -	t = btf_type_skip_modifiers(btf, arg->type, NULL);
>>> -	if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
>>> -		return false;
>>> -
>>>    	/* In the future, this can be ported to use BTF tagging */
>>>    	param_name = btf_name_by_offset(btf, arg->name_off);
>>>    	if (str_is_empty(param_name))
>>> @@ -6041,12 +6036,31 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
>>>    	if (len < sfx_len)
>>>    		return false;
>>>    	param_name += len - sfx_len;
>>> -	if (strncmp(param_name, "__sz", sfx_len))
>>> +	if (strncmp(param_name, suffix, sfx_len))
>>>    		return false;
>>>    	return true;
>>>    }
>>> +static bool is_kfunc_arg_ref(const struct btf *btf,
>>> +			     const struct btf_param *arg)
>>> +{
>>> +	return btf_param_match_suffix(btf, arg, "__ref");
>>
>> Do we also need to do btf_type_skip_modifiers and to ensure
>> the type after skipping modifiers are a pointer type?
>> The current implementation should work for
>> bpf_kfunc_call_test_ref(), but with additional checking
>> we may avoid some accidental mistakes.
>>
> 
> The point where this check happens, arg[i].type is already known to be a pointer
> type, after skipping modifiers.

Okay, maybe we can add a comment here like
	/* the arg has been verified as a pointer */
But anyway, this is minor.

Acked-by: Yonghong Song <yhs@fb.com>

> 
>>> +}
>>> +
>>> +static bool is_kfunc_arg_mem_size(const struct btf *btf,
>>> +				  const struct btf_param *arg,
>>> +				  const struct bpf_reg_state *reg)
>>> +{
>>> +	const struct btf_type *t;
>>> +
>>> +	t = btf_type_skip_modifiers(btf, arg->type, NULL);
>>> +	if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
>>> +		return false;
>>> +
>>> +	return btf_param_match_suffix(btf, arg, "__sz");
>>> +}
>>> +
>>>    static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>>>    				    const struct btf *btf, u32 func_id,
>>>    				    struct bpf_reg_state *regs,
>>> @@ -6115,6 +6129,12 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>>>    			return -EINVAL;
>>>    		}
>>> +		/* Check if argument must be a referenced pointer */
>>> +		if (is_kfunc && is_kfunc_arg_ref(btf, args + i) && !reg->ref_obj_id) {
>>> +			bpf_log(log, "R%d must be referenced\n", regno);
>>> +			return -EINVAL;
>>> +		}
>>> +
>>>    		ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
>>>    		ref_tname = btf_name_by_offset(btf, ref_t->name_off);
>>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>>> index 4d08cca771c7..adbc7dd18511 100644
>>> --- a/net/bpf/test_run.c
>>> +++ b/net/bpf/test_run.c
>>> @@ -690,6 +690,10 @@ noinline void bpf_kfunc_call_test_mem_len_fail2(u64 *mem, int len)
>>>    {
>>>    }
>>> +noinline void bpf_kfunc_call_test_ref(struct prog_test_ref_kfunc *p__ref)
>>> +{
>>> +}
>>> +
>>>    __diag_pop();
>> [...]
> 
> --
> Kartikeya