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 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.

> +}
> +
> +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();
>   
[...]