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Date:   Fri, 15 Jul 2022 21:29:07 -0700
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Subject: Re: [PATCH bpf-next v6 05/23] bpf/verifier: allow kfunc to return an
 allocated mem
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To:     Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Jiri Kosina <jikos@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Shuah Khan <shuah@kernel.org>,
        Dave Marchevsky <davemarchevsky@fb.com>,
        Joe Stringer <joe@cilium.io>, Jonathan Corbet <corbet@lwn.net>
Cc:     Tero Kristo <tero.kristo@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-input@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org
References: <20220712145850.599666-1-benjamin.tissoires@redhat.com>
 <20220712145850.599666-6-benjamin.tissoires@redhat.com>
From:   Yonghong Song <yhs@fb.com>
In-Reply-To: <20220712145850.599666-6-benjamin.tissoires@redhat.com>
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On 7/12/22 7:58 AM, Benjamin Tissoires wrote:
> When a kfunc is not returning a pointer to a struct but to a plain type,
> we can consider it is a valid allocated memory assuming that:
> - one of the arguments is either called rdonly_buf_size or
>    rdwr_buf_size
> - and this argument is a const from the caller point of view
> 
> We can then use this parameter as the size of the allocated memory.
> 
> The memory is either read-only or read-write based on the name
> of the size parameter.

If I understand correctly, this permits a kfunc like
    int *kfunc(..., int rdonly_buf_size);
    ...
    int *p = kfunc(..., 20);
so the 'p' points to a memory buffer with size 20.

This looks like a strange interface although probably there
is a valid reason for this as I didn't participated in
earlier discussions.

> 
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
> 
> ---
> 
> changes in v6:
> - code review from Kartikeya:
>    - remove comment change that had no reasons to be
>    - remove handling of PTR_TO_MEM with kfunc releases
>    - introduce struct bpf_kfunc_arg_meta
>    - do rdonly/rdwr_buf_size check in btf_check_kfunc_arg_match
>    - reverted most of the changes in verifier.c
>    - make sure kfunc acquire is using a struct pointer, not just a plain
>      pointer
>    - also forward ref_obj_id to PTR_TO_MEM in kfunc to not use after free
>      the allocated memory
> 
> changes in v5:
> - updated PTR_TO_MEM comment in btf.c to match upstream
> - make it read-only or read-write based on the name of size
> 
> new in v4
> ---
>   include/linux/bpf.h   | 10 ++++++-
>   include/linux/btf.h   | 12 ++++++++
>   kernel/bpf/btf.c      | 67 ++++++++++++++++++++++++++++++++++++++++---
>   kernel/bpf/verifier.c | 49 +++++++++++++++++++++++--------
>   4 files changed, 121 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2b21f2a3452f..5b8eadb6e7bc 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1916,12 +1916,20 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>   			   const char *func_name,
>   			   struct btf_func_model *m);
>   
> +struct bpf_kfunc_arg_meta {
> +	u64 r0_size;
> +	bool r0_rdonly;
> +	int ref_obj_id;
> +	bool multiple_ref_obj_id;
> +};
> +
>   struct bpf_reg_state;
>   int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
>   				struct bpf_reg_state *regs);
>   int btf_check_kfunc_arg_match(struct bpf_verifier_env *env,
>   			      const struct btf *btf, u32 func_id,
> -			      struct bpf_reg_state *regs);
> +			      struct bpf_reg_state *regs,
> +			      struct bpf_kfunc_arg_meta *meta);
>   int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
>   			  struct bpf_reg_state *reg);
>   int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 1bfed7fa0428..31da4273c2ec 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -420,4 +420,16 @@ static inline int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dt
>   }
>   #endif
>   
> +static inline bool btf_type_is_struct_ptr(struct btf *btf, const struct btf_type *t)
> +{
> +	/* t comes in already as a pointer */
> +	t = btf_type_by_id(btf, t->type);
> +
> +	/* allow const */
> +	if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
> +		t = btf_type_by_id(btf, t->type);
> +
> +	return btf_type_is_struct(t);
> +}
> +
>   #endif
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4423045b8ff3..552d7bc05a0c 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6168,10 +6168,36 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
>   	return true;
>   }
>   
> +static bool btf_is_kfunc_arg_mem_size(const struct btf *btf,
> +				      const struct btf_param *arg,
> +				      const struct bpf_reg_state *reg,
> +				      const char *name)
> +{
> +	int len, target_len = strlen(name);
> +	const struct btf_type *t;
> +	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;
> +
> +	param_name = btf_name_by_offset(btf, arg->name_off);
> +	if (str_is_empty(param_name))
> +		return false;
> +	len = strlen(param_name);
> +	if (len != target_len)
> +		return false;
> +	if (strncmp(param_name, name, target_len))

strcmp(param_name, name) is enough. len == target_len and both len and
target_len is computed from strlen(...).

> +		return false;
> +
> +	return true;
> +}
> +
>   static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>   				    const struct btf *btf, u32 func_id,
>   				    struct bpf_reg_state *regs,
> -				    bool ptr_to_mem_ok)
> +				    bool ptr_to_mem_ok,
> +				    struct bpf_kfunc_arg_meta *kfunc_meta)
>   {
>   	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>   	struct bpf_verifier_log *log = &env->log;
> @@ -6225,6 +6251,30 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>   
>   		t = btf_type_skip_modifiers(btf, args[i].type, NULL);
>   		if (btf_type_is_scalar(t)) {
> +			if (is_kfunc && kfunc_meta) {
> +				bool is_buf_size = false;
> +
> +				/* check for any const scalar parameter of name "rdonly_buf_size"
> +				 * or "rdwr_buf_size"
> +				 */
> +				if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> +							      "rdonly_buf_size")) {
> +					kfunc_meta->r0_rdonly = true;
> +					is_buf_size = true;
> +				} else if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> +								     "rdwr_buf_size"))
> +					is_buf_size = true;
> +
> +				if (is_buf_size) {
> +					if (kfunc_meta->r0_size) {
> +						bpf_log(log, "2 or more rdonly/rdwr_buf_size parameters for kfunc");
> +						return -EINVAL;
> +					}
> +
> +					kfunc_meta->r0_size = reg->var_off.value;

Did we check 'reg' is a constant somewhere?

> +				}
> +			}
> +
>   			if (reg->type == SCALAR_VALUE)
>   				continue;
>   			bpf_log(log, "R%d is not a scalar\n", regno);
> @@ -6246,6 +6296,14 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>   		if (ret < 0)
>   			return ret;
>   
> +		/* kptr_get is only valid for kfunc */
> +		if (kfunc_meta && reg->ref_obj_id) {
> +			/* check for any one ref_obj_id to keep track of memory */
> +			if (kfunc_meta->ref_obj_id)
> +				kfunc_meta->multiple_ref_obj_id = true;
> +			kfunc_meta->ref_obj_id = reg->ref_obj_id;
> +		}
> +
>   		/* kptr_get is only true for kfunc */
>   		if (i == 0 && kptr_get) {
>   			struct bpf_map_value_off_desc *off_desc;
> @@ -6441,7 +6499,7 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
>   		return -EINVAL;
>   
>   	is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
> -	err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global);
> +	err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, NULL);
>   
>   	/* Compiler optimizations can remove arguments from static functions
>   	 * or mismatched type can be passed into a global function.
[...]