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SubjectRe: [PATCH bpf-next v6 05/23] bpf/verifier: allow kfunc to return an allocated mem
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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.
[...]

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