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

On Sat, Jul 16, 2022 at 6:29 AM Yonghong Song yhs@fb.com wrote:

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.

Yes, exactly.

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

Well, the point is I need to be able to access a memory region that was allocated dynamically. For drivers, the incoming data can not usually be bound to a static value, and so we can not have the data statically defined in the matching struct. So this allows defining a kfunc to return any memory properly allocated and owned by the device.

...

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(...).

Ack, fixed locally

...

•         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?

I used to check for it in the previous version, but I think it got dropped in this revision. Re-adding this thanks to Kumar's help :)

Cheers, Benjamin

...

•                         }
  

•                 }
  

•                 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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