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Previously >> if users want to access some data available in kctx but not >> in uapi ctx, bpf_probe_read_kernel() helper is needed. >> The introduction of bpf_cast_to_kern_ctx() allows direct >> memory access which makes code simpler and easier to understand. >> >> Signed-off-by: Yonghong Song >> --- >> include/linux/btf.h | 5 +++++ >> kernel/bpf/btf.c | 25 +++++++++++++++++++++++++ >> kernel/bpf/helpers.c | 6 ++++++ >> kernel/bpf/verifier.c | 21 +++++++++++++++++++++ >> 4 files changed, 57 insertions(+) >> >> diff --git a/include/linux/btf.h b/include/linux/btf.h >> index d5b26380a60f..4b5d799f5d02 100644 >> --- a/include/linux/btf.h >> +++ b/include/linux/btf.h >> @@ -470,6 +470,7 @@ const struct btf_member * >> btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf, >> const struct btf_type *t, enum bpf_prog_type prog_type, >> int arg); >> +int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type); >> bool btf_types_are_same(const struct btf *btf1, u32 id1, >> const struct btf *btf2, u32 id2); >> #else >> @@ -514,6 +515,10 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf, >> { >> return NULL; >> } >> +static inline int get_kern_ctx_btf_id(struct bpf_verifier_log *log, >> + enum bpf_prog_type prog_type) { >> + return -EINVAL; >> +} >> static inline bool btf_types_are_same(const struct btf *btf1, u32 id1, >> const struct btf *btf2, u32 id2) >> { >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c >> index 0a3abbe56c5d..bef1b6cfe6b8 100644 >> --- a/kernel/bpf/btf.c >> +++ b/kernel/bpf/btf.c >> @@ -5603,6 +5603,31 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log, >> return kern_ctx_type->type; >> } >> >> +int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type) >> +{ >> + const struct btf_member *kctx_member; >> + const struct btf_type *conv_struct; >> + const struct btf_type *kctx_type; >> + u32 kctx_type_id; >> + >> + conv_struct = bpf_ctx_convert.t; >> + if (!conv_struct) { >> + bpf_log(log, "btf_vmlinux is malformed\n"); >> + return -EINVAL; >> + } > > If we get to this point this internal pointer would be already checked. > No need to check it again. Just use it. This is probably not true. Currently, conv_struct is tested in function btf_get_prog_ctx_type() which is called by get_kfunc_ptr_arg_type(). const struct btf_member * btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf, const struct btf_type *t, enum bpf_prog_type prog_type, int arg) { const struct btf_type *conv_struct; const struct btf_type *ctx_struct; const struct btf_member *ctx_type; const char *tname, *ctx_tname; conv_struct = bpf_ctx_convert.t; if (!conv_struct) { bpf_log(log, "btf_vmlinux is malformed\n"); return NULL; } ... } In get_kfunc_ptr_arg_type(), ... /* In this function, we verify the kfunc's BTF as per the argument type, * leaving the rest of the verification with respect to the register * type to our caller. When a set of conditions hold in the BTF type of * arguments, we resolve it to a known kfunc_ptr_arg_type. */ if (btf_get_prog_ctx_type(&env->log, meta->btf, t, resolve_prog_type(env->prog), argno)) return KF_ARG_PTR_TO_CTX; Note that if bpf_ctx_convert.t is NULL, btf_get_prog_ctx_type() simply returns NULL and the logic simply follows through. Should we actually add a NULL checking for bpf_ctx_convert.t in bpf_parse_vmlinux? ... err = btf_check_type_tags(env, btf, 1); if (err) goto errout; /* btf_parse_vmlinux() runs under bpf_verifier_lock */ bpf_ctx_convert.t = btf_type_by_id(btf, bpf_ctx_convert_btf_id[0]); bpf_struct_ops_init(btf, log); ... > >> + >> + /* get member for kernel ctx type */ >> + kctx_member = btf_type_member(conv_struct) + bpf_ctx_convert_map[prog_type] * 2 + 1; >> + kctx_type_id = kctx_member->type; >> + kctx_type = btf_type_by_id(btf_vmlinux, kctx_type_id); >> + if (!btf_type_is_struct(kctx_type)) { >> + bpf_log(log, "kern ctx type id %u is not a struct\n", kctx_type_id); >> + return -EINVAL; >> + } >> + >> + return kctx_type_id; >> +} >> + >> BTF_ID_LIST(bpf_ctx_convert_btf_id) >> BTF_ID(struct, bpf_ctx_convert) >> >> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c >> index eaae7f474eda..dc6e994feeb9 100644 >> --- a/kernel/bpf/helpers.c >> +++ b/kernel/bpf/helpers.c >> @@ -1824,6 +1824,11 @@ struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head) >> return __bpf_list_del(head, true); >> } >> >> +void *bpf_cast_to_kern_ctx(void *obj) >> +{ >> + return obj; >> +} >> + >> __diag_pop(); >> >> BTF_SET8_START(generic_btf_ids) >> @@ -1844,6 +1849,7 @@ static const struct btf_kfunc_id_set generic_kfunc_set = { >> }; >> >> BTF_SET8_START(common_btf_ids) >> +BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx) >> BTF_SET8_END(common_btf_ids) >> >> static const struct btf_kfunc_id_set common_kfunc_set = { >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index 195d24316750..a18b519c5225 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -8118,6 +8118,7 @@ enum special_kfunc_type { >> KF_bpf_list_push_back, >> KF_bpf_list_pop_front, >> KF_bpf_list_pop_back, >> + KF_bpf_cast_to_kern_ctx, >> }; >> >> BTF_SET_START(special_kfunc_set) >> @@ -8127,6 +8128,7 @@ BTF_ID(func, bpf_list_push_front) >> BTF_ID(func, bpf_list_push_back) >> BTF_ID(func, bpf_list_pop_front) >> BTF_ID(func, bpf_list_pop_back) >> +BTF_ID(func, bpf_cast_to_kern_ctx) >> BTF_SET_END(special_kfunc_set) >> >> BTF_ID_LIST(special_kfunc_list) >> @@ -8136,6 +8138,7 @@ BTF_ID(func, bpf_list_push_front) >> BTF_ID(func, bpf_list_push_back) >> BTF_ID(func, bpf_list_pop_front) >> BTF_ID(func, bpf_list_pop_back) >> +BTF_ID(func, bpf_cast_to_kern_ctx) >> >> static enum kfunc_ptr_arg_type >> get_kfunc_ptr_arg_type(struct bpf_verifier_env *env, >> @@ -8149,6 +8152,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env, >> struct bpf_reg_state *reg = ®s[regno]; >> bool arg_mem_size = false; >> >> + if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) >> + return KF_ARG_PTR_TO_CTX; >> + >> /* In this function, we verify the kfunc's BTF as per the argument type, >> * leaving the rest of the verification with respect to the register >> * type to our caller. When a set of conditions hold in the BTF type of >> @@ -8633,6 +8639,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_ >> verbose(env, "arg#%d expected pointer to ctx, but got %s\n", i, btf_type_str(t)); >> return -EINVAL; >> } >> + >> + if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) { >> + ret = get_kern_ctx_btf_id(&env->log, resolve_prog_type(env->prog)); >> + if (ret < 0) >> + return -EINVAL; >> + meta->arg_constant.value = ret; > > It's not an arg. So 'arg_constant' doesn't fit. > No need to save every byte in bpf_kfunc_call_arg_meta. > Let's add new filed like 'ret_btf_id'. Okay, I can do that. > >> + } >> break; >> case KF_ARG_PTR_TO_ALLOC_BTF_ID: >> if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) { >> @@ -8880,6 +8893,11 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, >> regs[BPF_REG_0].btf = field->list_head.btf; >> regs[BPF_REG_0].btf_id = field->list_head.value_btf_id; >> regs[BPF_REG_0].off = field->list_head.node_offset; >> + } else if (meta.func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) { >> + mark_reg_known_zero(env, regs, BPF_REG_0); >> + regs[BPF_REG_0].type = PTR_TO_BTF_ID; > > Let's use PTR_TO_BTF_ID | PTR_TRUSTED here. > PTR_TRUSTED was just recently added (hours ago :) > With that bpf_cast_to_kern_ctx() will return trusted pointer and we will be able > to pass it to kfuncs and helpers that expect valid args. Right, will add PTR_TRUSTED in the next revision. > >> + regs[BPF_REG_0].btf = desc_btf; >> + regs[BPF_REG_0].btf_id = meta.arg_constant.value; >> } else { >> verbose(env, "kernel function %s unhandled dynamic return type\n", >> meta.func_name); >> @@ -15130,6 +15148,9 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, >> insn_buf[1] = addr[1]; >> insn_buf[2] = *insn; >> *cnt = 3; >> + } else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) { >> + insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1); >> + *cnt = 1; > > Nice! Important optimization. > I guess we still need: > +void *bpf_cast_to_kern_ctx(void *obj) > +{ > + return obj; > +} > otherwise resolve_btfids will be confused? Right, we still need the above function definition so resolve_btfids can properly populate kfunc id for verification purpose.