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bpf: Add a kfunc for generic type cast
Implement bpf_rdonly_cast() which tries to cast the object to a specified type. This tries to support use case like below: #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB))) where skb_end_pointer(SKB) is a 'unsigned char *' and needs to be casted to 'struct skb_shared_info *'. The signature of bpf_rdonly_cast() looks like void *bpf_rdonly_cast(void *obj, __u32 btf_id) The function returns the same 'obj' but with PTR_TO_BTF_ID with btf_id. The verifier will ensure btf_id being a struct type. Since the supported type cast may not reflect what the 'obj' represents, the returned btf_id is marked as PTR_UNTRUSTED, so the return value and subsequent pointer chasing cannot be used as helper/kfunc arguments. Signed-off-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/r/20221120195437.3114585-1-yhs@fb.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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@ -1884,6 +1884,11 @@ void *bpf_cast_to_kern_ctx(void *obj)
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return obj;
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}
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void *bpf_rdonly_cast(void *obj__ign, u32 btf_id__k)
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{
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return obj__ign;
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}
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__diag_pop();
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BTF_SET8_START(generic_btf_ids)
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@ -1913,6 +1918,7 @@ BTF_ID(func, bpf_task_release)
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BTF_SET8_START(common_btf_ids)
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BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)
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BTF_ID_FLAGS(func, bpf_rdonly_cast)
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BTF_SET8_END(common_btf_ids)
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static const struct btf_kfunc_id_set common_kfunc_set = {
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@ -8153,6 +8153,7 @@ enum special_kfunc_type {
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KF_bpf_list_pop_front,
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KF_bpf_list_pop_back,
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KF_bpf_cast_to_kern_ctx,
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KF_bpf_rdonly_cast,
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};
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BTF_SET_START(special_kfunc_set)
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@ -8163,6 +8164,7 @@ BTF_ID(func, bpf_list_push_back)
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BTF_ID(func, bpf_list_pop_front)
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BTF_ID(func, bpf_list_pop_back)
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BTF_ID(func, bpf_cast_to_kern_ctx)
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BTF_ID(func, bpf_rdonly_cast)
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BTF_SET_END(special_kfunc_set)
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BTF_ID_LIST(special_kfunc_list)
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@ -8173,6 +8175,7 @@ BTF_ID(func, bpf_list_push_back)
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BTF_ID(func, bpf_list_pop_front)
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BTF_ID(func, bpf_list_pop_back)
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BTF_ID(func, bpf_cast_to_kern_ctx)
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BTF_ID(func, bpf_rdonly_cast)
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static enum kfunc_ptr_arg_type
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get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
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@ -8809,6 +8812,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
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u32 i, nargs, func_id, ptr_type_id;
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int err, insn_idx = *insn_idx_p;
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const struct btf_param *args;
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const struct btf_type *ret_t;
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struct btf *desc_btf;
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u32 *kfunc_flags;
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@ -8888,7 +8892,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
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if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {
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if (meta.func_id == special_kfunc_list[KF_bpf_obj_new_impl]) {
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const struct btf_type *ret_t;
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struct btf *ret_btf;
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u32 ret_btf_id;
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@ -8941,6 +8944,18 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
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regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
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regs[BPF_REG_0].btf = desc_btf;
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regs[BPF_REG_0].btf_id = meta.ret_btf_id;
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} else if (meta.func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
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ret_t = btf_type_by_id(desc_btf, meta.arg_constant.value);
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if (!ret_t || !btf_type_is_struct(ret_t)) {
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verbose(env,
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"kfunc bpf_rdonly_cast type ID argument must be of a struct\n");
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return -EINVAL;
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}
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mark_reg_known_zero(env, regs, BPF_REG_0);
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regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_UNTRUSTED;
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regs[BPF_REG_0].btf = desc_btf;
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regs[BPF_REG_0].btf_id = meta.arg_constant.value;
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} else {
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verbose(env, "kernel function %s unhandled dynamic return type\n",
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meta.func_name);
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@ -15194,7 +15209,8 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
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insn_buf[1] = addr[1];
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insn_buf[2] = *insn;
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*cnt = 3;
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} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
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} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
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desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
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insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
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*cnt = 1;
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}
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