Re: [PATCH v16 03/12] btf: Allow dynamic pointer parameters in kfuncs

From: Kumar Kartikeya Dwivedi
Date: Mon Sep 05 2022 - 22:34:04 EST

On Mon, 5 Sept 2022 at 16:34, Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> wrote:
>
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> Allow dynamic pointers (struct bpf_dynptr_kern *) to be specified as
> parameters in kfuncs. Also, ensure that dynamic pointers passed as argument
> are valid and initialized, are a pointer to the stack, and of the type
> local. More dynamic pointer types can be supported in the future.
>
> To properly detect whether a parameter is of the desired type, introduce
> the stringify_struct() macro to compare the returned structure name with
> the desired name. In addition, protect against structure renames, by
> halting the build with BUILD_BUG_ON(), so that developers have to revisit
> the code.
>
> To check if a dynamic pointer passed to the kfunc is valid and initialized,
> and if its type is local, export the existing functions
> is_dynptr_reg_valid_init() and is_dynptr_type_expected().
>
> Cc: Joanne Koong <joannelkoong@xxxxxxxxx>
> Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---

Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>

> include/linux/bpf_verifier.h | 5 +++++
> include/linux/btf.h | 9 +++++++++
> kernel/bpf/btf.c | 33 +++++++++++++++++++++++++++++++++
> kernel/bpf/verifier.c | 10 +++++-----
> 4 files changed, 52 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 1fdddbf3546b..dd58dfccd025 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -571,6 +571,11 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
> u32 regno);
> int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> u32 regno, u32 mem_size);
> +bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> + struct bpf_reg_state *reg);
> +bool is_dynptr_type_expected(struct bpf_verifier_env *env,
> + struct bpf_reg_state *reg,
> + enum bpf_arg_type arg_type);
>
> /* this lives here instead of in bpf.h because it needs to dereference tgt_prog */
> static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog,
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index ad93c2d9cc1c..f546d368ac5d 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -52,6 +52,15 @@
> #define KF_SLEEPABLE (1 << 5) /* kfunc may sleep */
> #define KF_DESTRUCTIVE (1 << 6) /* kfunc performs destructive actions */
>
> +/*
> + * Return the name of the passed struct, if exists, or halt the build if for
> + * example the structure gets renamed. In this way, developers have to revisit
> + * the code using that structure name, and update it accordingly.
> + */
> +#define stringify_struct(x) \
> + ({ BUILD_BUG_ON(sizeof(struct x) < 0); \
> + __stringify(x); })
> +
> struct btf;
> struct btf_member;
> struct btf_type;
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index e49b3b6d48ad..4266bff5fada 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6362,15 +6362,20 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>
> if (is_kfunc) {
> bool arg_mem_size = i + 1 < nargs && is_kfunc_arg_mem_size(btf, &args[i + 1], &regs[regno + 1]);
> + bool arg_dynptr = btf_type_is_struct(ref_t) &&
> + !strcmp(ref_tname,
> + stringify_struct(bpf_dynptr_kern));
>
> /* Permit pointer to mem, but only when argument
> * type is pointer to scalar, or struct composed
> * (recursively) of scalars.
> * When arg_mem_size is true, the pointer can be
> * void *.
> + * Also permit initialized local dynamic pointers.
> */
> if (!btf_type_is_scalar(ref_t) &&
> !__btf_type_is_scalar_struct(log, btf, ref_t, 0) &&
> + !arg_dynptr &&
> (arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
> bpf_log(log,
> "arg#%d pointer type %s %s must point to %sscalar, or struct with scalar\n",
> @@ -6378,6 +6383,34 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> return -EINVAL;
> }
>
> + if (arg_dynptr) {
> + if (reg->type != PTR_TO_STACK) {
> + bpf_log(log, "arg#%d pointer type %s %s not to stack\n",
> + i, btf_type_str(ref_t),
> + ref_tname);
> + return -EINVAL;
> + }
> +
> + if (!is_dynptr_reg_valid_init(env, reg)) {
> + bpf_log(log,
> + "arg#%d pointer type %s %s must be valid and initialized\n",
> + i, btf_type_str(ref_t),
> + ref_tname);
> + return -EINVAL;
> + }
> +
> + if (!is_dynptr_type_expected(env, reg,
> + ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_LOCAL)) {
> + bpf_log(log,
> + "arg#%d pointer type %s %s points to unsupported dynamic pointer type\n",
> + i, btf_type_str(ref_t),
> + ref_tname);
> + return -EINVAL;
> + }
> +
> + continue;
> + }
> +
> /* Check for mem, len pair */
> if (arg_mem_size) {
> if (check_kfunc_mem_size_reg(env, &regs[regno + 1], regno + 1)) {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 10b3c0a81d09..8f02729074c6 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -779,8 +779,8 @@ static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_
> return true;
> }
>
> -static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> - struct bpf_reg_state *reg)
> +bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> + struct bpf_reg_state *reg)
> {
> struct bpf_func_state *state = func(env, reg);
> int spi = get_spi(reg->off);
> @@ -799,9 +799,9 @@ static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> return true;
> }
>
> -static bool is_dynptr_type_expected(struct bpf_verifier_env *env,
> - struct bpf_reg_state *reg,
> - enum bpf_arg_type arg_type)
> +bool is_dynptr_type_expected(struct bpf_verifier_env *env,
> + struct bpf_reg_state *reg,
> + enum bpf_arg_type arg_type)
> {
> struct bpf_func_state *state = func(env, reg);
> enum bpf_dynptr_type dynptr_type;
> --
> 2.25.1
>