Re: [PATCH bpf-next v1 6/8] bpf: Introduce bpf_per_cpu_ptr()

[Andrii Nakryiko]

On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
> bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel
> except that it may return NULL. This happens when the cpu parameter is
> out of range. So the caller must check the returned value.
>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---

The logic looks correct, few naming nits, but otherwise:

Acked-by: Andrii Nakryiko <andriin@xxxxxx>

>  include/linux/bpf.h      |  3 ++
>  include/linux/btf.h      | 11 +++++++
>  include/uapi/linux/bpf.h | 14 +++++++++
>  kernel/bpf/btf.c         | 10 -------
>  kernel/bpf/verifier.c    | 64 ++++++++++++++++++++++++++++++++++++++--
>  kernel/trace/bpf_trace.c | 18 +++++++++++
>  6 files changed, 107 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 55f694b63164..613404beab33 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -268,6 +268,7 @@ enum bpf_arg_type {
>         ARG_PTR_TO_ALLOC_MEM,   /* pointer to dynamically allocated memory */
>         ARG_PTR_TO_ALLOC_MEM_OR_NULL,   /* pointer to dynamically allocated memory or NULL */
>         ARG_CONST_ALLOC_SIZE_OR_ZERO,   /* number of allocated bytes requested */
> +       ARG_PTR_TO_PERCPU_BTF_ID,       /* pointer to in-kernel percpu type */
>  };
>
>  /* type of values returned from helper functions */
> @@ -281,6 +282,7 @@ enum bpf_return_type {
>         RET_PTR_TO_SOCK_COMMON_OR_NULL, /* returns a pointer to a sock_common or NULL */
>         RET_PTR_TO_ALLOC_MEM_OR_NULL,   /* returns a pointer to dynamically allocated memory or NULL */
>         RET_PTR_TO_BTF_ID_OR_NULL,      /* returns a pointer to a btf_id or NULL */
> +       RET_PTR_TO_MEM_OR_BTF_OR_NULL,  /* returns a pointer to a valid memory or a btf_id or NULL */

I know it's already very long, but still let's use BTF_ID consistently

>  };
>
>  /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> @@ -360,6 +362,7 @@ enum bpf_reg_type {
>         PTR_TO_RDONLY_BUF_OR_NULL, /* reg points to a readonly buffer or NULL */
>         PTR_TO_RDWR_BUF,         /* reg points to a read/write buffer */
>         PTR_TO_RDWR_BUF_OR_NULL, /* reg points to a read/write buffer or NULL */
> +       PTR_TO_PERCPU_BTF_ID,    /* reg points to percpu kernel type */
>  };
>

[...]

> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 468376f2910b..c7e49a102ed2 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3415,6 +3415,19 @@ union bpf_attr {
>   *             A non-negative value equal to or less than *size* on success,
>   *             or a negative error in case of failure.
>   *
> + * void *bpf_per_cpu_ptr(const void *ptr, u32 cpu)
> + *     Description
> + *             Take the address of a percpu ksym and return a pointer pointing
> + *             to the variable on *cpu*. A ksym is an extern variable decorated
> + *             with '__ksym'. A ksym is percpu if there is a global percpu var
> + *             (either static or global) defined of the same name in the kernel.

The function signature has "ptr", not "ksym", but the description is
using "ksym". please make them consistent (might name param
"percpu_ptr")

> + *
> + *             bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the
> + *             kernel, except that bpf_per_cpu_ptr() may return NULL. This
> + *             happens if *cpu* is larger than nr_cpu_ids. The caller of
> + *             bpf_per_cpu_ptr() must check the returned value.
> + *     Return
> + *             A generic pointer pointing to the variable on *cpu*.
>   */

[...]

> +       } else if (arg_type == ARG_PTR_TO_PERCPU_BTF_ID) {
> +               expected_type = PTR_TO_PERCPU_BTF_ID;
> +               if (type != expected_type)
> +                       goto err_type;
> +               if (!reg->btf_id) {
> +                       verbose(env, "Helper has zero btf_id in R%d\n", regno);

nit: "invalid btf_id"?

> +                       return -EACCES;
> +               }
> +               meta->ret_btf_id = reg->btf_id;
>         } else if (arg_type == ARG_PTR_TO_BTF_ID) {
>                 expected_type = PTR_TO_BTF_ID;
>                 if (type != expected_type)
> @@ -4904,6 +4918,30 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>                 regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL;
>                 regs[BPF_REG_0].id = ++env->id_gen;
>                 regs[BPF_REG_0].mem_size = meta.mem_size;

[...]