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

[Andrii Nakryiko]

On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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)

btw, having bpf_this_cpu_ptr(const void *ptr) seems worthwhile as well, WDYT?

> > + *     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;
>
> [...]