Re: [PATCH bpf-next v2 5/6] bpf: Introduce bpf_this_cpu_ptr()

[Hao Luo]

Thanks for taking a look!

On Fri, Sep 4, 2020 at 1:09 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Sep 3, 2020 at 3:35 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > Add bpf_this_cpu_ptr() to help access percpu var on this cpu. This
> > helper always returns a valid pointer, therefore no need to check
> > returned value for NULL. Also note that all programs run with
> > preemption disabled, which means that the returned pointer is stable
> > during all the execution of the program.
> >
> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > ---
>
> looks good, few small things, but otherwise:
>
> Acked-by: Andrii Nakryiko <andriin@xxxxxx>
>
[...]
> >
> >  /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index d0ec94d5bdbf..e7ca91c697ed 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3612,6 +3612,19 @@ union bpf_attr {
> >   *             bpf_per_cpu_ptr() must check the returned value.
> >   *     Return
> >   *             A generic pointer pointing to the kernel percpu variable on *cpu*.
> > + *
> > + * void *bpf_this_cpu_ptr(const void *percpu_ptr)
> > + *     Description
> > + *             Take a pointer to a percpu ksym, *percpu_ptr*, and return a
> > + *             pointer to the percpu kernel variable on this cpu. See the
> > + *             description of 'ksym' in **bpf_per_cpu_ptr**\ ().
> > + *
> > + *             bpf_this_cpu_ptr() has the same semantic as this_cpu_ptr() in
> > + *             the kernel. Different from **bpf_per_cpu_ptr**\ (), it would
> > + *             never return NULL.
> > + *     Return
> > + *             A generic pointer pointing to the kernel percpu variable on
>
> what's "a generic pointer"? is it as opposed to sk_buff pointer or something?
>

Ack. "A pointer" should be good enough. I wrote "generic pointer"
because the per_cpu_ptr() in kernel code is a macro, whose returned
value is a typed pointer, IIUC. But here we are missing the type. This
is another difference between this helper and per_cpu_ptr(). But this
may not matter.

> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index a702600ff581..e070d2abc405 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5016,8 +5016,10 @@ 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;
> > -       } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL) {
> > +       } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL ||
> > +                  fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID) {
> >                 const struct btf_type *t;
> > +               bool not_null = fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID;
>
> nit: this is fine, but I'd inline it below
>

Ack.

> >
> >                 mark_reg_known_zero(env, regs, BPF_REG_0);
> >                 t = btf_type_skip_modifiers(btf_vmlinux, meta.ret_btf_id, NULL);
> > @@ -5034,10 +5036,12 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> >                                         tname, PTR_ERR(ret));
> >                                 return -EINVAL;
> >                         }
> > -                       regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL;
> > +                       regs[BPF_REG_0].type = not_null ?
> > +                               PTR_TO_MEM : PTR_TO_MEM_OR_NULL;
> >                         regs[BPF_REG_0].mem_size = tsize;
> >                 } else {
> > -                       regs[BPF_REG_0].type = PTR_TO_BTF_ID_OR_NULL;
> > +                       regs[BPF_REG_0].type = not_null ?
> > +                               PTR_TO_BTF_ID : PTR_TO_BTF_ID_OR_NULL;
> >                         regs[BPF_REG_0].btf_id = meta.ret_btf_id;
> >                 }
> >         } else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL) {
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index d474c1530f87..466acf82a9c7 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1160,6 +1160,18 @@ static const struct bpf_func_proto bpf_per_cpu_ptr_proto = {
> >         .arg2_type      = ARG_ANYTHING,
> >  };
> >
> > +BPF_CALL_1(bpf_this_cpu_ptr, const void *, percpu_ptr)
> > +{
> > +       return (u64)this_cpu_ptr(percpu_ptr);
>
> see previous comment, this might trigger unnecessary compilation
> warnings on 32-bit arches
>

Ack. Will cast to "unsigned long". Thanks for catching this!


> > +}
> > +
> > +static const struct bpf_func_proto bpf_this_cpu_ptr_proto = {
> > +       .func           = bpf_this_cpu_ptr,
> > +       .gpl_only       = false,
> > +       .ret_type       = RET_PTR_TO_MEM_OR_BTF_ID,
> > +       .arg1_type      = ARG_PTR_TO_PERCPU_BTF_ID,
> > +};
> > +
>
> [...]