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Date:   Sun, 13 Sep 2020 22:01:39 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Quentin Monnet <quentin@...valent.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Andrey Ignatov <rdna@...com>,
        Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [PATCH bpf-next v2 4/6] bpf: Introduce bpf_per_cpu_ptr()

Thanks for review, Andrii.

One question, should I add bpf_{per, this}_cpu_ptr() to the
bpf_base_func_proto() in kernel/bpf/helpers.c?

On Fri, Sep 4, 2020 at 1:04 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Sep 3, 2020 at 3:35 PM Hao Luo <haoluo@...gle.com> 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.
> >
> > Acked-by: Andrii Nakryiko <andriin@...com>
> > Signed-off-by: Hao Luo <haoluo@...gle.com>
> > ---
> >  include/linux/bpf.h            |  3 ++
> >  include/linux/btf.h            | 11 ++++++
> >  include/uapi/linux/bpf.h       | 17 +++++++++
> >  kernel/bpf/btf.c               | 10 ------
> >  kernel/bpf/verifier.c          | 66 +++++++++++++++++++++++++++++++---
> >  kernel/trace/bpf_trace.c       | 18 ++++++++++
> >  tools/include/uapi/linux/bpf.h | 17 +++++++++
> >  7 files changed, 128 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index c6d9f2c444f4..6b2034f7665e 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -292,6 +292,7 @@ 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 */
> > @@ -305,6 +306,7 @@ 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_ID_OR_NULL, /* returns a pointer to a valid memory or a btf_id or NULL */
> >  };
> >
> >  /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> > @@ -385,6 +387,7 @@ 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 */
> >  };
> >
> >  /* The information passed from prog-specific *_is_valid_access
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index 592373d359b9..07b7de1c05b0 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -71,6 +71,11 @@ btf_resolve_size(const struct btf *btf, const struct btf_type *type,
> >              i < btf_type_vlen(struct_type);                    \
> >              i++, member++)
> >
> > +#define for_each_vsi(i, struct_type, member)                   \
>
> datasec_type?
>

Hmmm, right. It seems to come when copy-pasted from "for_each_member".

> > +       for (i = 0, member = btf_type_var_secinfo(struct_type); \
> > +            i < btf_type_vlen(struct_type);                    \
> > +            i++, member++)
> > +
> >  static inline bool btf_type_is_ptr(const struct btf_type *t)
> >  {
> >         return BTF_INFO_KIND(t->info) == BTF_KIND_PTR;
> > @@ -155,6 +160,12 @@ static inline const struct btf_member *btf_type_member(const struct btf_type *t)
> >         return (const struct btf_member *)(t + 1);
> >  }
> >
> > +static inline const struct btf_var_secinfo *btf_type_var_secinfo(
> > +               const struct btf_type *t)
> > +{
> > +       return (const struct btf_var_secinfo *)(t + 1);
> > +}
> > +
> >  #ifdef CONFIG_BPF_SYSCALL
> >  const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);
> >  const char *btf_name_by_offset(const struct btf *btf, u32 offset);
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index ab00ad9b32e5..d0ec94d5bdbf 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3596,6 +3596,22 @@ union bpf_attr {
> >   *             the data in *dst*. This is a wrapper of copy_from_user().
> >   *     Return
> >   *             0 on success, or a negative error in case of failure.
> > + *
> > + * void *bpf_per_cpu_ptr(const void *percpu_ptr, u32 cpu)
> > + *     Description
> > + *             Take a pointer to a percpu ksym, *percpu_ptr*, and return a
> > + *             pointer to the percpu kernel variable on *cpu*. A ksym is an
> > + *             extern variable decorated with '__ksym'. For ksym, there is a
> > + *             global var (either static or global) defined of the same name
> > + *             in the kernel. The ksym is percpu if the global var is percpu.
> > + *             The returned pointer points to the global percpu var on *cpu*.
> > + *
> > + *             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 kernel percpu variable on *cpu*.
>
> Or NULL, if *cpu* is invalid.
>

Ack.

> >   */
> >  #define __BPF_FUNC_MAPPER(FN)          \
> >         FN(unspec),                     \
> > @@ -3747,6 +3763,7 @@ union bpf_attr {
> >         FN(inode_storage_delete),       \
> >         FN(d_path),                     \
> >         FN(copy_from_user),             \
> > +       FN(bpf_per_cpu_ptr),            \
> >         /* */
> >
>
> [...]
>
> > @@ -4003,6 +4008,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> >                         if (type != expected_type)
> >                                 goto err_type;
> >                 }
> > +       } 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 invalid btf_id in R%d\n", regno);
> > +                       return -EACCES;
> > +               }
> > +               meta->ret_btf_id = reg->btf_id;
> >         } else if (arg_type == ARG_PTR_TO_BTF_ID) {
> >                 bool ids_match = false;
> >
> > @@ -5002,6 +5016,30 @@ 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;
> > +       } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL) {
>
> Given this is internal implementation detail, this return type is
> fine, but I'm wondering if it would be better to just make
> PTR_TO_BTF_ID to allow not just structs? E.g., if we have an int, just
> allow reading those 4 bytes.
>
> Not sure what the implications are in terms of implementation, but
> conceptually that shouldn't be a problem, given we do have BTF type ID
> describing size and all.
>

Yeah. Totally agree. I looked at it initially. My take is
PTR_TO_BTF_ID is meant for struct types. It required some code
refactoring to break this assumption. I can add it to my TODO list and
investigate it if this makes more sense.

> > +               const struct btf_type *t;
> > +
> > +               mark_reg_known_zero(env, regs, BPF_REG_0);
> > +               t = btf_type_skip_modifiers(btf_vmlinux, meta.ret_btf_id, NULL);
> > +               if (!btf_type_is_struct(t)) {
> > +                       u32 tsize;
> > +                       const struct btf_type *ret;
> > +                       const char *tname;
> > +
> > +                       /* resolve the type size of ksym. */
> > +                       ret = btf_resolve_size(btf_vmlinux, t, &tsize);
> > +                       if (IS_ERR(ret)) {
> > +                               tname = btf_name_by_offset(btf_vmlinux, t->name_off);
> > +                               verbose(env, "unable to resolve the size of type '%s': %ld\n",
> > +                                       tname, PTR_ERR(ret));
> > +                               return -EINVAL;
> > +                       }
> > +                       regs[BPF_REG_0].type = 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].btf_id = meta.ret_btf_id;
> > +               }
> >         } else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL) {
> >                 int ret_btf_id;
> >
>
> [...]
>
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index b2a5380eb187..d474c1530f87 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1144,6 +1144,22 @@ static const struct bpf_func_proto bpf_d_path_proto = {
> >         .allowed        = bpf_d_path_allowed,
> >  };
> >
> > +BPF_CALL_2(bpf_per_cpu_ptr, const void *, ptr, u32, cpu)
> > +{
> > +       if (cpu >= nr_cpu_ids)
> > +               return 0;
> > +
> > +       return (u64)per_cpu_ptr(ptr, cpu);
>
> not sure, but on 32-bit arches this might cause compilation warning,
> case to (unsigned long) instead?
>

Ah, I see, good catch! Will fix, thanks.


> > +}
> > +
> > +static const struct bpf_func_proto bpf_per_cpu_ptr_proto = {
> > +       .func           = bpf_per_cpu_ptr,
> > +       .gpl_only       = false,
> > +       .ret_type       = RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL,
> > +       .arg1_type      = ARG_PTR_TO_PERCPU_BTF_ID,
> > +       .arg2_type      = ARG_ANYTHING,
> > +};
> > +
>
> [...]

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