Re: [PATCH 12/14] bpf: Add trampolines to kallsyms

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sat, Feb 8, 2020 at 7:43 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding trampolines to kallsyms. It's displayed as
>   bpf_trampoline_<ID> [bpf]
>
> where ID is the BTF id of the trampoline function.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/bpf.h     |  2 ++
>  kernel/bpf/trampoline.c | 23 +++++++++++++++++++++++
>  2 files changed, 25 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7a4626c8e747..b91bac10d3ea 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -502,6 +502,7 @@ struct bpf_trampoline {
>         /* Executable image of trampoline */
>         void *image;
>         u64 selector;
> +       struct bpf_ksym ksym;
>  };
>
>  #define BPF_DISPATCHER_MAX 48 /* Fits in 2048B */
> @@ -573,6 +574,7 @@ struct bpf_image {
>  #define BPF_IMAGE_SIZE (PAGE_SIZE - sizeof(struct bpf_image))
>  bool is_bpf_image_address(unsigned long address);
>  void *bpf_image_alloc(void);
> +void bpf_image_ksym_add(void *data, struct bpf_ksym *ksym);
>  /* Called only from code, so there's no need for stubs. */
>  void bpf_ksym_add(struct bpf_ksym *ksym);
>  void bpf_ksym_del(struct bpf_ksym *ksym);
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 6b264a92064b..1ee29907cbe5 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -96,6 +96,15 @@ bool is_bpf_image_address(unsigned long addr)
>         return ret;
>  }
>
> +void bpf_image_ksym_add(void *data, struct bpf_ksym *ksym)
> +{
> +       struct bpf_image *image = container_of(data, struct bpf_image, data);
> +
> +       ksym->start = (unsigned long) image;
> +       ksym->end = ksym->start + PAGE_SIZE;

this seems wrong, use BPF_IMAGE_SIZE instead of PAGE_SIZE?

> +       bpf_ksym_add(ksym);
> +}
> +
>  struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>  {
>         struct bpf_trampoline *tr;
> @@ -131,6 +140,7 @@ struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>         for (i = 0; i < BPF_TRAMP_MAX; i++)
>                 INIT_HLIST_HEAD(&tr->progs_hlist[i]);
>         tr->image = image;
> +       INIT_LIST_HEAD_RCU(&tr->ksym.lnode);
>  out:
>         mutex_unlock(&trampoline_mutex);
>         return tr;
> @@ -267,6 +277,15 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(enum bpf_attach_type t)
>         }
>  }
>
> +static void bpf_trampoline_kallsyms_add(struct bpf_trampoline *tr)
> +{
> +       struct bpf_ksym *ksym = &tr->ksym;
> +
> +       snprintf(ksym->name, KSYM_NAME_LEN, "bpf_trampoline_%llu",
> +                tr->key & ((u64) (1LU << 32) - 1));

why the 32-bit truncation? also, wouldn't it be more trivial as (u32)tr->key?

> +       bpf_image_ksym_add(tr->image, &tr->ksym);
> +}
> +
>  int bpf_trampoline_link_prog(struct bpf_prog *prog)
>  {
>         enum bpf_tramp_prog_type kind;
> @@ -311,6 +330,8 @@ int bpf_trampoline_link_prog(struct bpf_prog *prog)
>         if (err) {
>                 hlist_del(&prog->aux->tramp_hlist);
>                 tr->progs_cnt[kind]--;
> +       } else if (cnt == 0) {
> +               bpf_trampoline_kallsyms_add(tr);

You didn't handle BPF_TRAMP_REPLACE case above.

Also this if (err) { ... } else if (cnt == 0) { } pattern is a bit
convoluted. How about:

if (err) {
   ... whatever ...
   goto out;
}
if (cnt == 0) { ... }

>         }
>  out:
>         mutex_unlock(&tr->mutex);
> @@ -336,6 +357,8 @@ int bpf_trampoline_unlink_prog(struct bpf_prog *prog)
>         }
>         hlist_del(&prog->aux->tramp_hlist);
>         tr->progs_cnt[kind]--;
> +       if (!(tr->progs_cnt[BPF_TRAMP_FENTRY] + tr->progs_cnt[BPF_TRAMP_FEXIT]))
> +               bpf_ksym_del(&tr->ksym);

same, BPF_TRAMP_REPLACE case. I'd also introduce cnt for consistency
with bpf_trampoline_link_prog?

>         err = bpf_trampoline_update(prog->aux->trampoline);
>  out:
>         mutex_unlock(&tr->mutex);
> --
> 2.24.1
>