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

From: Jiri Olsa <[email-masked]>
Date: Wed, 12 Feb 2020 12:10:00 +0100

On Tue, Feb 11, 2020 at 10:51:27AM -0800, Andrii Nakryiko wrote:
> On Sat, Feb 8, 2020 at 7:43 AM Jiri Olsa <[email-masked]> 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 <[email-masked]>
> > ---
> >  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_IMAGE_SIZE is the size of the data portion of the image,
which is PAGE_SIZE - sizeof(struct bpf_image)

here we want to account the whole size = data + tree node (struct bpf_image)

> 
> > +       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?

tr->key can have the target prog id in upper 32 bits,
I'll try to use the casting as you suggest

> 
> > +       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.

ugh, right.. will add

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

yep, that's better

> 
> >         }
> >  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?

ok, thanks a lot for comments

jirka