Re: [RFC bpf-next v4 3/7] error-inject: add new type that carries if the function is non sleepable

[Benjamin Tissoires]

On Tue, Apr 26, 2022 at 6:11 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Apr 21, 2022 at 04:07:36PM +0200, Benjamin Tissoires wrote:
> > When using error-injection function through bpf to change the return
> > code, we need to know if the function is sleepable or not.
> >
> > Currently the code assumes that all error-inject functions are sleepable,
> > except for a few selected of them, hardcoded in kernel/bpf/verifier.c
> >
> > Add a new flag to error-inject so we can code that information where the
> > function is declared.
> >
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > ---
> >
> > new in v4:
> > - another approach would be to define a new kfunc_set, and register
> >   it with btf. But in that case, what program type would we use?
> >   BPF_PROG_TYPE_UNSPEC?
> > - also note that maybe we should consider all of the functions
> >   non-sleepable and only mark some as sleepable. IMO it makes more
> >   sense to be more restrictive by default.
>
> I think the approach in this patch is fine.
> We didn't have issues with check_non_sleepable_error_inject() so far,
> so I wouldn't start refactoring it.

OK... though I can't help but thinking that adding a new
error-inject.h enum value is going to be bad, because it's an API
change, and users might not expect NS_ERRNO.

OTOH, if we had a new kfunc_set, we keep the existing error-inject API
in place with all the variants and we just teach the verifier that the
function is non sleepable.

>
> > ---
> >  include/asm-generic/error-injection.h |  1 +
> >  kernel/bpf/verifier.c                 | 10 ++++++++--
> >  lib/error-inject.c                    |  2 ++
> >  3 files changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/error-injection.h b/include/asm-generic/error-injection.h
> > index fbca56bd9cbc..5974942353a6 100644
> > --- a/include/asm-generic/error-injection.h
> > +++ b/include/asm-generic/error-injection.h
> > @@ -9,6 +9,7 @@ enum {
> >       EI_ETYPE_ERRNO,         /* Return -ERRNO if failure */
> >       EI_ETYPE_ERRNO_NULL,    /* Return -ERRNO or NULL if failure */
> >       EI_ETYPE_TRUE,          /* Return true if failure */
> > +     EI_ETYPE_NS_ERRNO,      /* Return -ERRNO if failure and tag the function as non-sleepable */
>
> >  };
> >
> >  struct error_injection_entry {
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0f339f9058f3..45c8feea6478 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14085,6 +14085,11 @@ static int check_non_sleepable_error_inject(u32 btf_id)
> >       return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
> >  }
> >
> > +static int is_non_sleepable_error_inject(unsigned long addr)
> > +{
> > +     return get_injectable_error_type(addr) == EI_ETYPE_NS_ERRNO;
>
> It's a linear search. Probably ok. But would be good to double check
> that we're not calling it a lot.

IIUC, the kfunc_set approach would solve that, no?

Cheers,
Benjamin

>
> > +}
> > +
> >  int bpf_check_attach_target(struct bpf_verifier_log *log,
> >                           const struct bpf_prog *prog,
> >                           const struct bpf_prog *tgt_prog,
> > @@ -14281,8 +14286,9 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> >                               /* fentry/fexit/fmod_ret progs can be sleepable only if they are
> >                                * attached to ALLOW_ERROR_INJECTION and are not in denylist.
> >                                */
> > -                             if (!check_non_sleepable_error_inject(btf_id) &&
> > -                                 within_error_injection_list(addr))
> > +                             if (within_error_injection_list(addr) &&
> > +                                 !check_non_sleepable_error_inject(btf_id) &&
> > +                                 !is_non_sleepable_error_inject(addr))
> >                                       ret = 0;
> >                               break;
> >                       case BPF_PROG_TYPE_LSM:
> > diff --git a/lib/error-inject.c b/lib/error-inject.c
> > index 2ff5ef689d72..560c3b18f439 100644
> > --- a/lib/error-inject.c
> > +++ b/lib/error-inject.c
> > @@ -183,6 +183,8 @@ static const char *error_type_string(int etype)
> >               return "ERRNO_NULL";
> >       case EI_ETYPE_TRUE:
> >               return "TRUE";
> > +     case EI_ETYPE_NS_ERRNO:
> > +             return "NS_ERRNO";
> >       default:
> >               return "(unknown)";
> >       }
> > --
> > 2.35.1
> >
>