[PATCH v14 02/12] bpf: Move dynptr type check to is_dynptr_type_expected()

[Roberto Sassu]

On Tue, 2022-08-30 at 09:46 -0700, Joanne Koong wrote:
> On Tue, Aug 30, 2022 at 9:18 AM Roberto Sassu
> <roberto.sassu at huaweicloud.com> wrote:
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> > 
> > Move dynptr type check to is_dynptr_type_expected() from
> > is_dynptr_reg_valid_init(), so that callers can better determine
> > the cause
> > of a negative result (dynamic pointer not valid/initialized,
> > dynamic
> > pointer of the wrong type).
> > 
> > Also, splitting makes the code more readable, since checking the
> > dynamic
> > pointer type is not necessarily related to validity and
> > initialization.
> 
> I think it'd be helpful to also include that btf will be using these
> functions, which seems like the main motivation behind why this
> change
> is needed.

Ok, added.

> > Split the validity/initialization and dynamic pointer type check
> > also in
> > the verifier, and adjust the expected error message in the test (a
> > test for
> > an unexpected dynptr type passed to a helper cannot be added due to
> > missing
> > suitable helpers, but this case has been tested manually).
> 
> The bpf_ringbuf_submit_dynptr() and bpf_ringbuf_discard_dynptr()
> helpers take in only ringbuf-type dynptrs, so either of these would
> work for testing the case where an incorrect dynptr type is passed in
> :)

Uhm, that didn't work.

If I initialize a local dynptr, and call bpf_ringbuf_submit_dynptr(), I
get:

arg 1 is an unacquired reference

The only way to make this work was to create a custom helper, with
OBJ_RELEASE removed.

> > Cc: Joanne Koong <joannelkoong at gmail.com>
> > Cc: Kumar Kartikeya Dwivedi <memxor at gmail.com>
> > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > ---
> >  kernel/bpf/verifier.c                         | 35 ++++++++++++++-
> > ----
> >  .../testing/selftests/bpf/prog_tests/dynptr.c |  2 +-
> >  2 files changed, 28 insertions(+), 9 deletions(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0194a36d0b36..1b913db252a3 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -779,8 +779,8 @@ static bool is_dynptr_reg_valid_uninit(struct
> > bpf_verifier_env *env, struct bpf_
> >         return true;
> >  }
> > 
> > -static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> > struct bpf_reg_state *reg,
> > -                                    enum bpf_arg_type arg_type)
> > +static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env,
> > +                                    struct bpf_reg_state *reg)
> >  {
> >         struct bpf_func_state *state = func(env, reg);
> >         int spi = get_spi(reg->off);
> > @@ -796,11 +796,24 @@ static bool is_dynptr_reg_valid_init(struct
> > bpf_verifier_env *env, struct bpf_re
> >                         return false;
> >         }
> > 
> > +       return true;
> > +}
> > +
> > +static bool is_dynptr_type_expected(struct bpf_verifier_env *env,
> > +                                   struct bpf_reg_state *reg,
> > +                                   enum bpf_arg_type arg_type)
> > +{
> > +       struct bpf_func_state *state = func(env, reg);
> > +       int spi = get_spi(reg->off);
> > +       enum bpf_dynptr_type dynptr_type;
> 
> nit: the above 2 lines should be swapped to maintain reverse
> christmas
> tree order of declarations

Ok.

Thanks

Roberto

> 
> > +
> >         /* ARG_PTR_TO_DYNPTR takes any type of dynptr */
> >         if (arg_type == ARG_PTR_TO_DYNPTR)
> >                 return true;
> > 
> > -       return state->stack[spi].spilled_ptr.dynptr.type ==
> > arg_to_dynptr_type(arg_type);
> > +       dynptr_type = arg_to_dynptr_type(arg_type);
> > +
> > +       return state->stack[spi].spilled_ptr.dynptr.type ==
> > dynptr_type;
> >  }
> > 
> >  /* The reg state of a pointer or a bounded scalar was saved when
> > @@ -6050,21 +6063,27 @@ static int check_func_arg(struct
> > bpf_verifier_env *env, u32 arg,
> >                         }
> > 
> >                         meta->uninit_dynptr_regno = regno;
> > -               } else if (!is_dynptr_reg_valid_init(env, reg,
> > arg_type)) {
> > +               } else if (!is_dynptr_reg_valid_init(env, reg)) {
> > +                       verbose(env,
> > +                               "Expected an initialized dynptr as
> > arg #%d\n",
> > +                               arg + 1);
> > +                       return -EINVAL;
> > +               } else if (!is_dynptr_type_expected(env, reg,
> > arg_type)) {
> >                         const char *err_extra = "";
> > 
> >                         switch (arg_type & DYNPTR_TYPE_FLAG_MASK) {
> >                         case DYNPTR_TYPE_LOCAL:
> > -                               err_extra = "local ";
> > +                               err_extra = "local";
> >                                 break;
> >                         case DYNPTR_TYPE_RINGBUF:
> > -                               err_extra = "ringbuf ";
> > +                               err_extra = "ringbuf";
> >                                 break;
> >                         default:
> > +                               err_extra = "<unknown>";
> >                                 break;
> >                         }
> > -
> > -                       verbose(env, "Expected an initialized
> > %sdynptr as arg #%d\n",
> > +                       verbose(env,
> > +                               "Expected a dynptr of type %s as
> > arg #%d\n",
> >                                 err_extra, arg + 1);
> >                         return -EINVAL;
> >                 }
> > diff --git a/tools/testing/selftests/bpf/prog_tests/dynptr.c
> > b/tools/testing/selftests/bpf/prog_tests/dynptr.c
> > index bcf80b9f7c27..8fc4e6c02bfd 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/dynptr.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/dynptr.c
> > @@ -30,7 +30,7 @@ static struct {
> >         {"invalid_helper2", "Expected an initialized dynptr as arg
> > #3"},
> >         {"invalid_write1", "Expected an initialized dynptr as arg
> > #1"},
> >         {"invalid_write2", "Expected an initialized dynptr as arg
> > #3"},
> > -       {"invalid_write3", "Expected an initialized ringbuf dynptr
> > as arg #1"},
> > +       {"invalid_write3", "Expected an initialized dynptr as arg
> > #1"},
> >         {"invalid_write4", "arg 1 is an unacquired reference"},
> >         {"invalid_read1", "invalid read from stack"},
> >         {"invalid_read2", "cannot pass in dynptr at an offset"},
> > --
> > 2.25.1
> >