Re: [PATCH v1 bpf-next 2/5] bpf: Add bpf_dynptr_is_null and bpf_dynptr_is_rdonly

[Joanne Koong <joannelkoong@gmail.com>]

On Wed, Apr 12, 2023 at 2:50 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Apr 8, 2023 at 8:34 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > bpf_dynptr_is_null returns true if the dynptr is null / invalid
> > (determined by whether ptr->data is NULL), else false if
> > the dynptr is a valid dynptr.
> >
> > bpf_dynptr_is_rdonly returns true if the dynptr is read-only,
> > else false if the dynptr is read-writable.
> >
> > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > ---
> >  kernel/bpf/helpers.c | 23 +++++++++++++++++++----
> >  1 file changed, 19 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 51b4c4b5dbed..e4e84e92a4c6 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -1423,7 +1423,7 @@ static const struct bpf_func_proto bpf_kptr_xchg_proto = {
> >  #define DYNPTR_SIZE_MASK       0xFFFFFF
> >  #define DYNPTR_RDONLY_BIT      BIT(31)
> >
> > -static bool bpf_dynptr_is_rdonly(const struct bpf_dynptr_kern *ptr)
> > +static bool __bpf_dynptr_is_rdonly(const struct bpf_dynptr_kern *ptr)
> >  {
> >         return ptr->size & DYNPTR_RDONLY_BIT;
> >  }
> > @@ -1570,7 +1570,7 @@ BPF_CALL_5(bpf_dynptr_write, const struct bpf_dynptr_kern *, dst, u32, offset, v
> >         enum bpf_dynptr_type type;
> >         int err;
> >
> > -       if (!dst->data || bpf_dynptr_is_rdonly(dst))
> > +       if (!dst->data || __bpf_dynptr_is_rdonly(dst))
> >                 return -EINVAL;
> >
> >         err = bpf_dynptr_check_off_len(dst, offset, len);
> > @@ -1626,7 +1626,7 @@ BPF_CALL_3(bpf_dynptr_data, const struct bpf_dynptr_kern *, ptr, u32, offset, u3
> >         if (err)
> >                 return 0;
> >
> > -       if (bpf_dynptr_is_rdonly(ptr))
> > +       if (__bpf_dynptr_is_rdonly(ptr))
> >                 return 0;
> >
> >         type = bpf_dynptr_get_type(ptr);
> > @@ -2254,7 +2254,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
> >  __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 offset,
> >                                         void *buffer, u32 buffer__szk)
> >  {
> > -       if (!ptr->data || bpf_dynptr_is_rdonly(ptr))
> > +       if (!ptr->data || __bpf_dynptr_is_rdonly(ptr))
>
> seems like all the uses of __bpf_dynptr_is_rdonly check !ptr->data
> explicitly, so maybe move that ptr->data check inside and simplify all
> the callers?

i think combining it gets confusing in the case where ptr->data is
null, as to how the invoked places interpret the return value. I think
having the check spelled out more explicitly in the invoked places (eg
bpf_dynptr_write, bpf_dynptr_data, ...) makes it more clear as well
where the check for null is happening. for v2 I will leave this as is,
but also happy to change it if you prefer the two be combined

>
> Regardless, looks good:
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>
> >                 return NULL;
> >
> >         /* bpf_dynptr_slice_rdwr is the same logic as bpf_dynptr_slice.
> > @@ -2322,6 +2322,19 @@ __bpf_kfunc int bpf_dynptr_trim(struct bpf_dynptr_kern *ptr, u32 len)
> >         return bpf_dynptr_adjust(ptr, 0, len);
> >  }
> >
> > +__bpf_kfunc bool bpf_dynptr_is_null(struct bpf_dynptr_kern *ptr)
> > +{
> > +       return !ptr->data;
> > +}
> > +
> > +__bpf_kfunc bool bpf_dynptr_is_rdonly(struct bpf_dynptr_kern *ptr)
> > +{
> > +       if (!ptr->data)
> > +               return false;
> > +
> > +       return __bpf_dynptr_is_rdonly(ptr);
> > +}
> > +
> >  __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
> >  {
> >         return obj;
> > @@ -2396,6 +2409,8 @@ BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
> >  BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
> >  BTF_ID_FLAGS(func, bpf_dynptr_trim)
> >  BTF_ID_FLAGS(func, bpf_dynptr_advance)
> > +BTF_ID_FLAGS(func, bpf_dynptr_is_null)
> > +BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
> >  BTF_SET8_END(common_btf_ids)
> >
> >  static const struct btf_kfunc_id_set common_kfunc_set = {
> > --
> > 2.34.1
> >