Re: [PATCH v3 bpf-next 2/3] bpf: Add array support to btf_struct_access

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

On Thu, Nov 7, 2019 at 9:22 AM Martin Lau <kafai@fb.com> wrote:
>
> On Wed, Nov 06, 2019 at 06:41:15PM -0800, Andrii Nakryiko wrote:
> > On Wed, Nov 6, 2019 at 5:49 PM Martin KaFai Lau <kafai@fb.com> wrote:
> > >
> > > This patch adds array support to btf_struct_access().
> > > It supports array of int, array of struct and multidimensional
> > > array.
> > >
> > > It also allows using u8[] as a scratch space.  For example,
> > > it allows access the "char cb[48]" with size larger than
> > > the array's element "char".  Another potential use case is
> > > "u64 icsk_ca_priv[]" in the tcp congestion control.
> > >
> > > btf_resolve_size() is added to resolve the size of any type.
> > > It will follow the modifier if there is any.  Please
> > > see the function comment for details.
> > >
> > > This patch also adds the "off < moff" check at the beginning
> > > of the for loop.  It is to reject cases when "off" is pointing
> > > to a "hole" in a struct.
> > >
> > > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > > ---
> >
> > Looks good, just two small nits.
> >
> > Acked-by: Andrii Nakryiko <andriin@fb.com>
> >
> > >  kernel/bpf/btf.c | 187 +++++++++++++++++++++++++++++++++++++++--------
> > >  1 file changed, 157 insertions(+), 30 deletions(-)
> > >
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index 128d89601d73..5c4b6aa7b9f0 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -1036,6 +1036,82 @@ static const struct resolve_vertex *env_stack_peak(struct btf_verifier_env *env)
> > >         return env->top_stack ? &env->stack[env->top_stack - 1] : NULL;
> > >  }
> > >
> >
> > [...]
> >
> > > -               if (off + size <= moff / 8)
> > > -                       /* won't find anything, field is already too far */
> > > +               /* offset of the field in bytes */
> > > +               moff = btf_member_bit_offset(t, member) / 8;
> > > +               if (off + size <= moff)
> >
> > you dropped useful comment :(
> good catch. will undo.

thanks!

>
> >
> > >                         break;
> > > +               /* In case of "off" is pointing to holes of a struct */
> > > +               if (off < moff)
> > > +                       continue;
> > >
> >
> > [...]
> >
> > > +
> > > +               mtrue_end = moff + msize;
> >
> > nit: there is no other _end, so might be just mend (in line with moff)
> I prefer to keep it.  For array, this _end is not the end of mtype.
> The intention is to distinguish it from the mtype/msize convention
> such that it is the true_end of the current struct's member.  I will
> add some comments to clarify.

Ok, sure, no problem.

>
> >
> > > +               if (off >= mtrue_end)
> > >                         /* no overlap with member, keep iterating */
> > >                         continue;
> > > +
> >
> > [...]