linux-kernel - Re: [PATCH bpf-next 1/5] bpf: Add a ARG_PTR_TO_CONST

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Date:   Wed, 17 Mar 2021 01:45:57 +0100
From:   Florent Revest <revest@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/5] bpf: Add a ARG_PTR_TO_CONST_STR argument type

On Wed, Mar 17, 2021 at 1:35 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> On Tue, Mar 16, 2021 at 4:58 PM Florent Revest <revest@...omium.org> wrote:
> > On Tue, Mar 16, 2021 at 2:03 AM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > > On Wed, Mar 10, 2021 at 2:02 PM Florent Revest <revest@...omium.org> wrote:
> > > > +       } else if (arg_type == ARG_PTR_TO_CONST_STR) {
> > > > +               struct bpf_map *map = reg->map_ptr;
> > > > +               int map_off, i;
> > > > +               u64 map_addr;
> > > > +               char *map_ptr;
> > > > +
> > > > +               if (!map || !bpf_map_is_rdonly(map)) {
> > > > +                       verbose(env, "R%d does not point to a readonly map'\n", regno);
> > > > +                       return -EACCES;
> > > > +               }
> > > > +
> > > > +               if (!tnum_is_const(reg->var_off)) {
> > > > +                       verbose(env, "R%d is not a constant address'\n", regno);
> > > > +                       return -EACCES;
> > > > +               }
> > > > +
> > > > +               if (!map->ops->map_direct_value_addr) {
> > > > +                       verbose(env, "no direct value access support for this map type\n");
> > > > +                       return -EACCES;
> > > > +               }
> > > > +
> > > > +               err = check_helper_mem_access(env, regno,
> > > > +                                             map->value_size - reg->off,
> > > > +                                             false, meta);
> > >
> > > you expect reg to be PTR_TO_MAP_VALUE, so probably better to directly
> > > use check_map_access(). And double-check that register is of expected
> > > type. just the presence of ref->map_ptr might not be sufficient?
> >
> > Sorry, just making sure I understand your comment correctly, are you
> > suggesting that we:
> > 1- skip the check_map_access_type() currently done by
> > check_helper_mem_access()? or did you implicitly mean that we should
> > call it as well next to check_map_access() ?
>
> check_helper_mem_access() will call check_map_access() for
> PTR_TO_MAP_VALUE and we expect only PTR_TO_MAP_VALUE, right? So why go
> through check_helper_mem_access() if we know we need
> check_map_access()? Less indirection, more explicit. So I meant
> "replace check_helper_mem_access() with check_map_access()".

Mhh I suspect there's still a misunderstanding, these function names
are really confusing ahah.
What about check_map_access*_type*. which is also called by
check_helper_mem_access (before check_map_access):
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/verifier.c#n4329

Your message sounds like we should skip it so I was asking if that's
what you also implicitly meant or if you missed it?

> > 2- enforce (reg->type == PTR_TO_MAP_VALUE) even if currently
> > guaranteed by compatible_reg_types, just to stay on the safe side ?
>
> I can't follow compatible_reg_types :( If it does, then I guess it's
> fine without this check.

It's alright, I can keep an extra check just for safety. :)