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

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Date:   Tue, 16 Mar 2021 17:35:33 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Florent Revest <revest@...omium.org>
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 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()".

> 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.