netdev - Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting with error checking

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Date:   Fri, 23 Apr 2021 09:09:09 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Yonghong Song <yhs@...com>, Andrii Nakryiko <andrii@...nel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting
 with error checking

On Thu, Apr 22, 2021 at 10:11 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Apr 22, 2021 at 9:25 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Thu, Apr 22, 2021 at 7:54 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Thu, Apr 22, 2021 at 11:24 AM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > >
> > > > On Thu, Apr 22, 2021 at 9:50 AM Yonghong Song <yhs@...com> wrote:
> > > > >
> > > > >
> > > > >
> > > > > On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> > > > > > It should never fail, but if it does, it's better to know about this rather
> > > > > > than end up with nonsensical type IDs.
> > > > >
> > > > > So this is defensive programming. Maybe do another round of
> > > > > audit of the callers and if you didn't find any issue, you
> > > > > do not need to check not-happening condition here?
> > > >
> > > > It's far from obvious that this will never happen, because we do a
> > > > decently complicated BTF processing (we skip some types altogether
> > > > believing that they are not used, for example) and it will only get
> > > > more complicated with time. Just as there are "verifier bug" checks in
> > > > kernel, this prevents things from going wild if non-trivial bugs will
> > > > inevitably happen.
> > >
> > > I agree with Yonghong. This doesn't look right.
> >
> > I read it as Yonghong was asking about the entire patch. You seem to
> > be concerned with one particular check, right?
> >
> > > The callback will be called for all non-void types, right?
> > > so *type_id == 0 shouldn't never happen.
> > > If it does there is a bug somewhere that should be investigated
> > > instead of ignored.
> >
> > See btf_type_visit_type_ids() and btf_ext_visit_type_ids(), they call
> > callback for every field that contains type ID, even if it points to
> > VOID. So this can happen and is expected.
>
> I see. So something like 'extern cosnt void foo __ksym' would
> point to void type?
> But then why is it not a part of the id_map[] and has
> to be handled explicitly?

const void foo will be VAR -> CONST -> VOID. But any `void *` anywhere
will be PTR -> VOID. Any void bla(int x) would have return type VOID
(0), and so on. There are a lot of cases when we use VOID as type_id.
VOID always is handled specially, because it stays zero despite any
transformation: during BTF concatenation, BTF dedup, BTF generation,
etc.

>
> > > The
> > > if (new_id == 0) pr_warn
> > > bit makes sense.
> >
> > Right, and this is the point of this patch. id_map[] will have zeroes
> > for any unmapped type, so I just need to make sure I'm not false
> > erroring on id_map[0] (== 0, which is valid, but never used).
>
> Right, id_map[0] should be 0.
> I'm still missing something in this combination of 'if's.
> May be do it as:
> if (new_id == 0 && *type_id != 0) { pr_warn
> ?
> That was the idea?

That's the idea, there is just no need to do VOID -> VOID
transformation, but I'll rewrite it to a combined if if it makes it
easier to follow. Here's full source of remap_type_id with few
comments to added:

static int remap_type_id(__u32 *type_id, void *ctx)
{
        int *id_map = ctx;
        int new_id = id_map[*type_id];


/* Here VOID stays VOID, that's all */

        if (*type_id == 0)
                return 0;

/* This means whatever type we are trying to remap didn't get a new ID
assigned in linker->btf and that's an error */
        if (new_id == 0) {
                pr_warn("failed to find new ID mapping for original
BTF type ID %u\n", *type_id);
                return -EINVAL;
        }

        *type_id = id_map[*type_id];

        return 0;
}