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Date:   Wed, 27 Feb 2019 16:28:23 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/2] bpf: set inner_map_meta->spin_lock_off correctly

On Wed, Feb 27, 2019 at 4:19 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 2/27/19 3:34 PM, Andrii Nakryiko wrote:
> > On Wed, Feb 27, 2019 at 1:23 PM Yonghong Song <yhs@...com> wrote:
> >>
> >> Commit d83525ca62cf ("bpf: introduce bpf_spin_lock")
> >> introduced bpf_spin_lock and the field spin_lock_off
> >> in kernel internal structure bpf_map has the following
> >> meaning:
> >>    >=0 valid offset, <0 error
> >>
> >> For every map created, the kernel will ensure
> >> spin_lock_off has correct value.
> >>
> >> Currently, bpf_map->spin_lock_off is not copied
> >> from the inner map to the map_in_map inner_map_meta
> >> during a map_in_map type map creation, so
> >> inner_map_meta->spin_lock_off = 0.
> >> This will give verifier wrong information that
> >> inner_map has bpf_spin_lock and the bpf_spin_lock
> >> is defined at offset 0. An access to offset 0
> >> of a value pointer will trigger the following error:
> >>     bpf_spin_lock cannot be accessed directly by load/store
> >>
> >> This patch fixed the issue by copy inner map's spin_lock_off
> >> value to inner_map_meta->spin_lock_off.
> >>
> >> Fixes: d83525ca62cf ("bpf: introduce bpf_spin_lock")
> >> Signed-off-by: Yonghong Song <yhs@...com>

Acked-by: Andrii Nakryiko <andriin@...com>

> >> ---
> >>   kernel/bpf/map_in_map.c | 1 +
> >>   1 file changed, 1 insertion(+)
> >>
> >> diff --git a/kernel/bpf/map_in_map.c b/kernel/bpf/map_in_map.c
> >> index 583346a0ab29..3dff41403583 100644
> >> --- a/kernel/bpf/map_in_map.c
> >> +++ b/kernel/bpf/map_in_map.c
> >> @@ -58,6 +58,7 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
> >>          inner_map_meta->value_size = inner_map->value_size;
> >>          inner_map_meta->map_flags = inner_map->map_flags;
> >>          inner_map_meta->max_entries = inner_map->max_entries;
> >> +       inner_map_meta->spin_lock_off = inner_map->spin_lock_off;
> >
> > Looks like spinlock inside inner map is not supported: there is
> > specific check few lines above returning -ENOSUPP for such case. In
> > that case, maybe assign -1 here to make this explicit?
>
> -1 (-EPERM) probably not the best choice. The verifier already has
> knowledge that a particular tracked map is an inner map or not. So
> keeping the original error code (mostly -EINVAL) is preferred I think.

Ah, I actually missed the fact that verifier actually checks those
values (so it's not just >= 0 or < 0), so yeah, let's just pass
through. Btw, the value when there is no spinlock is actually -ENOENT.

>
> >
> > Though I guess that also brings up the question: is there any harm in
> > supporting spin lock for inner map and why it was disabled in the
> > first place?
>
> Not exactly sure about the reason. Maybe with this patch, it can get
> proper support. Not 100% sure.

No, it won't, because bpf_map_meta_alloc explicitly tests for it:

        if (map_value_has_spin_lock(inner_map)) {
                fdput(f);
                return ERR_PTR(-ENOTSUPP);
        }

Maybe Alexei can clarify?


>
> >
> >>
> >>          /* Misc members not needed in bpf_map_meta_equal() check. */
> >>          inner_map_meta->ops = inner_map->ops;
> >> --
> >> 2.17.1
> >>

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