netdev - Re: [PATCH bpf 1/3] bpf: don't assume build-id length is always 20 bytes

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Date:   Wed, 16 Jan 2019 09:50:52 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Song Liu <songliubraving@...com>
Cc:     Netdev <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf 1/3] bpf: don't assume build-id length is always 20 bytes

On Wed, Jan 16, 2019 at 9:45 AM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jan 15, 2019, at 2:54 PM, Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > Build-id length is not fixed to 20, it can be (`man ld` /--build-id):
> >  * 128-bit (uuid)
> >  * 160-bit (sha1)
> >  * any length specified in ld --build-id=0xhexstring
> >
> > To fix the issue of missing BPF_STACK_BUILD_ID_VALID for shorter build-ids,
> > assume that build-id is somewhere in the range of 1 .. 20.
> > Set the remaining bytes to zero.
> >
> > Fixes: 615755a77b24 ("bpf: extend stackmap to save binary_build_id+offset instead of address")
> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > ---
> > kernel/bpf/stackmap.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> > index d9e2483669d0..8975d1768dcb 100644
> > --- a/kernel/bpf/stackmap.c
> > +++ b/kernel/bpf/stackmap.c
> > @@ -180,11 +180,15 @@ static inline int stack_map_parse_build_id(void *page_addr,
> >
> >               if (nhdr->n_type == BPF_BUILD_ID &&
> >                   nhdr->n_namesz == sizeof("GNU") &&
> > -                 nhdr->n_descsz == BPF_BUILD_ID_SIZE) {
> > +                 nhdr->n_descsz > 0 &&
> > +                 nhdr->n_descsz <= BPF_BUILD_ID_SIZE) {
> > +                     __u32 len = min_t(__u32,
> > +                                       BPF_BUILD_ID_SIZE, nhdr->n_descsz);
>
> Given the check above, we only need len = nhdr->n_descsz, right?
Ah, correct, I'll fix in v2.
I initially had without `if (nhdr->n_descsz <= BPF_BUILD_ID_SIZE)` and
clamped it here, but then decided that clamping is probably bad as
well.
>
> Other than this,
>
> Acked-by: Song Liu <songliubraving@...com>
>
> Thanks for the fix!
>
> >                       memcpy(build_id,
> >                              note_start + note_offs +
> >                              ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
> > -                            BPF_BUILD_ID_SIZE);
> > +                            len);
> > +                     memset(build_id + len, 0, BPF_BUILD_ID_SIZE - len);
> >                       return 0;
> >               }
> >               new_offs = note_offs + sizeof(Elf32_Nhdr) +
> > --
> > 2.20.1.97.g81188d93c3-goog
> >
>