Re: [PATCH bpf] libbpf: fix passing uninitialized bytes to setsockopt

[Ilya Maximets]

On 09.10.2019 18:29, Andrii Nakryiko wrote:
> On Wed, Oct 9, 2019 at 8:43 AM Ilya Maximets <i.maximets@xxxxxxx> wrote:
> > 'struct xdp_umem_reg' has 4 bytes of padding at the end that makes
> > valgrind complain about passing uninitialized stack memory to the
> > syscall:
> >
> >    Syscall param socketcall.setsockopt() points to uninitialised byte(s)
> >      at 0x4E7AB7E: setsockopt (in /usr/lib64/libc-2.29.so)
> >      by 0x4BDE035: xsk_umem__create@@LIBBPF_0.0.4 (xsk.c:172)
> >    Uninitialised value was created by a stack allocation
> >      at 0x4BDDEBA: xsk_umem__create@@LIBBPF_0.0.4 (xsk.c:140)
> >
> > Padding bytes appeared after introducing of a new 'flags' field.
> >
> > Fixes: 10d30e301732 ("libbpf: add flags to umem config")
> > Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
> > ---
> >   tools/lib/bpf/xsk.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > index a902838f9fcc..26d9db783560 100644
> > --- a/tools/lib/bpf/xsk.c
> > +++ b/tools/lib/bpf/xsk.c
> > @@ -139,7 +139,7 @@ int xsk_umem__create_v0_0_4(struct xsk_umem **umem_ptr, void *umem_area,
> >                              const struct xsk_umem_config *usr_config)
> >   {
> >          struct xdp_mmap_offsets off;
> > -       struct xdp_umem_reg mr;
> > +       struct xdp_umem_reg mr = {};
>
> well, guess what, even with this explicit initialization, padding is
> not guaranteed to be initialized (and it's sometimes is not in
> practice, I ran into such problems), only since C11 standard it is
> specified that padding is also zero-initialized. You have to do memset
> to 0.

OK. Sure. I'll send v2.

> >          struct xsk_umem *umem;
> >          socklen_t optlen;
> >          void *map;
> > --
> > 2.17.1