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

From: Ilya Maximets
Date: Wed Oct 09 2019 - 12:40:02 EST

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 param socketcall.setsockopt() points to uninitialised byte(s)
at 0x4E7AB7E: setsockopt (in /usr/lib64/
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;