As Alexei points out, struct bpf_sk_lookup_kern has two 4-byte holes.
This leads to suboptimal instructions being generated (IPv4, x86):

    1372                    struct bpf_sk_lookup_kern ctx = {
       0xffffffff81b87f30 <+624>:   xor    %eax,%eax
       0xffffffff81b87f32 <+626>:   mov    $0x6,%ecx
       0xffffffff81b87f37 <+631>:   lea    0x90(%rsp),%rdi
       0xffffffff81b87f3f <+639>:   movl   $0x110002,0x88(%rsp)
       0xffffffff81b87f4a <+650>:   rep stos %rax,%es:(%rdi)
       0xffffffff81b87f4d <+653>:   mov    0x8(%rsp),%eax
       0xffffffff81b87f51 <+657>:   mov    %r13d,0x90(%rsp)
       0xffffffff81b87f59 <+665>:   incl   %gs:0x7e4970a0(%rip)
       0xffffffff81b87f60 <+672>:   mov    %eax,0x8c(%rsp)
       0xffffffff81b87f67 <+679>:   movzwl 0x10(%rsp),%eax
       0xffffffff81b87f6c <+684>:   mov    %ax,0xa8(%rsp)
       0xffffffff81b87f74 <+692>:   movzwl 0x38(%rsp),%eax
       0xffffffff81b87f79 <+697>:   mov    %ax,0xaa(%rsp)

Fix this by moving around sport and dport. pahole confirms there
are no more holes:

    struct bpf_sk_lookup_kern {
        u16                        family;       /*     0     2 */
        u16                        protocol;     /*     2     2 */
        __be16                     sport;        /*     4     2 */
        u16                        dport;        /*     6     2 */
        struct {
                __be32             saddr;        /*     8     4 */
                __be32             daddr;        /*    12     4 */
        } v4;                                    /*     8     8 */
        struct {
                const struct in6_addr  * saddr;  /*    16     8 */
                const struct in6_addr  * daddr;  /*    24     8 */
        } v6;                                    /*    16    16 */
        struct sock *              selected_sk;  /*    32     8 */
        bool                       no_reuseport; /*    40     1 */

        /* size: 48, cachelines: 1, members: 8 */
        /* padding: 7 */
        /* last cacheline: 48 bytes */
    };

The assembly also doesn't contain the pesky rep stos anymore:

    1372                    struct bpf_sk_lookup_kern ctx = {
       0xffffffff81b87f60 <+624>:   movzwl 0x10(%rsp),%eax
       0xffffffff81b87f65 <+629>:   movq   $0x0,0xa8(%rsp)
       0xffffffff81b87f71 <+641>:   movq   $0x0,0xb0(%rsp)
       0xffffffff81b87f7d <+653>:   mov    %ax,0x9c(%rsp)
       0xffffffff81b87f85 <+661>:   movzwl 0x38(%rsp),%eax
       0xffffffff81b87f8a <+666>:   movq   $0x0,0xb8(%rsp)
       0xffffffff81b87f96 <+678>:   mov    %ax,0x9e(%rsp)
       0xffffffff81b87f9e <+686>:   mov    0x8(%rsp),%eax
       0xffffffff81b87fa2 <+690>:   movq   $0x0,0xc0(%rsp)
       0xffffffff81b87fae <+702>:   movl   $0x110002,0x98(%rsp)
       0xffffffff81b87fb9 <+713>:   mov    %eax,0xa0(%rsp)
       0xffffffff81b87fc0 <+720>:   mov    %r13d,0xa4(%rsp)

1: https://lore.kernel.org/bpf/CAADnVQKE6y9h2fwX6OS837v-Uf+aBXnT_JXiN_bbo2gitZQ3tA@mail.gmail.com/

Fixes: e9ddbb77 ("bpf: Introduce SK_LOOKUP program type with a dedicated attach point")
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/20200910110248.198326-1-lmb@cloudflare.com