Re: [RFC bpf-next v3 5/5] selftests/bpf: Add test cases for sleepable BPF programs of the CGROUP_SOCKOPT type

[Kui-Feng Lee <sinquersw@xxxxxxxxx>]

On 8/15/23 16:37, Kui-Feng Lee wrote:
> On 8/15/23 13:57, Stanislav Fomichev wrote:
> > On 08/15, thinker.li@xxxxxxxxx wrote:
> > > From: Kui-Feng Lee <thinker.li@xxxxxxxxx>
> > >
> > > Do the same test as non-sleepable ones.
> > >
> > > Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
> > > ---
> > >   .../testing/selftests/bpf/bpf_experimental.h  |  36 +++
> > >   tools/testing/selftests/bpf/bpf_kfuncs.h      |  41 +++
> > >   .../selftests/bpf/prog_tests/sockopt_sk.c     | 112 +++++++-
> > >   .../testing/selftests/bpf/progs/sockopt_sk.c  | 257 ++++++++++++++++++
> > >   .../selftests/bpf/verifier/sleepable.c        |   2 +-
> > >   5 files changed, 445 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/bpf/bpf_experimental.h 
> > > b/tools/testing/selftests/bpf/bpf_experimental.h
> > > index 209811b1993a..9b5dfefe65dc 100644
> > > --- a/tools/testing/selftests/bpf/bpf_experimental.h
> > > +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> > > @@ -131,4 +131,40 @@ extern int bpf_rbtree_add_impl(struct 
> > > bpf_rb_root *root, struct bpf_rb_node *nod
> > >    */
> > >   extern struct bpf_rb_node *bpf_rbtree_first(struct bpf_rb_root 
> > > *root) __ksym;
> > > +/*
> > > + *     Description
> > > + *             Copy data from *ptr* to *sopt->optval*.
> > > + *     Return
> > > + *             >= 0 on success, or a negative error in case of failure.
> > > + */
> > > +extern int bpf_sockopt_dynptr_copy_to(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > > +/* Description
> > > + *    Allocate a buffer of 'size' bytes for being installed as optval.
> > > + * Returns
> > > + *    > 0 on success, the size of the allocated buffer
> > > + *    -ENOMEM or -EINVAL on failure
> > > + */
> > > +extern int bpf_sockopt_dynptr_alloc(struct bpf_sockopt *sopt, int size,
> > > +                    struct bpf_dynptr *ptr__uninit) __ksym;
> > > +
> > > +/* Description
> > > + *    Install the buffer pointed to by 'ptr' as optval.
> > > + * Returns
> > > + *    0 on success
> > > + *    -EINVAL if the buffer is too small
> > > + */
> > > +extern int bpf_sockopt_dynptr_install(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > > +/* Description
> > > + *    Release the buffer allocated by bpf_sockopt_dynptr_alloc.
> > > + * Returns
> > > + *    0 on success
> > > + *    -EINVAL if the buffer was not allocated by 
> > > bpf_sockopt_dynptr_alloc
> > > + */
> > > +extern int bpf_sockopt_dynptr_release(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > >   #endif
> > > diff --git a/tools/testing/selftests/bpf/bpf_kfuncs.h 
> > > b/tools/testing/selftests/bpf/bpf_kfuncs.h
> > > index 642dda0e758a..772040225257 100644
> > > --- a/tools/testing/selftests/bpf/bpf_kfuncs.h
> > > +++ b/tools/testing/selftests/bpf/bpf_kfuncs.h
> > > @@ -41,4 +41,45 @@ extern bool bpf_dynptr_is_rdonly(const struct 
> > > bpf_dynptr *ptr) __ksym;
> > >   extern __u32 bpf_dynptr_size(const struct bpf_dynptr *ptr) __ksym;
> > >   extern int bpf_dynptr_clone(const struct bpf_dynptr *ptr, struct 
> > > bpf_dynptr *clone__init) __ksym;
> > > +extern int bpf_sockopt_dynptr_copy_to(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > > +/* Description
> > > + *    Allocate a buffer of 'size' bytes for being installed as optval.
> > > + * Returns
> > > + *    > 0 on success, the size of the allocated buffer
> > > + *    -ENOMEM or -EINVAL on failure
> > > + */
> > > +extern int bpf_sockopt_dynptr_alloc(struct bpf_sockopt *sopt, int size,
> > > +                    struct bpf_dynptr *ptr__uninit) __ksym;
> > > +
> > > +/* Description
> > > + *    Install the buffer pointed to by 'ptr' as optval.
> > > + * Returns
> > > + *    0 on success
> > > + *    -EINVAL if the buffer is too small
> > > + */
> > > +extern int bpf_sockopt_dynptr_install(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > > +/* Description
> > > + *    Release the buffer allocated by bpf_sockopt_dynptr_alloc.
> > > + * Returns
> > > + *    0 on success
> > > + *    -EINVAL if the buffer was not allocated by 
> > > bpf_sockopt_dynptr_alloc
> > > + */
> > > +extern int bpf_sockopt_dynptr_release(struct bpf_sockopt *sopt,
> > > +                      struct bpf_dynptr *ptr) __ksym;
> > > +
> > > +/* Description
> > > + *    Initialize a dynptr to access the content of optval passing
> > > + *      to {get,set}sockopt()s.
> > > + * Returns
> > > + *    > 0 on success, the size of the allocated buffer
> > > + *    -ENOMEM or -EINVAL on failure
> > > + */
> > > +extern int bpf_sockopt_dynptr_from(struct bpf_sockopt *sopt,
> > > +                   struct bpf_dynptr *ptr__uninit,
> > > +                   unsigned int size) __ksym;
> > > +
> > >   #endif
> > > diff --git a/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c 
> > > b/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
> > > index 05d0e07da394..85255648747f 100644
> > > --- a/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
> > > +++ b/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
> > > @@ -92,6 +92,7 @@ static int getsetsockopt(void)
> > >       }
> > >       if (buf.u8[0] != 0x01) {
> > >           log_err("Unexpected buf[0] 0x%02x != 0x01", buf.u8[0]);
> > > +        log_err("optlen %d", optlen);
> > >           goto err;
> > >       }
> > > @@ -220,7 +221,7 @@ static int getsetsockopt(void)
> > >       return -1;
> > >   }
> > > -static void run_test(int cgroup_fd)
> > > +static void run_test_nonsleepable(int cgroup_fd)
> > >   {
> > >       struct sockopt_sk *skel;
> > > @@ -246,6 +247,106 @@ static void run_test(int cgroup_fd)
> > >       sockopt_sk__destroy(skel);
> > >   }
> > > +static void run_test_nonsleepable_mixed(int cgroup_fd)
> > > +{
> > > +    struct sockopt_sk *skel;
> > > +
> > > +    skel = sockopt_sk__open_and_load();
> > > +    if (!ASSERT_OK_PTR(skel, "skel_load"))
> > > +        goto cleanup;
> > > +
> > > +    skel->bss->page_size = getpagesize();
> > > +    skel->bss->skip_sleepable = 1;
> > > +
> > > +    skel->links._setsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._setsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._setsockopt_s, "setsockopt_link 
> > > (sleepable)"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._getsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._getsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._getsockopt_s, "getsockopt_link 
> > > (sleepable)"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._setsockopt =
> > > +        bpf_program__attach_cgroup(skel->progs._setsockopt, cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._setsockopt, "setsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._getsockopt =
> > > +        bpf_program__attach_cgroup(skel->progs._getsockopt, cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._getsockopt, "getsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    ASSERT_OK(getsetsockopt(), "getsetsockopt");
> > > +
> > > +cleanup:
> > > +    sockopt_sk__destroy(skel);
> > > +}
> > > +
> > > +static void run_test_sleepable(int cgroup_fd)
> > > +{
> > > +    struct sockopt_sk *skel;
> > > +
> > > +    skel = sockopt_sk__open_and_load();
> > > +    if (!ASSERT_OK_PTR(skel, "skel_load"))
> > > +        goto cleanup;
> > > +
> > > +    skel->bss->page_size = getpagesize();
> > > +
> > > +    skel->links._setsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._setsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._setsockopt_s, "setsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._getsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._getsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._getsockopt_s, "getsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    ASSERT_OK(getsetsockopt(), "getsetsockopt");
> > > +
> > > +cleanup:
> > > +    sockopt_sk__destroy(skel);
> > > +}
> > > +
> > > +static void run_test_sleepable_mixed(int cgroup_fd)
> > > +{
> > > +    struct sockopt_sk *skel;
> > > +
> > > +    skel = sockopt_sk__open_and_load();
> > > +    if (!ASSERT_OK_PTR(skel, "skel_load"))
> > > +        goto cleanup;
> > > +
> > > +    skel->bss->page_size = getpagesize();
> > > +    skel->bss->skip_nonsleepable = 1;
> > > +
> > > +    skel->links._setsockopt =
> > > +        bpf_program__attach_cgroup(skel->progs._setsockopt, cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._setsockopt, "setsockopt_link 
> > > (nonsleepable)"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._getsockopt =
> > > +        bpf_program__attach_cgroup(skel->progs._getsockopt, cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._getsockopt, "getsockopt_link 
> > > (nonsleepable)"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._setsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._setsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._setsockopt_s, "setsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    skel->links._getsockopt_s =
> > > +        bpf_program__attach_cgroup(skel->progs._getsockopt_s, 
> > > cgroup_fd);
> > > +    if (!ASSERT_OK_PTR(skel->links._getsockopt_s, "getsockopt_link"))
> > > +        goto cleanup;
> > > +
> > > +    ASSERT_OK(getsetsockopt(), "getsetsockopt");
> > > +
> > > +cleanup:
> > > +    sockopt_sk__destroy(skel);
> > > +}
> > > +
> > >   void test_sockopt_sk(void)
> > >   {
> > >       int cgroup_fd;
> > > @@ -254,6 +355,13 @@ void test_sockopt_sk(void)
> > >       if (!ASSERT_GE(cgroup_fd, 0, "join_cgroup /sockopt_sk"))
> > >           return;
> > > -    run_test(cgroup_fd);
> > > +    if (test__start_subtest("nonsleepable"))
> > > +        run_test_nonsleepable(cgroup_fd);
> > > +    if (test__start_subtest("sleepable"))
> > > +        run_test_sleepable(cgroup_fd);
> > > +    if (test__start_subtest("nonsleepable_mixed"))
> > > +        run_test_nonsleepable_mixed(cgroup_fd);
> > > +    if (test__start_subtest("sleepable_mixed"))
> > > +        run_test_sleepable_mixed(cgroup_fd);
> > >       close(cgroup_fd);
> > >   }
> > > diff --git a/tools/testing/selftests/bpf/progs/sockopt_sk.c 
> > > b/tools/testing/selftests/bpf/progs/sockopt_sk.c
> > > index cb990a7d3d45..efacd3b88c40 100644
> > > --- a/tools/testing/selftests/bpf/progs/sockopt_sk.c
> > > +++ b/tools/testing/selftests/bpf/progs/sockopt_sk.c
> > > @@ -5,10 +5,16 @@
> > >   #include <netinet/in.h>
> > >   #include <bpf/bpf_helpers.h>
> > > +typedef int bool;
> > > +#include "bpf_kfuncs.h"
> > > +
> > >   char _license[] SEC("license") = "GPL";
> > >   int page_size = 0; /* userspace should set it */
> > > +int skip_sleepable = 0;
> > > +int skip_nonsleepable = 0;
> > > +
> > >   #ifndef SOL_TCP
> > >   #define SOL_TCP IPPROTO_TCP
> > >   #endif
> > > @@ -34,6 +40,9 @@ int _getsockopt(struct bpf_sockopt *ctx)
> > >       struct sockopt_sk *storage;
> > >       struct bpf_sock *sk;
> > > +    if (skip_nonsleepable)
> > > +        return 1;
> > > +
> > >       /* Bypass AF_NETLINK. */
> > >       sk = ctx->sk;
> > >       if (sk && sk->family == AF_NETLINK)
> > > @@ -136,6 +145,134 @@ int _getsockopt(struct bpf_sockopt *ctx)
> > >       return 1;
> > >   }
> > > +SEC("cgroup/getsockopt.s")
> > > +int _getsockopt_s(struct bpf_sockopt *ctx)
> > > +{
> > > +    struct tcp_zerocopy_receive *zcvr;
> > > +    struct bpf_dynptr optval_dynptr;
> > > +    struct sockopt_sk *storage;
> > > +    __u8 *optval, *optval_end;
> > > +    struct bpf_sock *sk;
> > > +    char buf[1];
> > > +    __u64 addr;
> > > +    int ret;
> > > +
> > > +    if (skip_sleepable)
> > > +        return 1;
> > > +
> > > +    /* Bypass AF_NETLINK. */
> > > +    sk = ctx->sk;
> > > +    if (sk && sk->family == AF_NETLINK)
> > > +        return 1;
> > > +
> > > +    optval = ctx->optval;
> > > +    optval_end = ctx->optval_end;
> > > +
> > > +    /* Make sure bpf_get_netns_cookie is callable.
> > > +     */
> > > +    if (bpf_get_netns_cookie(NULL) == 0)
> > > +        return 0;
> > > +
> > > +    if (bpf_get_netns_cookie(ctx) == 0)
> > > +        return 0;
> > > +
> > > +    if (ctx->level == SOL_IP && ctx->optname == IP_TOS) {
> > > +        /* Not interested in SOL_IP:IP_TOS;
> > > +         * let next BPF program in the cgroup chain or kernel
> > > +         * handle it.
> > > +         */
> > > +        return 1;
> > > +    }
> > > +
> > > +    if (ctx->level == SOL_SOCKET && ctx->optname == SO_SNDBUF) {
> > > +        /* Not interested in SOL_SOCKET:SO_SNDBUF;
> > > +         * let next BPF program in the cgroup chain or kernel
> > > +         * handle it.
> > > +         */
> > > +        return 1;
> > > +    }
> > > +
> > > +    if (ctx->level == SOL_TCP && ctx->optname == TCP_CONGESTION) {
> > > +        /* Not interested in SOL_TCP:TCP_CONGESTION;
> > > +         * let next BPF program in the cgroup chain or kernel
> > > +         * handle it.
> > > +         */
> > > +        return 1;
> > > +    }
> > > +
> > > +    if (ctx->level == SOL_TCP && ctx->optname == 
> > > TCP_ZEROCOPY_RECEIVE) {
> > > +        /* Verify that TCP_ZEROCOPY_RECEIVE triggers.
> > > +         * It has a custom implementation for performance
> > > +         * reasons.
> > > +         */
> > > +
> > > +        bpf_sockopt_dynptr_from(ctx, &optval_dynptr, sizeof(*zcvr));
> > > +        zcvr = bpf_dynptr_data(&optval_dynptr, 0, sizeof(*zcvr));
> > > +        addr = zcvr ? zcvr->address : 0;
> > > +        bpf_sockopt_dynptr_release(ctx, &optval_dynptr);
> >
> > This starts to look more usable, thank you for the changes!
> > Let me poke the api a bit more, I'm not super familiar with the dynptrs.
> >
> > here: bpf_sockopt_dynptr_from should probably be called
> > bpf_dynptr_from_sockopt to match bpf_dynptr_from_mem?
>
> agree!
>
> > > +
> > > +        return addr != 0 ? 0 : 1;
> > > +    }
> > > +
> > > +    if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
> > > +        if (optval + 1 > optval_end)
> > > +            return 0; /* bounds check */
> > > +
> > > +        ctx->retval = 0; /* Reset system call return value to zero */
> > > +
> > > +        /* Always export 0x55 */
> > > +        buf[0] = 0x55;
> > > +        ret = bpf_sockopt_dynptr_alloc(ctx, 1, &optval_dynptr);
> > > +        if (ret >= 0) {
> > > +            bpf_dynptr_write(&optval_dynptr, 0, buf, 1, 0);
> > > +            ret = bpf_sockopt_dynptr_copy_to(ctx, &optval_dynptr);
> > > +        }
> > > +        bpf_sockopt_dynptr_release(ctx, &optval_dynptr);
> >
> > Does bpf_sockopt_dynptr_alloc and bpf_sockopt_dynptr_release need to be
> > sockopt specific? Seems like we should provide, instead, some generic
> > bpf_dynptr_alloc/bpf_dynptr_release and make
> > bpf_sockopt_dynptr_copy_to/install work with them? WDYT?
>
> I found that kmalloc can not be called in the context of nmi, slab or
> page alloc path. It is why we don't have functions like
> bpf_dynptr_alloc/bpf_dynptr_release yet. That means we need someone
> to implement an allocator for BPF programs. And, then, we can have
> bpf_dynptr_alloc unpon it. (There is an implementation of
> bpf_dynptr_alloc() in the early versions of the patchset of dynptr.
> But, be removed before landing.)
>
>
> > > +        if (ret < 0)
> > > +            return 0;
> > > +        ctx->optlen = 1;
> > > +
> > > +        /* Userspace buffer is PAGE_SIZE * 2, but BPF
> > > +         * program can only see the first PAGE_SIZE
> > > +         * bytes of data.
> > > +         */
> > > +        if (optval_end - optval != page_size && 0)
> > > +            return 0; /* unexpected data size */
> > > +
> > > +        return 1;
> > > +    }
> > > +
> > > +    if (ctx->level != SOL_CUSTOM)
> > > +        return 0; /* deny everything except custom level */
> > > +
> > > +    if (optval + 1 > optval_end)
> > > +        return 0; /* bounds check */
> > > +
> > > +    storage = bpf_sk_storage_get(&socket_storage_map, ctx->sk, 0,
> > > +                     BPF_SK_STORAGE_GET_F_CREATE);
> > > +    if (!storage)
> > > +        return 0; /* couldn't get sk storage */
> > > +
> > > +    if (!ctx->retval)
> > > +        return 0; /* kernel should not have handled
> > > +               * SOL_CUSTOM, something is wrong!
> > > +               */
> > > +    ctx->retval = 0; /* Reset system call return value to zero */
> > > +
> > > +    buf[0] = storage->val;
> > > +    ret = bpf_sockopt_dynptr_alloc(ctx, 1, &optval_dynptr);
> > > +    if (ret >= 0) {
> > > +        bpf_dynptr_write(&optval_dynptr, 0, buf, 1, 0);
> > > +        ret = bpf_sockopt_dynptr_copy_to(ctx, &optval_dynptr);
> > > +    }
> > > +    bpf_sockopt_dynptr_release(ctx, &optval_dynptr);
> > > +    if (ret < 0)
> > > +        return 0;
> > > +    ctx->optlen = 1;
> > > +
> > > +    return 1;
> > > +}
> > > +
> > >   SEC("cgroup/setsockopt")
> > >   int _setsockopt(struct bpf_sockopt *ctx)
> > >   {
> > > @@ -144,6 +281,9 @@ int _setsockopt(struct bpf_sockopt *ctx)
> > >       struct sockopt_sk *storage;
> > >       struct bpf_sock *sk;
> > > +    if (skip_nonsleepable)
> > > +        return 1;
> > > +
> > >       /* Bypass AF_NETLINK. */
> > >       sk = ctx->sk;
> > >       if (sk && sk->family == AF_NETLINK)
> > > @@ -236,3 +376,120 @@ int _setsockopt(struct bpf_sockopt *ctx)
> > >           ctx->optlen = 0;
> > >       return 1;
> > >   }
> > > +
> > > +SEC("cgroup/setsockopt.s")
> > > +int _setsockopt_s(struct bpf_sockopt *ctx)
> > > +{
> > > +    struct bpf_dynptr optval_buf;
> > > +    struct sockopt_sk *storage;
> > > +    __u8 *optval, *optval_end;
> > > +    struct bpf_sock *sk;
> > > +    __u8 tmp_u8;
> > > +    __u32 tmp;
> > > +    int ret;
> > > +
> > > +    if (skip_sleepable)
> > > +        return 1;
> > > +
> > > +    optval = ctx->optval;
> > > +    optval_end = ctx->optval_end;
> > > +
> > > +    /* Bypass AF_NETLINK. */
> > > +    sk = ctx->sk;
> > > +    if (sk && sk->family == AF_NETLINK)
> > > +        return -1;
> > > +
> > > +    /* Make sure bpf_get_netns_cookie is callable.
> > > +     */
> > > +    if (bpf_get_netns_cookie(NULL) == 0)
> > > +        return 0;
> > > +
> > > +    if (bpf_get_netns_cookie(ctx) == 0)
> > > +        return 0;
> > > +
> > > +    if (ctx->level == SOL_IP && ctx->optname == IP_TOS) {
> > > +        /* Not interested in SOL_IP:IP_TOS;
> > > +         * let next BPF program in the cgroup chain or kernel
> > > +         * handle it.
> > > +         */
> > > +        ctx->optlen = 0; /* bypass optval>PAGE_SIZE */
> > > +        return 1;
> > > +    }
> > > +
> > > +    if (ctx->level == SOL_SOCKET && ctx->optname == SO_SNDBUF) {
> > > +        /* Overwrite SO_SNDBUF value */
> > > +
> > > +        ret = bpf_sockopt_dynptr_alloc(ctx, sizeof(__u32),
> > > +                           &optval_buf);
> > > +        if (ret < 0)
> > > +            bpf_sockopt_dynptr_release(ctx, &optval_buf);
> > > +        else {
> > > +            tmp = 0x55AA;
> > > +            bpf_dynptr_write(&optval_buf, 0, &tmp, sizeof(tmp), 0);
> > > +            ret = bpf_sockopt_dynptr_install(ctx, &optval_buf);
> >
> > One thing I'm still slightly confused about is
> > bpf_sockopt_dynptr_install vs bpf_sockopt_dynptr_copy_to. I do
> > understand that it comes from getsockopt vs setsockopt (and the ability,
> > in setsockopt, to allocate larger buffers), but I wonder whether
> > we can hide everything under single bpf_sockopt_dynptr_copy_to call?
> >
> > For getsockopt, it stays as is. For setsockopt, it would work like
> > _install currently does. Would that work? From user perspective,
> > if we provide a simple call that does the right thing, seems a bit
> > more usable? The only problem is probably the fact that _install
> > explicitly moves the ownership, but I don't see why copy_to can't
> > have the same "consume" semantics?

Sorry for missing this part!
This overloading is counterintuitive for me.
*_copy_to() will not copy/overwrite the buffer, but replace the buffer
instead. And, it will change its side-effects according to its context.
I would prefer a different name instead of reusing *_copy_to().

We probably need a better name, instead of copy_to, in order to merge
these two functions if we want to.