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Date: Mon, 7 Oct 2019 22:38:55 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
Marek Majkowski <marek@...udflare.com>,
Lorenz Bauer <lmb@...udflare.com>,
Alan Maguire <alan.maguire@...cle.com>,
Jesper Dangaard Brouer <brouer@...hat.com>,
David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 2/5] bpf: Add support for setting chain call
sequence for programs
On Mon, Oct 07, 2019 at 07:20:37PM +0200, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> This adds support for setting and deleting bpf chain call programs through
> a couple of new commands in the bpf() syscall. The CHAIN_ADD and CHAIN_DEL
> commands take two eBPF program fds and a return code, and install the
> 'next' program to be chain called after the 'prev' program if that program
> returns 'retcode'. A retcode of -1 means "wildcard", so that the program
> will be executed regardless of the previous program's return code.
>
>
> The syscall command names are based on Alexei's prog_chain example[0],
> which Alan helpfully rebased on current bpf-next. However, the logic and
> program storage is obviously adapted to the execution logic in the previous
> commit.
>
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=prog_chain&id=f54f45d00f91e083f6aec2abe35b6f0be52ae85b&context=15
>
> Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
> include/uapi/linux/bpf.h | 10 ++++++
> kernel/bpf/syscall.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 88 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 1ce80a227be3..b03c23963af8 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -107,6 +107,9 @@ enum bpf_cmd {
> BPF_MAP_LOOKUP_AND_DELETE_ELEM,
> BPF_MAP_FREEZE,
> BPF_BTF_GET_NEXT_ID,
> + BPF_PROG_CHAIN_ADD,
> + BPF_PROG_CHAIN_DEL,
> + BPF_PROG_CHAIN_GET,
> };
>
> enum bpf_map_type {
> @@ -516,6 +519,13 @@ union bpf_attr {
> __u64 probe_offset; /* output: probe_offset */
> __u64 probe_addr; /* output: probe_addr */
> } task_fd_query;
> +
> + struct { /* anonymous struct used by BPF_PROG_CHAIN_* commands */
> + __u32 prev_prog_fd;
> + __u32 next_prog_fd;
> + __u32 retcode;
> + __u32 next_prog_id; /* output: prog_id */
> + };
> } __attribute__((aligned(8)));
>
> /* The description below is an attempt at providing documentation to eBPF
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index b8a203a05881..be8112e08a88 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2113,6 +2113,79 @@ static int bpf_prog_test_run(const union bpf_attr *attr,
> return ret;
> }
>
> +#define BPF_PROG_CHAIN_LAST_FIELD next_prog_id
> +
> +static int bpf_prog_chain(int cmd, const union bpf_attr *attr,
> + union bpf_attr __user *uattr)
> +{
> + struct bpf_prog *prog, *next_prog, *old_prog;
> + struct bpf_prog **array;
> + int ret = -EOPNOTSUPP;
> + u32 index, prog_id;
> +
> + if (CHECK_ATTR(BPF_PROG_CHAIN))
> + return -EINVAL;
> +
> + /* Index 0 is wildcard, encoded as ~0 by userspace */
> + if (attr->retcode == ((u32) ~0))
> + index = 0;
> + else
> + index = attr->retcode + 1;
> +
> + if (index >= BPF_NUM_CHAIN_SLOTS)
> + return -E2BIG;
> +
> + prog = bpf_prog_get(attr->prev_prog_fd);
> + if (IS_ERR(prog))
> + return PTR_ERR(prog);
> +
> + /* If the chain_calls bit is not set, that's because the chain call flag
> + * was not set on program load, and so we can't support chain calls.
> + */
> + if (!prog->chain_calls)
> + goto out;
> +
> + array = prog->aux->chain_progs;
> +
> + switch (cmd) {
> + case BPF_PROG_CHAIN_ADD:
> + next_prog = bpf_prog_get(attr->next_prog_fd);
> + if (IS_ERR(next_prog)) {
> + ret = PTR_ERR(next_prog);
> + break;
> + }
> + old_prog = xchg(array + index, next_prog);
> + if (old_prog)
> + bpf_prog_put(old_prog);
> + ret = 0;
> + break;
How are circular dependencies resolved here? Seems the situation is
not prevented, so progs unloaded via XDP won't get the __bpf_prog_free()
call where they then drop the references of all the other progs in the
chain.
> + case BPF_PROG_CHAIN_DEL:
> + old_prog = xchg(array + index, NULL);
> + if (old_prog) {
> + bpf_prog_put(old_prog);
> + ret = 0;
> + } else {
> + ret = -ENOENT;
> + }
> + break;
> + case BPF_PROG_CHAIN_GET:
> + old_prog = READ_ONCE(array[index]);
> + if (old_prog) {
> + prog_id = old_prog->aux->id;
> + if (put_user(prog_id, &uattr->next_prog_id))
> + ret = -EFAULT;
> + else
> + ret = 0;
> + } else
> + ret = -ENOENT;
> + break;
> + }
> +
> +out:
> + bpf_prog_put(prog);
> + return ret;
> +}
> +
> #define BPF_OBJ_GET_NEXT_ID_LAST_FIELD next_id
>
> static int bpf_obj_get_next_id(const union bpf_attr *attr,
> @@ -2885,6 +2958,11 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
> case BPF_PROG_TEST_RUN:
> err = bpf_prog_test_run(&attr, uattr);
> break;
> + case BPF_PROG_CHAIN_ADD:
> + case BPF_PROG_CHAIN_DEL:
> + case BPF_PROG_CHAIN_GET:
> + err = bpf_prog_chain(cmd, &attr, uattr);
> + break;
> case BPF_PROG_GET_NEXT_ID:
> err = bpf_obj_get_next_id(&attr, uattr,
> &prog_idr, &prog_idr_lock);
>
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