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Diffstat (limited to 'Documentation/bpf/prog_cgroup_sockopt.rst')
| -rw-r--r-- | Documentation/bpf/prog_cgroup_sockopt.rst | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/Documentation/bpf/prog_cgroup_sockopt.rst b/Documentation/bpf/prog_cgroup_sockopt.rst index c47d974629ae..1226a94af07a 100644 --- a/Documentation/bpf/prog_cgroup_sockopt.rst +++ b/Documentation/bpf/prog_cgroup_sockopt.rst @@ -86,8 +86,77 @@ then the next program in the chain (A) will see those changes, *not* the original input ``setsockopt`` arguments. The potentially modified values will be then passed down to the kernel. +Large optval +============ +When the ``optval`` is greater than the ``PAGE_SIZE``, the BPF program +can access only the first ``PAGE_SIZE`` of that data. So it has to options: + +* Set ``optlen`` to zero, which indicates that the kernel should + use the original buffer from the userspace. Any modifications + done by the BPF program to the ``optval`` are ignored. +* Set ``optlen`` to the value less than ``PAGE_SIZE``, which + indicates that the kernel should use BPF's trimmed ``optval``. + +When the BPF program returns with the ``optlen`` greater than +``PAGE_SIZE``, the userspace will receive original kernel +buffers without any modifications that the BPF program might have +applied. + Example ======= +Recommended way to handle BPF programs is as follows: + +.. code-block:: c + + SEC("cgroup/getsockopt") + int getsockopt(struct bpf_sockopt *ctx) + { + /* Custom socket option. */ + if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) { + ctx->retval = 0; + optval[0] = ...; + ctx->optlen = 1; + return 1; + } + + /* Modify kernel's socket option. */ + if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) { + ctx->retval = 0; + optval[0] = ...; + ctx->optlen = 1; + return 1; + } + + /* optval larger than PAGE_SIZE use kernel's buffer. */ + if (ctx->optlen > PAGE_SIZE) + ctx->optlen = 0; + + return 1; + } + + SEC("cgroup/setsockopt") + int setsockopt(struct bpf_sockopt *ctx) + { + /* Custom socket option. */ + if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) { + /* do something */ + ctx->optlen = -1; + return 1; + } + + /* Modify kernel's socket option. */ + if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) { + optval[0] = ...; + return 1; + } + + /* optval larger than PAGE_SIZE use kernel's buffer. */ + if (ctx->optlen > PAGE_SIZE) + ctx->optlen = 0; + + return 1; + } + See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example of BPF program that handles socket options. |