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bpf: Document EFAULT changes for sockopt 

ANBZ: #22860

commit 6b6a23d5d8 upstream.

And add examples for how to correctly handle large optlens.
This is less relevant now when we don't EFAULT anymore, but
that's still the correct thing to do.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230511170456.1759459-5-sdf@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Yinan Liu <yinan@linux.alibaba.com>
Reviewed-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Link: https://gitee.com/anolis/cloud-kernel/pulls/5523
This commit is contained in:
Stanislav Fomichev 2023-05-11 10:04:56 -07:00 committed by 小龙
parent 3507962043
commit 9e6c73f30c
1 changed files with 56 additions and 1 deletions
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57
Documentation/bpf/prog_cgroup_sockopt.rst
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@ -98,10 +98,65 @@ can access only the first ``PAGE_SIZE`` of that data. So it has to options:
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 ``EFAULT`` errno.
``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.