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bpf: Document EFAULT changes for sockopt

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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>
This commit is contained in:
Stanislav Fomichev 2023-05-11 10:04:56 -07:00 committed by Martin KaFai Lau
parent e01b4a72f1
commit 6b6a23d5d8
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.