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bpf: Document EFAULT changes for sockopt 

JIRA: https://issues.redhat.com/browse/RHEL-9957

commit 6b6a23d5d8e857e0dda1bbe5043cf4d5e9c711d3
Author: Stanislav Fomichev <sdf@google.com>
Date:   Thu May 11 10:04:56 2023 -0700

    bpf: Document EFAULT changes for sockopt
    
    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: Viktor Malik <vmalik@redhat.com>
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Viktor Malik 2023-10-11 13:01:12 +02:00
parent 67ddf1b217
commit a222660132
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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.