bpf: Document optval > PAGE_SIZE behavior for sockopt hooks

Extend existing doc with more details about requiring ctx->optlen = 0
for handling optval > PAGE_SIZE.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200617010416.93086-3-sdf@google.com
This commit is contained in:
Stanislav Fomichev 2020-06-16 18:04:16 -07:00 committed by Alexei Starovoitov
parent a0cb12b031
commit 8030e250d8
1 changed files with 14 additions and 0 deletions

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@ -86,6 +86,20 @@ 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 ``EFAULT`` errno.
Example
=======