4e950747ba
YiFei Zhu says:
====================
Right now, most cgroup hooks are best used for permission checks. They
can only reject a syscall with -EPERM, so a cause of a rejection, if
the rejected by eBPF cgroup hooks, is ambiguous to userspace.
Additionally, if the syscalls are implemented in eBPF, all permission
checks and the implementation has to happen within the same filter,
as programs executed later in the series of progs are unaware of the
return values return by the previous progs.
This patch series adds two helpers, bpf_get_retval and bpf_set_retval,
that allows hooks to get/set the return value of syscall to userspace.
This also allows later progs to retrieve retval set by previous progs.
For legacy programs that rejects a syscall without setting the retval,
for backwards compatibility, if a prog rejects without itself or a
prior prog setting retval to an -err, the retval is set by the kernel
to -EPERM.
For getsockopt hooks that has ctx->retval, this variable mirrors that
that accessed by the helpers.
Additionally, the following user-visible behavior for getsockopt
hooks has changed:
- If a prior filter rejected the syscall, it will be visible
in ctx->retval.
- Attempting to change the retval arbitrarily is now allowed and
will not cause an -EFAULT.
- If kernel rejects a getsockopt syscall before running the hooks,
the error will be visible in ctx->retval. Returning 0 from the
prog will not overwrite the error to -EPERM unless there is an
explicit call of bpf_set_retval(-EPERM)
Tests have been added in this series to test the behavior of the helper
with cgroup setsockopt getsockopt hooks.
Patch 1 changes the API of macros to prepare for the next patch and
should be a no-op.
Patch 2 moves ctx->retval to a struct pointed to by current
task_struct.
Patch 3 implements the helpers.
Patch 4 tests the behaviors of the helpers.
Patch 5 updates a test after the test broke due to the visible changes.
v1 -> v2:
- errno -> retval
- split one helper to get & set helpers
- allow retval to be set arbitrarily in the general case
- made the helper retval and context retval mirror each other
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.