[PATCH bpf-next 0/3] bpf: allow cgroup progs to export custom errnos to userspace

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: YiFei Zhu <zhuyifei@google.com>

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 a helper, bpf_export_errno, that allows hooks
to get/set the errno exported by eBPF to userspace. Invoking the helper
with a positive errno will set the exported errno, and invoking the
helper with zero will return the previously set errno. This means
that an errno, once set, can be overridden but cannot be unset. This
also allows later progs to retrieve errnos set by previous progs.

For legacy programs that rejects a syscall without setting the exported
errno, for backwards compatibility, if a prog rejects without itself
or a prior prog setting errno, the errno is set by the kernel to -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 implements the helper and the tracking of errno.
Patch 3 tests the behaviors of the helper.

YiFei Zhu (3):
  bpf: Make BPF_PROG_RUN_ARRAY return -errno instead of allow boolean
  bpf: Add cgroup helper bpf_export_errno to get/set exported errno
    value
  selftests/bpf: Test bpf_export_errno behavior with cgroup/sockopt

 include/linux/bpf.h                           |  27 +-
 include/uapi/linux/bpf.h                      |  14 +
 kernel/bpf/cgroup.c                           |  65 ++-
 security/device_cgroup.c                      |   2 +-
 tools/include/uapi/linux/bpf.h                |  14 +
 .../bpf/prog_tests/cgroup_export_errno.c      | 472 ++++++++++++++++++
 .../progs/cgroup_export_errno_getsockopt.c    |  45 ++
 .../progs/cgroup_export_errno_setsockopt.c    |  52 ++
 8 files changed, 651 insertions(+), 40 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_export_errno.c
 create mode 100644 tools/testing/selftests/bpf/progs/cgroup_export_errno_getsockopt.c
 create mode 100644 tools/testing/selftests/bpf/progs/cgroup_export_errno_setsockopt.c

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2.33.0