[PATCH bpf 0/3] bpf: avoid using/returning file descriptor value zero

Jesper Dangaard Brouer
2020-06-08T16:51:23Z

Make it easier to handle UAPI/kABI extensions by avoid BPF using/returning
file descriptor value zero. Use this in recent devmap extension to keep
older applications compatible with newer kernels.

For special type maps (e.g. devmap and cpumap) the map-value data-layout is
a configuration interface. This is a kernel Application Binary Interface
(kABI) that can only be tail extended. Thus, new members (and thus features)
can only be added to the end of this structure, and the kernel uses the
map->value_size from userspace to determine feature set 'version'.

For this kind of kABI to be extensible and backward compatible, is it common
that new members/fields (that represent a new feature) in the struct are
initialised as zero, which indicate that the feature isn't used. This makes
it possible to write userspace applications that are unaware of new kernel
features, but just include latest uapi headers, zero-init struct and
populate features it knows about.

The recent extension of devmap with a bpf_prog.fd requires end-user to
supply the file-descriptor value minus-1 to communicate that the features
isn't used. This isn't compatible with the described kABI extension model.

---

Jesper Dangaard Brouer (3):
bpf: syscall to start at file-descriptor 1
bpf: devmap adjust uapi for attach bpf program
bpf: selftests and tools use struct bpf_devmap_val from uapi

fs/file.c | 2 +
include/linux/file.h | 1 +
include/uapi/linux/bpf.h | 13 +++++++
kernel/bpf/devmap.c | 17 ++-------
kernel/bpf/syscall.c | 38 +++++++++++++++++---
tools/include/uapi/linux/bpf.h | 13 +++++++
.../selftests/bpf/prog_tests/xdp_devmap_attach.c | 8 ----
.../selftests/bpf/progs/test_xdp_devmap_helpers.c | 2 +
.../bpf/progs/test_xdp_with_devmap_helpers.c | 3 +-
9 files changed, 66 insertions(+), 31 deletions(-)

--