prog: Allow explicit ifindex specification

[twaapo]

fixes #1841

[lmb]

Is it possible to bind an XDP program to lo? If yes, please add a unit test.

[twaapo]

It seems to be possible to bind XDP programs to lo. However with the kfunc I get.
load program: operation not supported

[lmb]

It seems to be possible to bind XDP programs to lo. However with the kfunc I get.
load program: operation not supported

How about adding a test which just loads a minimal XDP program which is bound to lo, without the kfunc? (There are plenty examples in the code base) I think that is enough to validate that binding works. Kfuncs already have coverage otherwise.

[twaapo]

Theres a caveat:

greybox ebpf (main*) # go test -v --run TestProgramLoadBound.+
=== RUN   TestProgramLoadBoundToDevice
    prog_test.go:36: out:
                0: LdImmDW dst: r0 imm: 2
                2: Exit

    prog_test.go:37: bind on ifi 1
    prog_test.go:52: load program: operation not supported
--- FAIL: TestProgramLoadBoundToDevice (0.00s)
FAIL
exit status 1
FAIL    github.com/cilium/ebpf  0.003s
greybox ebpf (main*) # go test -v --run TestProgramLoadBound.+
=== RUN   TestProgramLoadBoundToDevice
    prog_test.go:36: out:
                0: LdImmDW dst: r0 imm: 2
                2: Exit

    prog_test.go:37: bind on ifi 2
    prog_test.go:52: load program: operation not supported
--- FAIL: TestProgramLoadBoundToDevice (0.00s)
FAIL
exit status 1
FAIL    github.com/cilium/ebpf  0.003s
greybox ebpf (main*) #
greybox ebpf (main*) # go test -v --run TestProgramLoadBound.+
=== RUN   TestProgramLoadBoundToDevice
    prog_test.go:36: out:
                0: LdImmDW dst: r0 imm: 2
                2: Exit

    prog_test.go:37: bind on ifi 3
--- PASS: TestProgramLoadBoundToDevice (0.00s)
PASS
ok      github.com/cilium/ebpf  0.003s
greybox ebpf (main*) # ethtool -i eno1
driver: e1000e
greybox ebpf (main*) # ethtool -i enp1s0f0np0
driver: i40e
greybox ebpf (main) #

Where ifindex=3 is Intel XL710 NIC. Seems not to work with lo nor the other physical NIC at ifi=2.

[lmb]

Turned the test into a negative check instead.