[PATCH] net/tap: Modified TAP BPF program as per the new Kernel-version upgrade requirements.

[Stephen Hemminger]

On Thu,  4 Jan 2024 22:57:56 +0530
madhuker.mythri at oracle.com wrote:

> adhuker Mythri <madhuker.mythri at oracle.com>
> 
> When multiple queues configured, internally RSS will be enabled and thus TAP BPF RSS byte-code will be loaded on to the Kernel using BPF system calls.
> 
> Here, the problem is loading the existing BPF byte-code to the Kernel-5.15 and above versions throws errors, i.e: Kernel BPF verifier not accepted this existing BPF byte-code and system calls return error code "-7" as follows:
> ------------------------
> rss_add_actions(): Failed to load BPF section l3_l4 (7): Argument list too long
> ------------------------
> 
> RCA:  These errors started coming after from the Kernel-5.15 version, in which lots of new BPF verification restrictions were added for safe execution of byte-code on to the Kernel, due to which existing BPF program verification does not pass.
> Here are the major BPF verifier restrictions observed:
> 1) Need to use new BPF maps structure.
> 2) Kernel SKB data pointer access not allowed.
> 3) Undefined loops were not allowed(which are bounded by a variable value).
> 4) unreachable instructions(like: undefined array access).
> 
> After addressing all these Kernel BPF verifier restrictions able to load the BPF byte-code onto the Kernel successfully.
> 
> Note: This new BPF changes supports from Kernel:4.10 version.
> 
> Bugzilla Id: 1329
> 
> Signed-off-by: Madhuker Mythri <madhuker.mythri at oracle.com>

I tried this version on Debian testing which has:
	kernel 6.5.0-5-amd64
	clang 16.0.6

If build and run with the pre-compiled BPF then it will load the
example flow  (see https://doc.dpdk.org/guides/nics/tap.html)

But if I recompile the bpf program by using make in the tap/bpf
directory, then the resulting bpf instructions will not make
it past verifier.

With modified tap_bpf_api can get the log message as:

testpmd> flow create 0 priority 4 ingress pattern eth dst is 0a:0b:0c:0d:0e:0f  / ipv4 / tcp / end actions rss queues 0 1 2 3 end / end
rss_add_actions(): Failed to load BPF section l3_l4 (13): func#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
0: (bf) r6 = r1                       ; R1=ctx(off=0,imm=0) R6_w=ctx(off=0,imm=0)
1: (18) r1 = 0x300                    ; R1_w=768
3: (63) *(u32 *)(r10 -84) = r1        ; R1_w=768 R10=fp0 fp-88=mmmm????
4: (bf) r2 = r10                      ; R2_w=fp0 R10=fp0
5: (07) r2 += -84                     ; R2_w=fp-84
6: (18) r1 = 0xfd                     ; R1_w=253
8: (85) call bpf_map_lookup_elem#1
R1 type=scalar expected=map_ptr
processed 7 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

port_flow_complain(): Caught PMD error type 16 (specific action): cause: 0x7ffcef37e678, action not supported: Operation not supported