Re: [net-next V2 PATCH 5/5] samples/bpf: add cpumap sample program xdp_redirect_cpu

On Mon, Oct 02, 2017 at 02:07:23PM +0200, Jesper Dangaard Brouer wrote:

On Fri, 29 Sep 2017 20:06:09 -0700
Alexei Starovoitov [off-list ref] wrote:

quoted

quoted

+/*** Trace point code ***/
+
+/* Tracepoint format: /sys/kernel/debug/tracing/events/xdp/xdp_redirect/format
+ * Code in:                kernel/include/trace/events/xdp.h
+ */
+struct xdp_redirect_ctx {
+	unsigned short common_type;	//	offset:0;  size:2; signed:0;
+	unsigned char common_flags;	//	offset:2;  size:1; signed:0;
+	unsigned char common_preempt_count;//	offset:3;  size:1; signed:0;
+	int common_pid;			//	offset:4;  size:4; signed:1;  

this part is not right. First 8 bytes are not accessible by bpf code.
Please use __u64 pad; or similar here.

I've corrected this in V3.

Can you explain why BPF cannot access these (first 8 bytes) struct members?

in the very first patch adding bpf to tracepoint we allowed the access,
but then people complained that common_* stuff is not abi and since it was
slow to populate anyway due to local_save_flags (which is never used by
scripts we had so far) and for pid there was a helper already, I was happy
to get rid of it and save these 8 bytes for future use. which ended up
being used by hidden(inaccessible) pt_regs pointer.
Layout description is in
commit 98b5c2c65c29 ("perf, bpf: allow bpf programs attach to tracepoints")
The work on dynamic bits of tracepoints is still tbd.