Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be attached to events

On Fri, Aug 15, 2014 at 12:02 PM, Andy Lutomirski [off-list ref] wrote:

quoted

correct. eBPF program would be using 8-byte read on 64-bit kernel
and 4-byte read on 32-bit kernel. Same with access to ptrace fields
and pretty much all other fields in the kernel. The program will be
different on different kernels.
Say, this bpf_context struct doesn't exist at all. The programs would
still need to be different to walk in-kernel data structures...

Hmm.  I guess this isn't so bad.

What's the actual difficulty with using u64?  ISTM that, if the clang
front-end can't deal with u64, there's a bigger problem.  Or is it
something else I don't understand.

clang/llvm has no problem with u64 :)
This bpf_context struct for tracing is trying to answer the question:
 'what's the most convenient way to access tracepoint arguments
from a script'.
When kernel code has something like:
 trace_kfree_skb(skb, net_tx_action);
the script needs to be able to access this 'skb' and 'net_tx_action'
values through _single_ data structure.
In this proposal they are ctx->arg1 and ctx->arg2.
I've considered having different bpf_context's for every event, but
the complexity explodes. I need to hack all event definitions and so on.
imo it's better to move complexity to userspace, so program author
or high level language abstracts these details.