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

From: Alexei Starovoitov
Date: Fri Aug 15 2014 - 14:56:19 EST

On Fri, Aug 15, 2014 at 11:50 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Aug 15, 2014 10:36 AM, "Alexei Starovoitov" <ast@xxxxxxxxxxxx> wrote:
>> On Fri, Aug 15, 2014 at 10:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> > The downside of this approach is that compat support might be
>> > difficult or impossible.
>> Would do you mean by compat? 32-bit programs on 64-bit kernels?
>> There is no such concept for eBPF. All eBPF programs are always
>> operating on 64-bit registers.
> Doesn't the eBPF program need to know sizeof(long) to read these
> fields correctly? Or am I misunderstanding what the code does?

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...
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