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

From: Andy Lutomirski
Date: Fri Aug 15 2014 - 15:03:26 EST

On Fri, Aug 15, 2014 at 11:56 AM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
> 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...

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.

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