[RFC][PATCH bpf] tools: bpftool: Fix tags for bpf-to-bpf calls

[Alexei Starovoitov]

On 3/1/18 12:51 AM, Naveen N. Rao wrote:
> Daniel Borkmann wrote:
>> On 02/27/2018 01:13 PM, Sandipan Das wrote:
>>> With this patch, it will look like this:
>>>    0: (85) call pc+2#bpf_prog_8f85936f29a7790a+3
>>
>> (Note the +2 is the insn->off already.)
>>
>>>    1: (b7) r0 = 1
>>>    2: (95) exit
>>>    3: (b7) r0 = 2
>>>    4: (95) exit
>>>
>>> where 8f85936f29a7790a is the tag of the bpf program and 3 is
>>> the offset to the start of the subprog from the start of the
>>> program.
>>
>> The problem with this approach would be that right now the name is
>> something like bpf_prog_5f76847930402518_F where the subprog tag is
>> just a placeholder so in future, this may well adapt to e.g. the actual
>> function name from the elf file. Note that when kallsyms is enabled
>> then a name like bpf_prog_5f76847930402518_F will also appear in stack
>> traces, perf records, etc, so for correlation/debugging it would really
>> help to have them the same everywhere.
>>
>> Worst case if there's nothing better, potentially what one could do in
>> bpf_prog_get_info_by_fd() is to dump an array of full addresses and
>> have the imm part as the index pointing to one of them, just unfortunate
>> that it's likely only needed in ppc64.
>
> Ok. We seem to have discussed a few different aspects in this thread.
> Let me summarize the different aspects we have discussed:
> 1. Passing address of JIT'ed function to the JIT engines:
>    Two approaches discussed:
>    a. Existing approach, where the subprog address is encoded as an
> offset from __bpf_call_base() in imm32 field of the BPF call
> instruction. This requires the JIT'ed function to be within 2GB of
> __bpf_call_base(), which won't be true on ppc64, at the least. So,
> this won't on ppc64 (and any other architectures where vmalloc'ed
> (module_alloc()) memory is from a different, far, address range).

it looks like ppc64 doesn't guarantee today that all of module_alloc()
will be within 32-bit, but I think it should be trivial to add such
guarantee. If so, we can define another __bpf_call_base specifically
for bpf-to-bpf calls when jit is on.
Then jit_subprogs() math will fit:
insn->imm = func[subprog]->bpf_func - __bpf_call_base_for_jited_progs;
and will make it easier for ppc64 jit to optimize and use
near calls for bpf-to-bpf calls while still using trampoline
for bpf-to-kernel.
Also it solves bpftool issue.
For all other archs we can keep
__bpf_call_base_for_jited_progs == __bpf_call_base

>    There is a third option we can consider:
>    c. Convert BPF pseudo call instruction into a 2-instruction sequence
>    (similar to BPF_DW) and encode the full 64-bit call target in the
> second bpf instruction. To distinguish this from other instruction
> forms, we can set imm32 to -1.

Adding new instruction just for that case looks like overkill.