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

Alexei Starovoitov wrote:

On 3/1/18 12:51 AM, Naveen N. Rao wrote:

quoted

Daniel Borkmann wrote:

quoted

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 unfortunat=

e

quoted

quoted

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).

=20
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.

Ok, we prefer not to do that for powerpc (atleast, not for all of=20
module_alloc()) at this point.

And since option (c) below is not preferable, I think we will implement=20
what Daniel suggested above. This patchset already handles communicating=20
the BPF function addresses to the JIT engine, and enhancing=20
bpf_prog_get_info_by_fd() should address the concerns with bpftool.


- Naveen

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