powerpc/bpf: Move out dummy_tramp_addr after Long branch stub

    Move the long branch address space to the bottom of the long
    branch stub. This allows uninterrupted disassembly until the
    last 8 bytes. Exclude these last bytes from the overall
    program length to prevent failure in assembly generation.
    Also, align dummy_tramp_addr field with 8-byte boundary.

    Following is disassembler output for test program with moved down
    dummy_tramp_addr field:
    .....
    .....
    pc:68    left:44     a6 03 08 7c  :  mtlr 0
    pc:72    left:40     bc ff ff 4b  :  b .-68
    pc:76    left:36     a6 02 68 7d  :  mflr 11
    pc:80    left:32     05 00 9f 42  :  bcl 20, 31, .+4
    pc:84    left:28     a6 02 88 7d  :  mflr 12
    pc:88    left:24     14 00 8c e9  :  ld 12, 20(12)
    pc:92    left:20     a6 03 89 7d  :  mtctr 12
    pc:96    left:16     a6 03 68 7d  :  mtlr 11
    pc:100   left:12     20 04 80 4e  :  bctr
    pc:104   left:8      c0 34 1d 00  :

    Failure log:
    Can't disasm instruction at offset 104: c0 34 1d 00 00 00 00 c0
    Disassembly logic can truncate at 104, ignoring last 8 bytes.

    Update the dummy_tramp_addr field offset calculation from the end
    of the program to reflect its new location, for bpf_arch_text_poke()
    to update the actual trampoline's address in this field.

    All BPF trampoline selftests continue to pass with this patch applied.

    Signed-off-by: Abhishek Dubey [off-list ref]

This looks like a bug fix for the bpf trampoline implementation on
powerpc64. Should this include a Fixes tag?

The bug was introduced in commit d243b62b7bd3 which added support for bpf
trampolines on powerpc64. That commit placed the dummy_tramp_addr field at
the beginning of the long branch stub, causing the disassembly failures and
incorrect offset calculations that this patch fixes.

Suggested:

  Fixes: d243b62b7bd3 ("powerpc64/bpf: Add support for bpf trampolines")


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