[Crash-utility] [BUG?] DWARF unwind is always off (and seems to be broken)

All, 1. Crash version 7.0.8 (debian8) never actually sets DWARF_UNWIND due to a bug in initialization order: first, `kernel_init` is called that checks whether the DWARF_UNWIND bit is set and, if not, sets NO_DWARF_UNWIND. Then the `init_unwind_table` called that sets DWARF_UNWIND only if NO_DWARF_UNWIND is not set. So, the resulting `kt->flags` are (DWARF_UNWIND_EH_FRAME | NO_DWARF_UNWIND) and the `x86_64_low_budge_back_trace_cmd` is always called despite having the DWARF tables loaded. 2. When setting DWARF_UNWIND by gdb, the only result that backtrace shows is the address inside the `__schedule` derived via `x86_64_thread_return_init`. This is due to the two things: First, `thread_return` points to the address after `callq __switch_to` and this address is not on the stack of the original thread since the stacks were switched already early. Setting `thread_return` to the address just before stacks are swapped wont help either because of the below. Second, since the Kernel lacks CFI instructions in `context_switch` macro the stack state before the `rsp` swap is slightly different comparing to what is described in the debug info, at exactly two longs because of `pushfq; push %rbp`. Manually adjusting `frame.regs.rsp` at the entry to the `unwind` fixes this, yielding in a correct DWARF_UNWIND output from the `bt` command. 3. I'm asking for an advice on how to fix that. My proposal is to switch `thread_return` to point to the code right before stacks are swapped since it seems that the current implementation wrongly assumes that the `thread_return` value will be on the stack of original thread. This, however, is not required to fix DWARF_UNWIND. Next, the stack should be adjusted right before we enter the `x86_64_dwarf_back_trace_cmd` routine based on what the disassembly of `context_switch` macro. Does this seems correct to you guys? -- Pavel