Re: [Crash-utility] [BUG?] DWARF unwind is always off (and seems to be broken)
----- Original Message -----
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.
Correct -- DWARF_UNWIND only gets set if the user wants it, so by default it
should never be set. The check in kernel_init() exists to handle the case
where DWARF_UNWIND was explicitly set by a "set unwind on" command
in a .crashrc file.
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.
Right. x86_64_low_budge_back_trace_cmd() is the default backtrace function.
The alternative x86_64_dwarf_back_trace_cmd() backtrace function was added
by IBM back in the 4.0-3.8 timeframe in 2006. It has not been maintained
much since then (if at all), so I don't even know how well it even works.
Personally, I've never utilized it.
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`.
You can enter "set unwind on" during runtime.
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?
As to advice, you're on your own, unless the original author -- or someone
else who actually still uses it -- wants to step up and resurrect support
of the functionality. Feel free to file a patch. All I ask is that any
changes you make do not affect the default functionality.
Thanks,
Dave