Re: [rfa] Attach vsyscall support for GNU/Linux

[Andrew Cagney <cagney at gnu dot org>]

Mark Kettenis wrote:
>    Date: Sun, 24 Oct 2004 19:16:36 -0400
>    From: Daniel Jacobowitz <drow@false.org>
>
> Thanks for the explanation Daniel!
>
>    Unwinding bugs aside, I think it's valuable for GDB to know that it is
>    at a signal trampoline.  I think the custom display in backtrace is
>    valuable.  That means we should look for signal handlers before looking
>    for CFI.  For NPTL that value judgement would fall the other way - not
>    having to special-case signal handlers is a clear win.
>
> Given the fact that it is desirable for GDB to know that it's dealing
> with a signal handler, I think the correct approach is to extend the
> DWARF2 unwinder with a method to get the frame type, similar to what
> we already do for pre-initializing the register state.

The ``correct approach'' is both more complicated and simplier (the 
complexity comes with the amount of refactoring, the simplicity from the 
result):

At present we've the relationship:

   FRAME <>---- UNWINDER
     <>
      |
   FUNCTION ---> SYMBOL

where the UNWINDER is providing attributes and methods such as:

  - unwind register
  - callee frame?

while the FUNCTION provides attributes such as:

  - code start/range
  - function name

It needs to be changed so more like:

                   SYMBOL
                    /|\
                     |
   FRAME <>----- FUNCTION <>------ UNWINDER

and have the FUNCTION provide:

  - signal trampoline?
  - code start/range
  - function name

and the FUNCTION's unwinder just supply registers.  For the case being 
discussed, this would let us equally implement:

    FRAME <>----- SIGTRAMP-FUNCTION <>----- DWARF2-UNWINDER

    FRAME <>----- SIGTRAMP-FUNCTION <>----- SIGTRAMP-UNWINDER

    FRAME <>----- SIGTRAMP-FUNCTION <>----- UNWINDER-PROXY

(for the last case, the actual unwinder being selected if/when needed).

On the down side, this means replacing the frame identification 
heuristic found in frame-unwind with robust logic in the symtab, and 
expanding symbol.

On the upside, this means that the function symbol starts correctly 
identifying these probed frames (at present the're all b-).

Andrew