[rfa] Attach vsyscall support for GNU/Linux

[Andrew Cagney]

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