Re: RFA symtab: Fix for PR c++/1267 ("next" and shared libraries)

[Jim Blandy <jimb at redhat dot com>]

I think this is a great idea.  How widely have you tested it?

Daniel Jacobowitz <drow@mvista.com> writes:

> This patch fixes c++/1267, a bug where stepping over a function call that
> went through the PLT (as happens when a -fPIC function makes a call to a
> globally visible symbol) would lose control of the inferior.  I'll spare you
> the complete debugging session, as it really doesn't make much sense.  But
> here's the root of the problem:
> 
> When we called frame_pc_unwind on the sentinel frame, we got an address in
> the PLT.  But when we called frame_func_unwind, we got "_init", in ".init",
> which is generally located right before the PLT.  Then, we'd run the
> new-and-improved prologue unwinder on _init, and get some completely bogus
> information, since things weren't actually saved on the stack where it
> thought they were.  This led to the unwound stack pointer being wrong for
> the step_resume breakpoint, so when we hit the step_resume breakpoint we
> kept going.
> 
> I fixed this by changing lookup_minimal_symbol_pc_section to be paranoid
> about returning a minsym in the same section as the PC.  Technically, at
> least on ELF targets, that doesn't _have_ to be true.  I've never
> encountered an exception or a good reason for one, though.  Does anyone see
> any pitfalls for this change?  Symtab maintainers, is this patch OK?
> 
> I believe this patch should also fix shlibs/1237, and may also fix
> shlibs/1280.  Adam, could you check those?
> 
> 
> 
> 
> 
> By the way, I'm convinced that all is not well in step_over_function.  This
> comment,
> 
>   /* NOTE: cagney/2003-04-06:
> 
>      The intent of DEPRECATED_SAVED_PC_AFTER_CALL was to:
> 
>      - provide a very light weight equivalent to frame_unwind_pc()
>      (nee FRAME_SAVED_PC) that avoids the prologue analyzer
> 
>      - avoid handling the case where the PC hasn't been saved in the
>      prologue analyzer
> 
>      Unfortunatly, not five lines further down, is a call to
>      get_frame_id() and that is guarenteed to trigger the prologue
>      analyzer.
> 
> is either incorrect or has gotten out of sync with the code:
> 
>   if (DEPRECATED_SAVED_PC_AFTER_CALL_P ())
>     sr_sal.pc = ADDR_BITS_REMOVE (DEPRECATED_SAVED_PC_AFTER_CALL (get_current_frame ()));
>   else
>     sr_sal.pc = ADDR_BITS_REMOVE (frame_pc_unwind (get_current_frame ()));
>   sr_sal.section = find_pc_overlay (sr_sal.pc);
> 
>   check_for_old_step_resume_breakpoint ();
>   step_resume_breakpoint =
>     set_momentary_breakpoint (sr_sal, get_frame_id (get_current_frame ()),
>                               bp_step_resume);
> 
> 
> Note that get_frame_id unwinds from the NEXT frame, and
> frame_pc_unwind/DEPRECATED_SAVED_PC_AFTER_CALL unwind from THIS frame.
> This throws me a loop every time I have to work in this function.  Also, I
> have the nagging feeling we're saving the wrong frame.  I have an old MIPS
> patch where I needed to use get_prev_frame in step_over_function.  As soon
> as I have time to revisit that patch I'll be back to clean this up some
> more.
> 
> -- 
> Daniel Jacobowitz
> MontaVista Software                         Debian GNU/Linux Developer
> 
> 2003-07-19  Daniel Jacobowitz  <drow@mvista.com>
> 
> 	PR c++/1267
> 	* minsyms.c (lookup_minimal_symbol_by_pc_section): If SECTION is
> 	NULL, default to the section containing PC.
> 
> Index: minsyms.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/minsyms.c,v
> retrieving revision 1.31
> diff -u -p -r1.31 minsyms.c
> --- minsyms.c	15 May 2003 22:23:24 -0000	1.31
> +++ minsyms.c	19 Jul 2003 18:03:08 -0000
> @@ -403,12 +403,22 @@ lookup_minimal_symbol_by_pc_section (COR
>    struct objfile *objfile;
>    struct minimal_symbol *msymbol;
>    struct minimal_symbol *best_symbol = NULL;
> +  struct obj_section *pc_section;
>  
>    /* pc has to be in a known section. This ensures that anything beyond
>       the end of the last segment doesn't appear to be part of the last
>       function in the last segment.  */
> -  if (find_pc_section (pc) == NULL)
> +  pc_section = find_pc_section (pc);
> +  if (pc_section == NULL)
>      return NULL;
> +
> +  /* If no section was specified, then just make sure that the PC is in
> +     the same section as the minimal symbol we find.  */
> +  if (section == NULL)
> +    section = pc_section->the_bfd_section;
> +
> +  /* FIXME drow/2003-07-19: Should we also check that PC is in SECTION
> +     if we were passed a non-NULL SECTION argument?  */
>  
>    for (objfile = object_files;
>         objfile != NULL;