Re: Macro code crasher on re-run

[Daniel Jacobowitz <drow@mvista.com>]

On Sun, Nov 02, 2003 at 11:48:05PM -0500, Jim Blandy wrote:
> 
> Daniel Jacobowitz <drow@mvista.com> writes:
> > There's a bug in default_macro_scope.
> 
> Only one?!?  :)

One at a time, one at a time.

> 
> > 97
> > 98        /* If all else fails, fall back to the current listing position.  */
> > 99        else
> > 100         {
> > 101           /* Don't call select_source_symtab here.  That can raise an
> > 102              error if symbols aren't loaded, but GDB calls the expression
> > 103              evaluator in all sorts of contexts.
> > 104
> > 105              For example, commands like `set width' call the expression
> > 106              evaluator to evaluate their numeric arguments.  If the
> > 107              current language is C, then that may call this function to
> > 108              choose a scope for macro expansion.  If you don't have any
> > 109              symbol files loaded, then get_current_or_default would raise an
> > 110              error.  But `set width' shouldn't raise an error just because
> > 111              it can't decide which scope to macro-expand its argument in.  */
> > 112           struct symtab_and_line cursal = 
> > 113                             get_current_source_symtab_and_line ();
> > 114           
> > 115           sal.symtab = cursal.symtab;
> > 116           sal.line = cursal.line;
> > 117         }
> > 
> > So we initialize just the symtab and line pointers.
> > 118
> > 119       return sal_macro_scope (sal);
> > 120     }
> > 
> > 
> > 39        if (! sal.symtab
> > 40            || ! sal.symtab->macro_table)
> > 41          return 0;
> > 
> > Oops, uninitialized memory read.  That else case can't work; Jim, should we
> > just return 0 from default_macro_scope if the target isn't running, or
> > is there a function I don't see somewhere to find the macrotab and
> > initialize the rest of the symtab?  Should it be "sal = cursal"?
> 
> I don't follow.  All default_macro_scope's callers check for null
> return; it's documented to return zero at times.  So you must be
> talking about that code in sal_macro_scope itself.  Line 39 refers to
> sal.symtab, initialized by line 115, so you must be talking about line
> 40.  But sal.symtab must be non-zero, or else we wouldn't reach the
> right operand of the ||.
> 
> Or should I sleep and try again?
> 

Right.  sal.symtab is nonzero.  sal.symtab->macro_table, however, is
uninitialized.  The short-circuit if on line 40 does not trigger, but
the remainder of sal_macro_scope assumes that sal.symtab->macro_table
is valid.

In my case it happens to be 0xB.


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer