Re: [patch] Fix a glitch in debugging 32-bit process with 64-bit GDB.

[Joel Brobecker <brobecker at adacore dot com>]

> > It seems to me that this should work without negative effect
> > (we have to be a little careful because there is a 64bit stabs
> > extension on Tru64).
> 
> AFAICT, Tru64 goes through ECOFF reader in mdebugread.c, and so
> wouldn't be affected by a change to dbxread.c

You're probably right. I'm a little fuzzy now about what each module
does and I'm a little tied up to investigate further. Regardless,
I think that this won't affect Tru64 anyway.

> 2008-12-12  Paul Pluzhnikov  <ppluzhnikov@google.com>
> 
>         * dbxread.c (read_ofile_symtab): Sign-extend 32-bit N_LSYM and
>         N_PSYM STABS values for 64-bit GDB.

This looks reasonable to me, but I'd really like to have a more detailed
comment:

> +	  if (sizeof (nlist.n_value) > 4
> +	      && (type == N_LSYM || type == N_PSYM))
> +	    /* These are very likely to be 32-bit negative.
> +	       Sign-extend them for 64-bit GDB.  */
> +	    nlist.n_value = (nlist.n_value ^ 0x80000000) - 0x80000000;

In particular:

  . Why are we testing the size of nlist.n_value? (to detect that we
    are using a 64bit debugger)
  . You mention that this is very likely to be 32-bit negative, but
    offsets on some platforms may be positive.

I'd like to see the comment detail a little the situation that we are
trying to solve (64bit gdb debugging a 32bit app). Just as a starting
point, I suggest for instance:

  if (sizeof (nlist.n_value) > 4)
    /* We are a 64bit debugger debugging a 32bit program.
       We have to be a little careful with the n_value in the case
       of N_LSYM and N_PSYM entries, because they are signed offsets
       that we actually read as an unsigned 32bit value. This is
       not a problem with 32bit debuggers where negative values end
       up being interpreted correctly.  But we need to sign-extend
       the sign bit on 64bit debuggers.  Otherwise, we'll end up
       interpreting negative values as very large positive values...  */
    if (type == N_LSYM || type == N_PSYM)
      nlist.n_value = (nlist.n_value ^ 0x80000000) - 0x80000000;

-- 
Joel