Re: [PATCH] Do not redirect ld stdout/stderr in collect2 with -debug

[Thomas Schwinge]

Hi Richard!

On Mon, 10 Jun 2013 14:01:53 +0200 (CEST), Richard Biener <rguent...@suse.de> 
wrote:
> This fixes one very annoying thing collect2 does when trying to
> debug LTO WPA issues.  Even with -v you need to wait until all
> LTRANS stages completed to see the lto1 -fwpa invocation which
> is because collect2 buffers and replays stdout/stderr of ld
> (to avoid duplicating that in some cases).  But I really want
> to see the output immediately but there is no way to do that.
> The easiest is to disable the buffering with -debug (that is,
> -Wl,-debug to the -flto driver command line).
> 
> Tested with/without -debug.
> 
> Ok for trunk?

>       * collect2.c (main): Do not redirect ld stdout/stderr when
>       debugging.

> --- gcc/collect2.c    (revision 199732)
> +++ gcc/collect2.c    (working copy)
> @@ -1189,8 +1189,11 @@ main (int argc, char **argv)
>  #ifdef COLLECT_EXPORT_LIST
>    export_file = make_temp_file (".x");
>  #endif
> -  ldout = make_temp_file (".ld");
> -  lderrout = make_temp_file (".le");
> +  if (!debug)
> +    {
> +      ldout = make_temp_file (".ld");
> +      lderrout = make_temp_file (".le");
> +    }
>    *c_ptr++ = c_file_name;
>    *c_ptr++ = "-x";
>    *c_ptr++ = "c";

This change (r199936) is problematic, given the usage of ldout and
lderrout in gcc/tlink.c:do_tlink.  If, for -debug, they're not
initialized, they'll be NULL, which in do_tlink, the tlink_execute call
will handle fine (as I understand it), but after that, for example,
dump_ld_file will attempt to fopen (NULL), which will cause a SIGSEGV on
GNU Hurd at least.  (Correct me if I'm wrong -- I have not yet read the
relevant standards in detail -- but from what I remember, that's
appropriate: NULL is not a string naming a file.)

I found this when running binutils' gold testsuite:

    $ gcc-4.9 -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow 
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -fmerge-constants -g -O2 
-fno-use-linker-plugin -o incremental_test -Bgcctestdir/ -Wl,--incremental-full 
incremental_test_1.o incremental_test_2.o -v -Wl,-debug
    [...]
    gcc-4.9: internal compiler error: Segmentation fault (program collect2)
    [...]

In do_tlink, the last four uses of ldout and lderrout should be guarded
by NULL and empty string checks (as done in gcc/collect2.c:tool_cleanup),
but I'm not sure what the correct fix is in the »if (ret)« block:

    void
    do_tlink (char **ld_argv, char **object_lst ATTRIBUTE_UNUSED)
    {
      int ret = tlink_execute ("ld", ld_argv, ldout, lderrout,
                               HAVE_GNU_LD && at_file_supplied);
    
      tlink_init ();
    
      if (ret)
        {
          int i = 0;
    
          /* Until collect does a better job of figuring out which are object
             files, assume that everything on the command line could be.  */
          if (read_repo_files (ld_argv))
            while (ret && i++ < MAX_ITERATIONS)
              {
                if (tlink_verbose >= 3)
                  {
                    dump_ld_file (ldout, stdout);
                    dump_ld_file (lderrout, stderr);
                  }
                demangle_new_symbols ();
                if (! scan_linker_output (ldout)
                    && ! scan_linker_output (lderrout))
                  break;
                if (! recompile_files ())
                  break;
                if (tlink_verbose)
                  fprintf (stderr, _("collect: relinking\n"));
                ret = tlink_execute ("ld", ld_argv, ldout, lderrout,
                                     HAVE_GNU_LD && at_file_supplied);
              }
        }
    
      dump_ld_file (ldout, stdout);
      unlink (ldout);
      dump_ld_file (lderrout, stderr);
      unlink (lderrout);
    [...]


Grüße,
 Thomas

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