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

[Thomas Schwinge <thomas at codesourcery dot com>]

Hi!

On Sat, 21 Feb 2015 17:22:40 +0100, I wrote:
> On Mon, 10 Jun 2013 14:01:53 +0200 (CEST), Richard Biener <rguenther@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 Âif (ret)Â 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);
>     [...]

By the way, to make progress without having to rebuild Debian's gcc-4.9
package, I had the idea of creating a LD_PRELOAD wrapper to wrap the
fopen (NULL) and unlink (NULL) calls, but this turned out to be an
interesting exercise in its own right: I relatively quickly realized that
I actually need to wrap fopen64 instead of fopen, but it took me longer
to realize why the unlink wrapper just didn't work.  GCC has been happily
optimizing away my path != NULL check, because of:

/usr/include/unistd.h:

    extern int unlink (const char *__name) __THROW __nonnull ((1));

/usr/include/i386-gnu/sys/cdefs.h:

    /* The nonull function attribute allows to mark pointer parameters which
       must not be NULL.  */
    #if __GNUC_PREREQ (3,3)
    # define __nonnull(params) __attribute__ ((__nonnull__ params))
    #else
    # define __nonnull(params)
    #endif

Certainly this is another indication that unlink (NULL) really isn't
meant to be done.  ;-)

Got this resolved by defusing the __nonnull macro.  (See attached.  Not
sure if I'm using the atomic builtins correctly.)


GrÃÃe,
 Thomas


#include <sys/cdefs.h>
#undef __nonnull
#define __nonnull(params)
#include <dlfcn.h>
#include <stdio.h>
#include <unistd.h>

FILE *fopen64(const char *path, const char *mode)
{
  if (path == NULL)
    return NULL;

  static FILE *(*fopen64_)(const char*, const char*) = NULL;
  if (__atomic_load_n (&fopen64_, __ATOMIC_SEQ_CST) == NULL)
    {
      void *fopen64__ = dlsym(RTLD_NEXT, __FUNCTION__);
      __atomic_store_n (&fopen64_, fopen64__, __ATOMIC_SEQ_CST);
    }

  return __atomic_load_n (&fopen64_, __ATOMIC_SEQ_CST)(path, mode);
}

int unlink(const char *path)
{
  if (path == NULL)
    return 0;

  static int (*unlink_)(const char*) = NULL;
  if (__atomic_load_n (&unlink_, __ATOMIC_SEQ_CST) == NULL)
    {
      void *unlink__ = dlsym(RTLD_NEXT, __FUNCTION__);
      __atomic_store_n (&unlink_, unlink__, __ATOMIC_SEQ_CST);
    }

  return __atomic_load_n (&unlink_, __ATOMIC_SEQ_CST)(path);
}

/* gcc -shared -o fopen,unlink-NULL-wrapper.so fopen,unlink-NULL-wrapper.c -Wall -D_GNU_SOURCE -O2 -fPIC -ldl */

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