Re: [PATCH] dlsym: Add RTLD_PROBE to dlsym only probe symbol without add dependency.

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Nov 21, 2022 at 6:36 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Nov 21, 2022 at 6:31 PM Wang Bing via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > Signed-off-by: Wang Bing <wangbing6@huawei.com>
> > ---
> >  dlfcn/dlfcn.h              |  4 ++++
> >  elf/Makefile               |  1 +
> >  elf/dl-sym.c               | 13 +++++++++---
> >  elf/tst-dlsym-rtld-probe.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 65 insertions(+), 3 deletions(-)
> >  create mode 100644 elf/tst-dlsym-rtld-probe.c
> >
> > diff --git a/dlfcn/dlfcn.h b/dlfcn/dlfcn.h
> > index 6f7cad8682..ab709883a6 100644
> > --- a/dlfcn/dlfcn.h
> > +++ b/dlfcn/dlfcn.h
> > @@ -49,6 +49,10 @@ typedef long int Lmid_t;
> >     is returned.  */
> >  #define RTLD_DEFAULT   ((void *) 0)
> >
> > +/* If only find sym in the global scope, but will not use it, do not
> > +   set sym dependency. */
> > +# define RTLD_PROBE    ((void *) -2l)
> > +
> >  __BEGIN_DECLS
> >
> >  /* Open the shared object FILE and map it in; return a handle that can be
> > diff --git a/elf/Makefile b/elf/Makefile
> > index eca7b28ab5..f9fc9fbebb 100644
> > --- a/elf/Makefile
> > +++ b/elf/Makefile
> > @@ -472,6 +472,7 @@ tests += \
> >    unload7 \
> >    unload8 \
> >    valgrind-test \
> > +  tst-dlsym-rtld-probe \
> >    # tests
> >  tests-cxx = \
> >    tst-dlopen-nodelete-reloc \
> > diff --git a/elf/dl-sym.c b/elf/dl-sym.c
> > index b1cf42f36d..bc95b12a19 100644
> > --- a/elf/dl-sym.c
> > +++ b/elf/dl-sym.c
> > @@ -92,10 +92,17 @@ do_sym (void *handle, const char *name, void *who,
> >    /* Link map of the caller if needed.  */
> >    struct link_map *match = NULL;
> >
> > -  if (handle == RTLD_DEFAULT)
> > +  int def_flags;
> > +
> > +  if (handle == RTLD_DEFAULT || handle == RTLD_PROBE)
> >      {
> >        match = _dl_sym_find_caller_link_map (caller);
> >
> > +      def_flags = flags
> > +      if (def_flags == RTLD_DEFAULT) {
> > +          def_flags != DL_LOOKUP_ADD_DEPENDENCY;
>
> What does it do?

Did you mean

def_flags |= DL_LOOKUP_ADD_DEPENDENCY;


> > +      }
> > +
> >        /* Search the global scope.  We have the simple case where
> >          we look up in the scope of an object which was part of
> >          the initial binary.  And then the more complex part
> > @@ -104,7 +111,7 @@ do_sym (void *handle, const char *name, void *who,
> >        if (RTLD_SINGLE_THREAD_P)
> >         result = GLRO(dl_lookup_symbol_x) (name, match, &ref,
> >                                            match->l_scope, vers, 0,
> > -                                          flags | DL_LOOKUP_ADD_DEPENDENCY,
> > +                                          def_flags,
> >                                            NULL);
> >        else
> >         {
> > @@ -113,7 +120,7 @@ do_sym (void *handle, const char *name, void *who,
> >           args.map = match;
> >           args.vers = vers;
> >           args.flags
> > -           = flags | DL_LOOKUP_ADD_DEPENDENCY | DL_LOOKUP_GSCOPE_LOCK;
> > +           = def_flags | DL_LOOKUP_GSCOPE_LOCK;
> >           args.refp = &ref;
> >
> >           THREAD_GSCOPE_SET_FLAG ();
> > diff --git a/elf/tst-dlsym-rtld-probe.c b/elf/tst-dlsym-rtld-probe.c
> > new file mode 100644
> > index 0000000000..c72ceaa182
> > --- /dev/null
> > +++ b/elf/tst-dlsym-rtld-probe.c
> > @@ -0,0 +1,50 @@
> > +/* Test RTLD_PROBE for dlsym.
> > +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> > +   This file is part of the GNU C Library.
> > +
> > +   The GNU C Library is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU Lesser General Public
> > +   License as published by the Free Software Foundation; either
> > +   version 2.1 of the License, or (at your option) any later version.
> > +
> > +   The GNU C Library is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > +   Lesser General Public License for more details.
> > +
> > +   You should have received a copy of the GNU Lesser General Public
> > +   License along with the GNU C Library; if not, see
> > +   <https://www.gnu.org/licenses/>.  */
> > +
> > +#include <dlfcn.h>
> > +#include <gnu/lib-names.h>
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <string.h>
> > +#include <support/check.h>
> > +#include <support/xdlfcn.h>
> > +
> > +static int
> > +do_test (void)
> > +{
> > +  int *iptr;
> > +  int ret;
> > +  void *handle;
> > +
> > +  handle = dlopen (LIBM_SO, RTLD_LAZY);
> > +  TEST_VERIFY (handle == NULL);
> > +  iptr = (int *)dlsym (RTLD_PROBE, "finite");  // get sym but not call --detect if symbol exist
> > +  ret = dlclose (handle);
> > +  TEST_VERIFY (ret != 0);
> > +  ret = 0;
> > +
> > +  handle = dlopen (LIBM_SO, RTLD_LAZY);
> > +  TEST_VERIFY (handle == NULL);
> > +  iptr = (int *)dlsym (RTLD_DEFAULT, "finite"); // get sym and keep
> > +  ret = dlcose (handle);
> > +  TEST_VERIFY (ret == 0);
> > +  return 0;
> > +}
> > +
> > +
> > +#include <support/test-driver.c>
> > --
> > 2.12.3
> >
>
>
> --
> H.J.



-- 
H.J.