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

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

[Wangbing(wangbing,RTOS/Poincare Lab)]

dlsym with RTLD_DEFAULT add dependency for target symbol, if program 
use dlsym only to detect if a symbol exist, and will not use it.

this operation will make unable to dlclose so file containing target 
symbol, add RTLD_PROBE to support symbol probe.

The reason why not useing struct link_map * as an argument to dlsym is 
that there may be more than one same-named symbol in different so 
files(provided by different vendors or components) in one process, and 
by -Wl,-z,now，only the first symbol is needed, and we cannot be sure 
which struct link_map* should be send to dlsym.


-----邮件原件-----
发件人: H.J. Lu [mailto:hjl.tools@gmail.com] 
发送时间: 2022年11月22日 10:37
收件人: Wangbing(wangbing,RTOS/Poincare Lab) <wangbing6@huawei.com>
抄送: libc-alpha@sourceware.org; szabolcs.nagy@arm.com; Nixiaoming <nixiaoming@huawei.com>; zhongjubin <zhongjubin@huawei.com>; Yanhuijun (DOPRA SSP) <yanhuijun@huawei.com>
主题: Re: [PATCH] dlsym: Add RTLD_PROBE to dlsym only probe symbol without add dependency.

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?

> +      }
> +
>        /* 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.