Re: Define __start/__stop symbols when there is only a dynamic def

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

On Mon, Jan 29, 2018 at 10:28 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 29 Jan 2018, H.J. Lu wrote:
>
>> > % cat app.c
>> > #define _GNU_SOURCE
>> > #include <stdlib.h>
>> > #include <stdio.h>
>> > #include <dlfcn.h>
>> >
>> > extern int __start___verbose[];
>> > extern int __stop___verbose[];
>> > int bar (void)
>> > {
>> >   static int my_var __attribute__((section("__verbose"))) = 6;
>> >   int *ptr;
>> >   ptr = (int*) dlsym(RTLD_DEFAULT, "__start___verbose");
>> >   if (!ptr || *ptr != 6)
>> >     return -1;
>> >   return 0;
>> > }
>> >
>> > int main()
>> > {
>> >   if (bar () != 0)
>> >     {
>> >       printf("main: wrong __start___verbose\n");
>> >       exit(2);
>> >     }
>> >   return 0;
>> > }
>>
>> Just to verify.  This is the correct testcase:
>>
>> [hjl@gnu-6 orphan-4]$ cat app.c
>> #define _GNU_SOURCE
>> #include <stdio.h>
>> #include <dlfcn.h>
>>
>> extern int __start___verbose[];
>> extern int __stop___verbose[];
>> int bar (void)
>> {
>>   static int my_var __attribute__((section("__verbose"))) = 6;
>>   int *ptr;
>>   ptr = (int*) dlsym(RTLD_DEFAULT, "__start___verbose");
>>   if (!ptr || *ptr != 6)
>>     return -1;
>>   return 0;
>> }
>>
>> int main()
>> {
>>   if (bar () != 0)
>>     {
>>       printf("main: wrong __start___verbose\n");
>>       return 2;
>>     }
>>   return 0;
>> }
>> [hjl@gnu-6 orphan-4]$ make
>> cc -g   -c -o app.o app.c
>> cc -B./ -o app app.o -ldl -Wl,-R,.
>> ./app
>> main: wrong __start___verbose
>> make: *** [Makefile:6: all] Error 2
>> [hjl@gnu-6 orphan-4]$
>
> Yes, that's it.  (the associated lib.c needs to have a regular ref symbol
> to __start___symbol and that lib needs to have something else than "6" in
> its __verbose section, in order to test the exact situation of pacemaker).
> With the patch from Alan ./app will have __start___verbose in its symbol
> table, but not yet in the dynamic symbol table.  So with only Alans patch
> -E would be necessary.  With the addition of the
> bfd_elf_link_record_dynamic_symbol() call to bfd_elf_define_start_stop the
> above testcase will start working as is even without -E.

I don't think Alan's patch nor yours are correct.   Here is mine with your
testcase.   OK for master?

Thanks.

-- 
H.J.

From 64a0765807ad0cbd178b5b1aade1ab1f20e2be32 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Mon, 29 Jan 2018 13:07:34 -0800
Subject: [PATCH] ELF: Always make __start and __stop symbols dynamic

This patch always makes __start and __stop symbols dynamic to support
dlsym on __start and __stop symbols when dynamic sections are created.

NB: --gc-sections will remove __start and __stop symbols in the
executable if they aren't referenced within the executable.

bfd/

	PR ld/21964
	* elflink.c (bfd_elf_define_start_stop): Always make __start and
	__stop symbols dynamic when dynamic sections are created.

ld/

	PR ld/21964
	* testsuite/ld-elf/pr21964-3.c: New file.
	* testsuite/ld-elf/shared.exp: Run pr21964-3 test on Linux.
	* testsuite/ld-x86-64/pr18176.d: Updated.
---
 bfd/elflink.c                    | 28 +++++++++++++++++++++++++---
 ld/testsuite/ld-elf/pr21964-3.c  | 23 +++++++++++++++++++++++
 ld/testsuite/ld-elf/shared.exp   | 12 ++++++++++++
 ld/testsuite/ld-x86-64/pr18176.d |  2 +-
 4 files changed, 61 insertions(+), 4 deletions(-)
 create mode 100644 ld/testsuite/ld-elf/pr21964-3.c

diff --git a/bfd/elflink.c b/bfd/elflink.c
index e81f6c6d82..0ece18094f 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -14332,13 +14332,32 @@ bfd_elf_define_start_stop (struct bfd_link_info *info,
 			   const char *symbol, asection *sec)
 {
   struct elf_link_hash_entry *h;
+  struct elf_link_hash_table *htab = elf_hash_table (info);
+  /* Always make __start and __stop symbols dynamic to support dlsym
+     on __start and __stop symbols when dynamic sections are created.
+
+     NB: --gc-sections will remove __start and __stop symbols in the
+     executable if they aren't referenced within the executable.  */
+  bfd_boolean create_p
+    = (htab != NULL
+       && htab->dynamic_sections_created
+       && elf_section_data (sec)->this_hdr.bfd_section != NULL
+       && symbol[0] != '.');
 
   h = elf_link_hash_lookup (elf_hash_table (info), symbol,
 			    FALSE, FALSE, TRUE);
+  if (h == NULL && create_p)
+    {
+      /* lang_init_start_stop will free SYMBOL later.  */
+      symbol = xstrdup (symbol);
+      h = elf_link_hash_lookup (elf_hash_table (info), symbol,
+				TRUE, FALSE, TRUE);
+    }
   if (h != NULL
-      && (h->root.type == bfd_link_hash_undefined
+      && (create_p
+	  || h->root.type == bfd_link_hash_undefined
 	  || h->root.type == bfd_link_hash_undefweak
-	  || ((h->ref_regular || h->def_dynamic) && !h->def_regular)))
+	  || (h->ref_regular && !h->def_regular)))
     {
       h->root.type = bfd_link_hash_defined;
       h->root.u.def.section = sec;
@@ -14355,7 +14374,10 @@ bfd_elf_define_start_stop (struct bfd_link_info *info,
 	  (*bed->elf_backend_hide_symbol) (info, h, TRUE);
 	}
       else if (ELF_ST_VISIBILITY (h->other) == STV_DEFAULT)
-	h->other = (h->other & ~ELF_ST_VISIBILITY (-1)) | STV_PROTECTED;
+	{
+	  bfd_elf_link_record_dynamic_symbol (info, h);
+	  h->other = (h->other & ~ELF_ST_VISIBILITY (-1)) | STV_PROTECTED;
+	}
       return &h->root;
     }
   return NULL;
diff --git a/ld/testsuite/ld-elf/pr21964-3.c b/ld/testsuite/ld-elf/pr21964-3.c
new file mode 100644
index 0000000000..424c3e50ea
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr21964-3.c
@@ -0,0 +1,23 @@
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <stdio.h>
+
+extern int __start___verbose[];
+extern int __stop___verbose[];
+int bar (void)
+{
+  static int my_var __attribute__ ((section("__verbose"), used)) = 6;
+  int *ptr;
+  ptr = (int*) dlsym (RTLD_DEFAULT, "__start___verbose");
+  if (!ptr || *ptr != 6)
+    return -1;
+  return 0;
+}
+
+int
+main ()
+{
+  if (bar () == 0)
+    printf ("PASS\n");
+  return 0;
+}
diff --git a/ld/testsuite/ld-elf/shared.exp b/ld/testsuite/ld-elf/shared.exp
index 5c4d78aa2d..3eafd9846e 100644
--- a/ld/testsuite/ld-elf/shared.exp
+++ b/ld/testsuite/ld-elf/shared.exp
@@ -1077,6 +1077,18 @@ if { [istarget *-*-linux*]
 	    "pr22393-2-static" \
 	    "pass.out" \
 	] \
+	[list \
+	    "Run pr21964-3" \
+	    "" \
+	    "" \
+	    {pr21964-3.c} \
+	    "pr21964-3" \
+	    "pass.out" \
+	    "" \
+	    "" \
+	    "" \
+	    "-ldl" \
+	] \
     ]
 }
 
diff --git a/ld/testsuite/ld-x86-64/pr18176.d b/ld/testsuite/ld-x86-64/pr18176.d
index 4e3ad9ff08..4ff07bc23e 100644
--- a/ld/testsuite/ld-x86-64/pr18176.d
+++ b/ld/testsuite/ld-x86-64/pr18176.d
@@ -5,5 +5,5 @@
 #target: x86_64-*-linux*
 
 #...
-  GNU_RELRO      0x04bd17 0x000000000024bd17 0x000000000024bd17 0x0022e9 0x0022e9 R   0x1
+  GNU_RELRO      0x04cd17 0x000000000024cd17 0x000000000024cd17 0x0022e9 0x0022e9 R   0x1
 #pass
-- 
2.14.3