elf: Restore support for _r_debug interpositions and copy relocations

The changes in commit a93d9e03a31ec14405cb3a09aa95413b67067380
("Extend struct r_debug to support multiple namespaces [BZ #15971]")
break the dyninst dynamic instrumentation tool.  It brings its
own definition of _r_debug (rather than a declaration).

Furthermore, it turns out it is rather hard to use the proposed
handshake for accessing _r_debug via DT_DEBUG. If applications want
to access _r_debug, they can do so directly if the relevant code has
been built as PIC.  To protect against harm from accidental copy
relocations due to linker relaxations, this commit restores copy
relocation support by adjusting both copies if interposition or
copy relocations are in play.  Therefore, it is possible to
use a hidden reference in ld.so to access _r_debug.

Only perform the copy relocation initialization if libc has been
loaded.  Otherwise, the ld.so search scope can be empty, and the
lookup of the _r_debug symbol mail fail.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

diff --git a/elf/Makefile b/elf/Makefile
index 8a31a291914d26aa0a07b5102d7df8c84bacbc51..44b9f192f22be6d03e3cffda21d21816ce92b6e1 100644 (file)
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -419,6 +419,8 @@ tests += \
   tst-dlmopen1 \
   tst-dlmopen3 \
   tst-dlmopen4 \
+  tst-dlmopen4-nonpic \
+  tst-dlmopen4-pic \
   tst-dlopen-auditdup \
   tst-dlopen-constructor-null \
   tst-dlopen-self \
@@ -2258,6 +2260,13 @@ $(objpfx)tst-dlmopen3.out: $(objpfx)tst-dlmopen1mod.so
 
 $(objpfx)tst-dlmopen4.out: $(objpfx)tst-dlmopen1mod.so
 
+CFLAGS-tst-dlmopen4-pic.c += -fPIC
+$(objpfx)tst-dlmopen4-pic.out: $(objpfx)tst-dlmopen1mod.so
+
+CFLAGS-tst-dlmopen4-nonpic.c += -fno-pie
+tst-dlmopen4-nonpic-no-pie = yes
+$(objpfx)tst-dlmopen4-nonpic.out: $(objpfx)tst-dlmopen1mod.so
+
 $(objpfx)tst-audit1.out: $(objpfx)tst-auditmod1.so
 tst-audit1-ENV = LD_AUDIT=$(objpfx)tst-auditmod1.so
 

diff --git a/elf/dl-debug-symbols.S b/elf/dl-debug-symbols.S
index 7bcb035826a8fca023737e6ddececa4ab122f310..d789f4e35a2b6852407b9764f4a679aa6f083369 100644 (file)
--- a/elf/dl-debug-symbols.S
+++ b/elf/dl-debug-symbols.S
@@ -38,3 +38,4 @@
 _r_debug:
 _r_debug_extended:
        .zero   R_DEBUG_EXTENDED_SIZE
+rtld_hidden_def (_r_debug)

diff --git a/elf/dl-debug.c b/elf/dl-debug.c
index b033723679ab3b3de8a579695f98058c17e260e3..38a5b9a718fd5dd2a5538aceb3608f1840baf937 100644 (file)
--- a/elf/dl-debug.c
+++ b/elf/dl-debug.c
@@ -16,6 +16,7 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <assert.h>
 #include <ldsodefs.h>
 
 
@@ -37,6 +38,37 @@ extern const int verify_link_map_members[(VERIFY_MEMBER (l_addr)
    to LM_ID_BASE + 1.  See elf/dl-debug-symbols.S.  */
 struct r_debug_extended _r_debug_array[DL_NNS - 1];
 
+/* If not null, pointer to the _r_debug in the main executable.  */
+static struct r_debug *_r_debug_main;
+
+void
+_dl_debug_post_relocate (struct link_map *main_map)
+{
+  /* Perform a full symbol search in all objects, to maintain
+     compatibility if interposed _r_debug definitions.  The lookup
+     cannot fail because there is a definition in ld.so, and this
+     function is only called if the ld.so search scope is not empty.  */
+  const ElfW(Sym) *sym = NULL;
+  lookup_t result =_dl_lookup_symbol_x ("_r_debug", main_map, &sym,
+                                       main_map->l_scope, NULL, 0, 0, NULL);
+  if (sym->st_size >= sizeof (struct r_debug))
+    {
+      struct r_debug *main_r_debug = DL_SYMBOL_ADDRESS (result, sym);
+      if (main_r_debug != &_r_debug_extended.base)
+       {
+         /* The extended version of the struct is not available in
+            the main executable because a copy relocation has been
+            used.  r_map etc. have already been copied as part of the
+            copy relocation processing.  */
+         main_r_debug->r_version = 1;
+
+          /* Record that dual updates of the initial link map are
+             required.  */
+          _r_debug_main = main_r_debug;
+       }
+    }
+}
+
 /* Return the r_debug object for the namespace NS.  */
 static inline struct r_debug_extended *
 get_rdebug (Lmid_t ns)
@@ -71,6 +103,11 @@ void
 _dl_debug_change_state (struct r_debug *r, int state)
 {
   atomic_store_release (&r->r_state, state);
+#ifdef SHARED
+  if (r == &_r_debug_extended.base && _r_debug_main != NULL)
+    /* Update the copy-relocation of _r_debug.  */
+    atomic_store_release (&_r_debug_main->r_state, state);
+#endif
   _dl_debug_state ();
 }
 
@@ -103,7 +140,9 @@ _dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
          if (ns - 1 == LM_ID_BASE)
            {
              atomic_store_release (&_r_debug_extended.r_next, r);
-             /* Now there are multiple namespaces.  */
+             /* Now there are multiple namespaces.  Note that this
+                deliberately does not update the copy in the main
+                executable (if it exists).  */
              atomic_store_release (&_r_debug_extended.base.r_version, 2);
            }
          else
@@ -116,8 +155,15 @@ _dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
     }
 
   if (r->base.r_map == NULL)
-    atomic_store_release (&r->base.r_map,
-                         (void *) GL(dl_ns)[ns]._ns_loaded);
+    {
+      struct link_map_public *l = (void *) GL(dl_ns)[ns]._ns_loaded;
+      atomic_store_release (&r->base.r_map, l);
+#ifdef SHARED
+      if (ns == LM_ID_BASE && _r_debug_main != NULL)
+       /* Update the copy-relocation of _r_debug.  */
+       atomic_store_release (&_r_debug_main->r_map, l);
+#endif
+    }
 
   return &r->base;
 }

diff --git a/elf/rtld.c b/elf/rtld.c
index eec11bf79025740548b95e3fcc40f714c2dc2f19..f889f01b4a4b4abadb01081782725b16d58ebf03 100644 (file)
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -2313,6 +2313,9 @@ dl_main (const ElfW(Phdr) *phdr,
 
       __rtld_mutex_init ();
       __rtld_malloc_init_real (main_map);
+
+      /* Update copy-relocated _r_debug if necessary.  */
+      _dl_debug_post_relocate (main_map);
     }
 
   /* All ld.so initialization is complete.  Apply RELRO.  */

diff --git a/elf/tst-dlmopen4-nonpic.c b/elf/tst-dlmopen4-nonpic.c
new file mode 100644 (file)
index 0000000..ad4e409
--- /dev/null
+++ b/elf/tst-dlmopen4-nonpic.c
@@ -0,0 +1,2 @@
+#define BUILD_FOR_NONPIC
+#include "tst-dlmopen4.c"

diff --git a/elf/tst-dlmopen4-pic.c b/elf/tst-dlmopen4-pic.c
new file mode 100644 (file)
index 0000000..919fa85
--- /dev/null
+++ b/elf/tst-dlmopen4-pic.c
@@ -0,0 +1,2 @@
+#define BUILD_FOR_PIC
+#include "tst-dlmopen4.c"

diff --git a/elf/tst-dlmopen4.c b/elf/tst-dlmopen4.c
index 64e007e3dcbe8f25c96f92a0c500f78fcc7ed638..5cda024fdf2bd64dfcfb1b484ae31751813ea835 100644 (file)
--- a/elf/tst-dlmopen4.c
+++ b/elf/tst-dlmopen4.c
@@ -46,6 +46,15 @@ do_test (void)
   TEST_COMPARE (debug->base.r_version, 1);
   TEST_VERIFY_EXIT (debug->r_next == NULL);
 
+#ifdef BUILD_FOR_PIC
+  /* In a PIC build, using _r_debug directly should give us the same
+     object.  */
+  TEST_VERIFY (&_r_debug == &debug->base);
+#endif
+#ifdef BUILD_FOR_NONPIC
+  TEST_COMPARE (_r_debug.r_version, 1);
+#endif
+
   void *h = xdlmopen (LM_ID_NEWLM, "$ORIGIN/tst-dlmopen1mod.so",
                      RTLD_LAZY);
 
@@ -57,6 +66,19 @@ do_test (void)
   const char *name = basename (debug->r_next->base.r_map->l_name);
   TEST_COMPARE_STRING (name, "tst-dlmopen1mod.so");
 
+#ifdef BUILD_FOR_NONPIC
+  /* If a copy relocation is used, it must be at version 1.  */
+  if (&_r_debug != &debug->base)
+    {
+      TEST_COMPARE (_r_debug.r_version, 1);
+      TEST_COMPARE ((uintptr_t) _r_debug.r_map,
+                   (uintptr_t) debug->base.r_map);
+      TEST_COMPARE (_r_debug.r_brk, debug->base.r_brk);
+      TEST_COMPARE (_r_debug.r_state, debug->base.r_state);
+      TEST_COMPARE (_r_debug.r_ldbase, debug->base.r_ldbase);
+    }
+#endif
+
   xdlclose (h);
 
   return 0;

diff --git a/include/link.h b/include/link.h
index 518bfd1670a98027ef871c3f1a8dc2006e452632..41e5e54a7b150c673432c7bb809dd3e8d563035f 100644 (file)
--- a/include/link.h
+++ b/include/link.h
@@ -365,6 +365,8 @@ struct auditstate
    dynamic linker.  */
 extern struct r_debug_extended _r_debug_extended attribute_hidden;
 
+rtld_hidden_proto (_r_debug)
+
 #if __ELF_NATIVE_CLASS == 32
 # define symbind symbind32
 # define LA_SYMBIND "la_symbind32"

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 74931fec25437a805baa57382c3a1f3ce660e1d7..74025f1f93ff934ee3eaae8e87e4d509f76f30d1 100644 (file)
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1098,6 +1098,10 @@ rtld_hidden_proto (_dl_debug_state)
 extern struct r_debug *_dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
      attribute_hidden;
 
+/* This is called after relocation processing to handle a potential
+   copy relocation for _r_debug.  */
+void _dl_debug_post_relocate (struct link_map *main_map) attribute_hidden;
+
 /* Update the `r_map' member and return the address of `struct r_debug'
    of the namespace NS.  */
 extern struct r_debug *_dl_debug_update (Lmid_t ns) attribute_hidden;