[v3,3/3] elf: Restore support for _r_debug interpositions and copy relocations

Message ID 0dc61973ff556b5fb8048e952123036ff1088432.1667826585.git.fweimer@redhat.com
State New
Headers
Series Restore support for _r_debug copy relocations & interposition |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Florian Weimer Nov. 7, 2022, 1:11 p.m. UTC
  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.
---
 elf/Makefile               |  9 +++++++
 elf/dl-debug.c             | 52 +++++++++++++++++++++++++++++++++++---
 elf/rtld.c                 |  3 +++
 elf/tst-dlmopen4-nonpic.c  |  2 ++
 elf/tst-dlmopen4-pic.c     |  2 ++
 elf/tst-dlmopen4.c         | 22 ++++++++++++++++
 sysdeps/generic/ldsodefs.h |  4 +++
 7 files changed, 91 insertions(+), 3 deletions(-)
 create mode 100644 elf/tst-dlmopen4-nonpic.c
 create mode 100644 elf/tst-dlmopen4-pic.c
  

Patch

diff --git a/elf/Makefile b/elf/Makefile
index eca7b28ab5..b0c0c4f6ff 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -403,6 +403,8 @@  tests += \
   tst-dlmopen1 \
   tst-dlmopen3 \
   tst-dlmopen4 \
+  tst-dlmopen4-nonpic \
+  tst-dlmopen4-pic \
   tst-dlmopen-dlerror \
   tst-dlmopen-gethostbyname \
   tst-dlmopen-twice \
@@ -1993,6 +1995,13 @@  $(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.c b/elf/dl-debug.c
index 47961236cf..2762e01c16 100644
--- 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 faul because there is a definition in ld.so.  */
+  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);
+  assert (sym != 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 a85daaa732..48003f6b43 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -2332,6 +2332,9 @@  dl_main (const ElfW(Phdr) *phdr,
     /* We must prepare the profiling.  */
     _dl_start_profile ();
 
+  /* Update _r_debug if necessary.  */
+  _dl_debug_post_relocate (main_map);
+
   if ((!was_tls_init_tp_called && GL(dl_tls_max_dtv_idx) > 0)
       || count_modids != _dl_count_modids ())
     ++GL(dl_tls_generation);
diff --git a/elf/tst-dlmopen4-nonpic.c b/elf/tst-dlmopen4-nonpic.c
new file mode 100644
index 0000000000..ad4e409953
--- /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
index 0000000000..919fa85c25
--- /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 8456f89053..6db928950d 100644
--- 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/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index d62998b5ec..f4957fc2b5 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1075,6 +1075,10 @@  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;