elf: Support recursive use of dynamic TLS in interposed malloc

It turns out that quite a few applications use bundled mallocs that
have been built to use global-dynamic TLS (instead of the recommended
initial-exec TLS).  The previous workaround from
commit afe42e935b ("elf: Avoid some
free (NULL) calls in _dl_update_slotinfo") does not fix all
encountered cases unfortunatelly.

This change avoids the TLS generation update for recursive use
of TLS from a malloc that was called during a TLS update.  This
is possible because an interposed malloc has a fixed module ID and
TLS slot.  (It cannot be unloaded.)  If an initially-loaded module ID
is encountered in __tls_get_addr and the dynamic linker is already
in the middle of a TLS update, use the outdated DTV, thus avoiding
another call into malloc.  It's still necessary to update the
DTV to the most recent generation, to get out of the slow path,
which is why the check for recursion is needed.

The bookkeeping is done using a global counter instead of per-thread
flag because TLS access in the dynamic linker is tricky.

All this will go away once the dynamic linker stops using malloc
for TLS, likely as part of a change that pre-allocates all TLS
during pthread_create/dlopen.

Fixes commit d2123d6827 ("elf: Fix slow
tls access after dlopen [BZ #19924]").

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

elf/Makefile

@@ -445,6 +445,7 @@ tests += \
   tst-p_align1 \
   tst-p_align2 \
   tst-p_align3 \
+  tst-recursive-tls \
   tst-relsort1 \
   tst-ro-dynamic \
   tst-rtld-run-static \
@@ -892,6 +893,23 @@ modules-names += \
   tst-null-argv-lib \
   tst-p_alignmod-base \
   tst-p_alignmod3 \
+  tst-recursive-tlsmallocmod \
+  tst-recursive-tlsmod0 \
+  tst-recursive-tlsmod1 \
+  tst-recursive-tlsmod2 \
+  tst-recursive-tlsmod3 \
+  tst-recursive-tlsmod4 \
+  tst-recursive-tlsmod5 \
+  tst-recursive-tlsmod6 \
+  tst-recursive-tlsmod7 \
+  tst-recursive-tlsmod8 \
+  tst-recursive-tlsmod9 \
+  tst-recursive-tlsmod10 \
+  tst-recursive-tlsmod11 \
+  tst-recursive-tlsmod12 \
+  tst-recursive-tlsmod13 \
+  tst-recursive-tlsmod14 \
+  tst-recursive-tlsmod15 \
   tst-relsort1mod1 \
   tst-relsort1mod2 \
   tst-ro-dynamic-mod \
@@ -3099,3 +3117,11 @@ CFLAGS-tst-gnu2-tls2mod0.c += -mtls-dialect=$(have-mtls-descriptor)
 CFLAGS-tst-gnu2-tls2mod1.c += -mtls-dialect=$(have-mtls-descriptor)
 CFLAGS-tst-gnu2-tls2mod2.c += -mtls-dialect=$(have-mtls-descriptor)
 endif
+
+$(objpfx)tst-recursive-tls: $(objpfx)tst-recursive-tlsmallocmod.so
+# More objects than DTV_SURPLUS, to trigger DTV reallocation.
+$(objpfx)tst-recursive-tls.out: \
+  $(patsubst %,$(objpfx)tst-recursive-tlsmod%.so, \
+    0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
+$(objpfx)tst-recursive-tlsmod%.os: tst-recursive-tlsmodN.c
+	$(compile-command.c) -DVAR=thread_$* -DFUNC=get_threadvar_$*

elf/dl-tls.c

@@ -75,6 +75,31 @@
 /* Default for dl_tls_static_optional.  */
 #define OPTIONAL_TLS 512
 
+/* Used to count the number of threads currently executing dynamic TLS
+   updates.  Used to avoid recursive malloc calls in __tls_get_addr
+   for an interposed malloc that uses global-dynamic TLS (which is not
+   recommended); see _dl_tls_allocate_active checks.  This could be a
+   per-thread flag, but would need TLS access in the dynamic linker.  */
+unsigned int _dl_tls_threads_in_update;
+
+static inline void
+_dl_tls_allocate_begin (void)
+{
+  atomic_fetch_add_relaxed (&_dl_tls_threads_in_update, 1);
+}
+
+static inline void
+_dl_tls_allocate_end (void)
+{
+  atomic_fetch_add_relaxed (&_dl_tls_threads_in_update, -1);
+}
+
+static inline bool
+_dl_tls_allocate_active (void)
+{
+  return atomic_load_relaxed (&_dl_tls_threads_in_update) > 0;
+}
+
 /* Compute the static TLS surplus based on the namespace count and the
    TLS space that can be used for optimizations.  */
 static inline int
@@ -425,12 +450,18 @@ _dl_allocate_tls_storage (void)
   size += TLS_PRE_TCB_SIZE;
 #endif
 
-  /* Perform the allocation.  Reserve space for the required alignment
-     and the pointer to the original allocation.  */
+  /* Reserve space for the required alignment and the pointer to the
+     original allocation.  */
   size_t alignment = GLRO (dl_tls_static_align);
+
+  /* Perform the allocation.  */
+  _dl_tls_allocate_begin ();
   void *allocated = malloc (size + alignment + sizeof (void *));
   if (__glibc_unlikely (allocated == NULL))
+    {
+      _dl_tls_allocate_end ();
       return NULL;
+    }
 
   /* Perform alignment and allocate the DTV.  */
 #if TLS_TCB_AT_TP
@@ -466,6 +497,8 @@ _dl_allocate_tls_storage (void)
   result = allocate_dtv (result);
   if (result == NULL)
     free (allocated);
+
+  _dl_tls_allocate_end ();
   return result;
 }
 
@@ -483,6 +516,7 @@ _dl_resize_dtv (dtv_t *dtv, size_t max_modid)
   size_t newsize = max_modid + DTV_SURPLUS;
   size_t oldsize = dtv[-1].counter;
 
+  _dl_tls_allocate_begin ();
   if (dtv == GL(dl_initial_dtv))
     {
       /* This is the initial dtv that was either statically allocated in
@@ -502,6 +536,7 @@ _dl_resize_dtv (dtv_t *dtv, size_t max_modid)
       if (newp == NULL)
 	oom ();
     }
+  _dl_tls_allocate_end ();
 
   newp[0].counter = newsize;
 
@@ -676,7 +711,9 @@ allocate_dtv_entry (size_t alignment, size_t size)
   if (powerof2 (alignment) && alignment <= _Alignof (max_align_t))
     {
       /* The alignment is supported by malloc.  */
+      _dl_tls_allocate_begin ();
       void *ptr = malloc (size);
+      _dl_tls_allocate_end ();
       return (struct dtv_pointer) { ptr, ptr };
     }
 
@@ -688,7 +725,10 @@ allocate_dtv_entry (size_t alignment, size_t size)
 
   /* Perform the allocation.  This is the pointer we need to free
      later.  */
+  _dl_tls_allocate_begin ();
   void *start = malloc (alloc_size);
+  _dl_tls_allocate_end ();
+
   if (start == NULL)
     return (struct dtv_pointer) {};
 
@@ -826,7 +866,11 @@ _dl_update_slotinfo (unsigned long int req_modid, size_t new_gen)
 		 free implementation.  Checking here papers over at
 		 least some dynamic TLS usage by interposed mallocs.  */
 	      if (dtv[modid].pointer.to_free != NULL)
+		{
+		  _dl_tls_allocate_begin ();
 		  free (dtv[modid].pointer.to_free);
+		  _dl_tls_allocate_end ();
+		}
 	      dtv[modid].pointer.val = TLS_DTV_UNALLOCATED;
 	      dtv[modid].pointer.to_free = NULL;
 
@@ -955,12 +999,24 @@ __tls_get_addr (GET_ADDR_ARGS)
      user code doing the TLS access so relaxed mo read is enough).  */
   size_t gen = atomic_load_relaxed (&GL(dl_tls_generation));
   if (__glibc_unlikely (dtv[0].counter != gen))
+    {
+      if (_dl_tls_allocate_active ()
+	  && GET_ADDR_MODULE < _dl_tls_initial_modid_limit)
+	  /* This is a reentrant __tls_get_addr call, but we can
+	     satisfy it because it's an initially-loaded module ID.
+	     These TLS slotinfo slots do not change, so the
+	     out-of-date generation counter does not matter.  However,
+	     if not in a TLS update, still update_get_addr below, to
+	     get off the slow path eventually.  */
+	;
+      else
 	{
 	  /* Update DTV up to the global generation, see CONCURRENCY NOTES
 	     in _dl_update_slotinfo.  */
 	  gen = atomic_load_acquire (&GL(dl_tls_generation));
 	  return update_get_addr (GET_ADDR_PARAM, gen);
 	}
+    }
 
   void *p = dtv[GET_ADDR_MODULE].pointer.val;
 
@@ -969,7 +1025,7 @@ __tls_get_addr (GET_ADDR_ARGS)
 
   return (char *) p + GET_ADDR_OFFSET;
 }
-#endif
+#endif /* SHARED */
 
 
 /* Look up the module's TLS block as for __tls_get_addr,
@@ -1018,6 +1074,25 @@ _dl_tls_get_addr_soft (struct link_map *l)
   return data;
 }
 
+size_t _dl_tls_initial_modid_limit;
+
+void
+_dl_tls_initial_modid_limit_setup (void)
+{
+  struct dtv_slotinfo_list *listp = GL(dl_tls_dtv_slotinfo_list);
+  size_t idx;
+  for (idx = 0; idx < listp->len; ++idx)
+    {
+      struct link_map *l = listp->slotinfo[idx].map;
+      if (l == NULL
+	  /* The object can be unloaded, so its modid can be
+	     reassociated.  */
+	  || !(l->l_type == lt_executable || l->l_type == lt_library))
+	break;
+    }
+  _dl_tls_initial_modid_limit = idx;
+}
+
 
 void
 _dl_add_to_slotinfo (struct link_map *l, bool do_add)
@@ -1050,9 +1125,11 @@ _dl_add_to_slotinfo (struct link_map *l, bool do_add)
 	 the first slot.  */
       assert (idx == 0);
 
+      _dl_tls_allocate_begin ();
       listp = (struct dtv_slotinfo_list *)
 	malloc (sizeof (struct dtv_slotinfo_list)
 		+ TLS_SLOTINFO_SURPLUS * sizeof (struct dtv_slotinfo));
+      _dl_tls_allocate_end ();
       if (listp == NULL)
 	{
 	  /* We ran out of memory while resizing the dtv slotinfo list.  */

elf/rtld.c

@@ -788,6 +788,8 @@ init_tls (size_t naudit)
     _dl_fatal_printf ("\
 cannot allocate TLS data structures for initial thread\n");
 
+  _dl_tls_initial_modid_limit_setup ();
+
   /* Store for detection of the special case by __tls_get_addr
      so it knows not to pass this dtv to the normal realloc.  */
   GL(dl_initial_dtv) = GET_DTV (tcbp);

elf/tst-recursive-tls.c

@@ -0,0 +1,60 @@
+/* Test with interposed malloc with dynamic TLS.
+   Copyright (C) 2024 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 <array_length.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+
+/* Defined in tst-recursive-tlsmallocmod.so.  */
+extern __thread unsigned int malloc_subsytem_counter;
+
+static int
+do_test (void)
+{
+  /* 16 is large enough to exercise the DTV resizing case.  */
+  void *handles[16];
+
+  for (unsigned int i = 0; i < array_length (handles); ++i)
+    {
+      /* Re-use the TLS slot for module 0.  */
+      if (i > 0)
+        xdlclose (handles[0]);
+
+      char soname[30];
+      snprintf (soname, sizeof (soname), "tst-recursive-tlsmod%u.so", i);
+      handles[i] = xdlopen (soname, RTLD_NOW);
+
+      if (i > 0)
+        {
+          handles[0] = xdlopen ("tst-recursive-tlsmod0.so", RTLD_NOW);
+          int (*fptr) (void) = xdlsym (handles[0], "get_threadvar_0");
+          /* May trigger TLS storage allocation using malloc.  */
+          TEST_COMPARE (fptr (), 0);
+        }
+    }
+
+  for (unsigned int i = 0; i < array_length (handles); ++i)
+    xdlclose (handles[i]);
+
+  printf ("info: malloc subsystem calls: %u\n", malloc_subsytem_counter);
+  TEST_VERIFY (malloc_subsytem_counter > 0);
+  return 0;
+}
+
+#include <support/test-driver.c>

elf/tst-recursive-tlsmallocmod.c

@@ -0,0 +1,64 @@
+/* Interposed malloc with dynamic TLS.
+   Copyright (C) 2024 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 <stdlib.h>
+#include <dlfcn.h>
+
+__thread unsigned int malloc_subsytem_counter;
+
+static __typeof (malloc) *malloc_fptr;
+static __typeof (free) *free_fptr;
+static __typeof (calloc) *calloc_fptr;
+static __typeof (realloc) *realloc_fptr;
+
+static void __attribute__ ((constructor))
+init (void)
+{
+  malloc_fptr = dlsym (RTLD_NEXT, "malloc");
+  free_fptr = dlsym (RTLD_NEXT, "free");
+  calloc_fptr = dlsym (RTLD_NEXT, "calloc");
+  realloc_fptr = dlsym (RTLD_NEXT, "realloc");
+}
+
+void *
+malloc (size_t size)
+{
+  ++malloc_subsytem_counter;
+  return malloc_fptr (size);
+}
+
+void
+free (void *ptr)
+{
+  ++malloc_subsytem_counter;
+  return free_fptr (ptr);
+}
+
+void *
+calloc (size_t a, size_t b)
+{
+  ++malloc_subsytem_counter;
+  return calloc_fptr (a, b);
+}
+
+void *
+realloc (void *ptr, size_t size)
+{
+  ++malloc_subsytem_counter;
+  return realloc_fptr (ptr, size);
+}

elf/tst-recursive-tlsmodN.c

@@ -0,0 +1,28 @@
+/* Test module with global-dynamic TLS.  Used to trigger DTV reallocation.
+   Copyright (C) 2024 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/>.  */
+
+/* Compiled with VAR and FUNC set via -D.  FUNC requires some
+   relocation against TLS variable VAR.  */
+
+__thread int VAR;
+
+int
+FUNC (void)
+{
+  return VAR;
+}

sysdeps/generic/ldsodefs.h

@@ -1256,6 +1256,20 @@ extern struct link_map *_dl_update_slotinfo (unsigned long int req_modid,
 					     size_t gen)
      attribute_hidden;
 
+/* The last TLS module ID that is initially loaded, plus 1.  TLS
+   addresses for modules with IDs lower than that can be obtained from
+   the DTV even if its generation is outdated.  */
+extern size_t _dl_tls_initial_modid_limit attribute_hidden attribute_relro;
+
+/* Compute _dl_tls_initial_modid_limit.  To be called after initial
+   relocation.  */
+void _dl_tls_initial_modid_limit_setup (void) attribute_hidden;
+
+/* Number of threads currently in a TLS update.  This is used to
+   detect reentrant __tls_get_addr calls without a per-thread
+   flag.  */
+extern unsigned int _dl_tls_threads_in_update attribute_hidden;
+
 /* Look up the module's TLS block as for __tls_get_addr,
    but never touch anything.  Return null if it's not allocated yet.  */
 extern void *_dl_tls_get_addr_soft (struct link_map *l) attribute_hidden;

sysdeps/x86_64/dl-tls.c

@@ -41,7 +41,10 @@ __tls_get_addr_slow (GET_ADDR_ARGS)
   dtv_t *dtv = THREAD_DTV ();
 
   size_t gen = atomic_load_acquire (&GL(dl_tls_generation));
-  if (__glibc_unlikely (dtv[0].counter != gen))
+  if (__glibc_unlikely (dtv[0].counter != gen)
+      /* See comment in __tls_get_addr in elf/dl-tls.c.  */
+      && !(_dl_tls_allocate_active ()
+           && GET_ADDR_MODULE < _dl_tls_initial_modid_limit))
     return update_get_addr (GET_ADDR_PARAM, gen);
 
   return tls_get_addr_tail (GET_ADDR_PARAM, dtv, NULL);