elf: Add TLS modid reuse test for bug 29039

This is a minimal regression test for bug 29039 which only affects
targets with TLSDESC and a reproducer requires that

1) Have modid gaps (closed modules) with old generation.
2) Update a DTV to a newer generation (needs a newer dlopen).
3) But do not update the closed gap entry in that DTV.
4) Reuse the modid gap for a new module (another dlopen).
5) Use dynamic TLSDESC in that new module with old generation (bug).
6) Access TLS via this TLSDESC and the now outdated DTV.

However step (3) in practice rarely happens: during DTV update the
entries for closed modids are initialized to "unallocated" and then
dynamic TLSDESC calls __tls_get_addr independently of its generation.
The only exception to this is DTV setup at thread creation (gaps are
initialized to NULL instead of unallocated) or DTV resize where the
gap entries are outside the previous DTV array (again NULL instead
of unallocated, and this requires loading > DTV_SURPLUS modules).

So the bug can only cause NULL (+ offset) dereference, not use after
free. And the easiest way to get (3) is via thread creation.

Note that step (5) requires that the newly loaded module has larger
TLS than the remaining optional static TLS. And for (6) there cannot
be other TLS access or dlopen in the thread that updates the DTV.

Tested on aarch64-linux-gnu.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 92 insertions, 0 deletions

diff --git a/elf/tst-tlsgap.c b/elf/tst-tlsgap.c
new file mode 100644
index 0000000..4932885
--- /dev/null
+++ b/elf/tst-tlsgap.c
@@ -0,0 +1,92 @@
+/* TLS modid gap reuse regression test for bug 29039.
+   Copyright (C) 2023 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <dlfcn.h>
+#include <pthread.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+#include <support/check.h>
+
+static void *mod[3];
+#define MOD(i) "tst-tlsgap-mod" #i ".so"
+static const char *modname[3] = { MOD(0), MOD(1), MOD(2) };
+#undef MOD
+
+static void
+open_mod (int i)
+{
+  mod[i] = xdlopen (modname[i], RTLD_LAZY);
+  printf ("open %s\n", modname[i]);
+}
+
+static void
+close_mod (int i)
+{
+  xdlclose (mod[i]);
+  mod[i] = NULL;
+  printf ("close %s\n", modname[i]);
+}
+
+static void
+access_mod (int i, const char *sym)
+{
+  int *(*f) (void) = xdlsym (mod[i], sym);
+  int *p = f ();
+  printf ("access %s: %s() = %p\n", modname[i], sym, p);
+  TEST_VERIFY_EXIT (p != NULL);
+  ++*p;
+}
+
+static void *
+start (void *arg)
+{
+  /* The DTV generation is at the last dlopen of mod0 and the
+     entry for mod1 is NULL.  */
+
+  open_mod (1); /* Reuse modid of mod1. Uses dynamic TLS.  */
+
+  /* DTV is unchanged: dlopen only updates the DTV to the latest
+     generation if static TLS is allocated for a loaded module.
+
+     With bug 29039, the TLSDESC relocation in mod1 uses the old
+     dlclose generation of mod1 instead of the new dlopen one so
+     DTV is not updated on TLS access.  */
+
+  access_mod (1, "f1");
+
+  return arg;
+}
+
+static int
+do_test (void)
+{
+  open_mod (0);
+  open_mod (1);
+  open_mod (2);
+  close_mod (0);
+  close_mod (1); /* Create modid gap at mod1.  */
+  open_mod (0); /* Reuse modid of mod0, bump generation count.  */
+
+  /* Create a thread where DTV of mod1 is NULL.  */
+  pthread_t t = xpthread_create (NULL, start, NULL);
+  xpthread_join (t);
+  return 0;
+}
+
+#include <support/test-driver.c>