ld.so: Introduce delayed relocation processingfw/bug21242

This makes it possible to use IFUNC resolvers which depend
on relocations themselves, as long as these reloctions do
not depend on IFUNCs.

So far, delayed relocation processing is only implemented for
x86-64.

1 files changed, 247 insertions, 0 deletions

diff --git a/elf/dl-delayed-reloc.c b/elf/dl-delayed-reloc.c
new file mode 100644
index 0000000000..39c864fc64
--- /dev/null
+++ b/elf/dl-delayed-reloc.c
@@ -0,0 +1,247 @@
+/* Delayed relocation processing.
+   Copyright (C) 2018 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/>.  */
+
+#if HAVE_IFUNC
+
+# include <assert.h>
+# include <dl-delayed-reloc.h>
+# include <errno.h>
+# include <ldsodefs.h>
+# include <sys/mman.h>
+# include <unistd.h>
+
+/* Machine-specific definitions.  */
+# include <dl-delayed-reloc-machine.h>
+
+/* This struct covers a whole page containing individual struct
+   dl_delayed_reloc elements, which are allocated individually by
+   allocate_reloc below.  */
+struct dl_delayed_reloc_array
+{
+  struct dl_delayed_reloc_array *next;
+  struct dl_delayed_reloc data[];
+};
+
+/* Pointer to global state.  We use this indirection so that we do not
+   have to add the entire struct to the BSS segment.  */
+static struct dl_delayed_reloc_global *global;
+
+/* Allocate a new struct dl_delayed_reloc_array object.  Update global
+   *accordingly.  */
+static void
+allocate_array (void)
+{
+  size_t page_size = GLRO(dl_pagesize);
+  void *ptr = __mmap (NULL, page_size, PROT_READ | PROT_WRITE,
+                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (ptr == MAP_FAILED)
+    _dl_signal_error (ENOMEM, NULL, NULL,
+                      "cannot allocate IFUNC resolver information");
+  struct dl_delayed_reloc_array *new_head = ptr;
+
+  if (global->array_list_tail == NULL)
+    {
+      /* First allocation.  */
+      global->array_list_head = new_head;
+      global->array_list_tail = new_head;
+      global->array_limit
+        = (page_size - offsetof (struct dl_delayed_reloc_array, data))
+            / sizeof (new_head->data[0]);
+    }
+  else
+    {
+      global->array_list_tail->next = new_head;
+      global->array_list_tail = new_head;
+      global->tail_array_count = 0;
+    }
+}
+
+/* Allocate one struct dl_delayed_reloc element from the active
+   allocation array.  */
+static struct dl_delayed_reloc *
+allocate_reloc (void)
+{
+  assert (global != NULL);
+
+  /* Allocate a new array if none exists or the current array is
+     full.  */
+  if (global->tail_array_count == global->array_limit)
+    allocate_array ();
+  assert (global->tail_array_count < global->array_limit);
+  return &global->array_list_tail->data[global->tail_array_count++];
+}
+
+/* Deallocate the list of array allocations starting at
+   array_list.  */
+static void
+free_allocations (void)
+{
+  size_t page_size = GLRO(dl_pagesize);
+  struct dl_delayed_reloc_array *p = global->array_list_head;
+  while (p != NULL)
+    {
+      struct dl_delayed_reloc_array *next = p->next;
+      __munmap (p, page_size);
+      p = next;
+    }
+  /* The caller needs to call _dl_delayed_reloc_init again to start
+     over.  */
+  global = NULL;
+}
+
+/* Called in debugging mode to print details about a delayed
+   relocation.  */
+static void
+report_delayed_relocation (struct link_map **current_map,
+                           struct dl_delayed_reloc *dr)
+{
+  if (dr->map != *current_map)
+    {
+      *current_map = dr->map;
+
+      /* l_name is NULL for the main executable.  */
+      const char *map_name;
+      if (dr->map->l_name != NULL && *dr->map->l_name != '\0')
+        map_name = dr->map->l_name;
+      else
+        map_name = "<executable>";
+
+      _dl_debug_printf ("applying delayed relocations for %s\n", map_name);
+    }
+
+  if (dr->sym != NULL)
+    {
+      const char *strtab
+        = (const char *) D_PTR (dr->sym_map, l_info[DT_STRTAB]);
+      if (dr->sym_map->l_name != NULL)
+        _dl_debug_printf ("delayed relocation of symbol %s in %s\n",
+                          strtab + dr->sym->st_name, dr->sym_map->l_name);
+      else
+        _dl_debug_printf ("delayed relocation of symbol %s\n",
+                          strtab + dr->sym->st_name);
+    }
+  else
+    {
+      unsigned long int where = (uintptr_t) dr->reloc_addr;
+      _dl_debug_printf ("delayed relative relocation at 0x%lx\n", where);
+    }
+}
+
+/* Process all delayed IFUNC resolutions for IFUNC_MAP alone.  */
+static void
+apply_relocations (void)
+{
+  size_t array_limit = global->array_limit;
+  if (array_limit == 0)
+    /* No delayed relocations have been allocated, so there is nothing
+       to do.  */
+    return;
+
+  /* Used for debugging output, to report switches from relocated
+     object to another.  */
+  struct link_map *current_map = NULL;
+  unsigned long int count = 0;
+
+  for (struct dl_delayed_reloc_array *list = global->array_list_head;
+       list != NULL; list = list->next)
+    {
+      for (size_t index = 0; index < array_limit; ++index)
+        {
+          struct dl_delayed_reloc *dr = list->data + index;
+          if (dr->reloc == NULL)
+            /* An incompletely filled array marks the end of the
+               list.  */
+            goto out;
+
+          if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_BINDINGS))
+            report_delayed_relocation (&current_map, dr);
+          _dl_delayed_reloc_machine (dr);
+
+          /* Mark the object as fully relocated, for subsequent dlopen
+             calls.  This will clear the flag even if there are still
+             pending relocations to process, but we keep executing the
+             loop, so this is not a problem.  */
+          dr->map->l_delayed_relocations = false;
+
+          ++count;
+        }
+
+    }
+
+ out:
+  if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_BINDINGS))
+    _dl_debug_printf ("%lu delayed relocations performed\n", count);
+}
+
+void
+_dl_delayed_reloc_init (struct dl_delayed_reloc_global *new_global)
+{
+  assert (global == NULL);
+  global = new_global;
+  *global = (struct dl_delayed_reloc_global) { };
+}
+
+void
+_dl_delayed_reloc_record (struct link_map *map,
+                          const ElfW(Sym) *refsym,
+                          const ElfW(Rela) *reloc,
+                          ElfW(Addr) *reloc_addr,
+                          struct link_map *sym_map,
+                          const ElfW(Sym) *sym)
+{
+  /* reloc == NULL is a marker to find the end of the allocations.  */
+  assert (reloc != NULL);
+
+  /* Add the delayed relocation to the global list.  */
+  struct dl_delayed_reloc *dr = allocate_reloc ();
+  *dr = (struct dl_delayed_reloc)
+    {
+      .map = map,
+      .refsym = refsym,
+      .reloc = reloc,
+      .reloc_addr = reloc_addr,
+      .sym = sym,
+      .sym_map = sym_map,
+    };
+
+  /* The map containing the relocation will now need special
+     processing for future copy and relative IFUNC relocations.  */
+  map->l_delayed_relocations = true;
+}
+
+void
+_dl_delayed_reloc_apply (void)
+{
+  assert (global != NULL);
+
+  apply_relocations ();
+  free_allocations ();
+}
+
+void
+_dl_delayed_reloc_clear (void)
+{
+  /* This can be called from error handling, where the initialization
+     may not yet have happened.  */
+  if (global == NULL)
+    return;
+
+  free_allocations ();
+}
+
+#endif /* HAVE_IFUNC */