elf: Implement a string table for ldconfig, with tail merging

This will be used in ldconfig to reduce the ld.so.cache size slightly.

Tail merging is an optimization where a pointer points into another
string if the first string is a suffix of the second string.

The hash function FNV-1a was chosen because it is simple and achieves
good dispersion even for short strings (so that the hash table bucket
count can be a power of two).  It is clearly superior to the hsearch
hash and the ELF hash in this regard.

The hash table uses chaining for collision resolution.

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

1 files changed, 209 insertions, 0 deletions

diff --git a/elf/stringtable.c b/elf/stringtable.c
new file mode 100644
index 0000000000..099347d73e
--- /dev/null
+++ b/elf/stringtable.c
@@ -0,0 +1,209 @@
+/* String tables for ld.so.cache construction.  Implementation.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published
+   by the Free Software Foundation; version 2 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+#include <error.h>
+#include <ldconfig.h>
+#include <libintl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stringtable.h>
+
+static void
+stringtable_init (struct stringtable *table)
+{
+  table->count = 0;
+
+  /* This needs to be a power of two.  128 is sufficient to keep track
+     of 42 DSOs without resizing (assuming two strings per DSOs).
+     glibc itself comes with more than 20 DSOs, so 64 would likely to
+     be too small.  */
+  table->allocated = 128;
+
+  table->entries = xcalloc (table->allocated, sizeof (table->entries[0]));
+}
+
+/* 32-bit FNV-1a hash function.  */
+static uint32_t
+fnv1a (const char *string, size_t length)
+{
+  const unsigned char *p = (const unsigned char *) string;
+  uint32_t hash = 2166136261U;
+  for (size_t i = 0; i < length; ++i)
+    {
+      hash ^= p[i];
+      hash *= 16777619U;
+    }
+  return hash;
+}
+
+/* Double the capacity of the hash table.  */
+static void
+stringtable_rehash (struct stringtable *table)
+{
+  /* This computation cannot overflow because the old total in-memory
+     size of the hash table is larger than the computed value.  */
+  uint32_t new_allocated = table->allocated * 2;
+  struct stringtable_entry **new_entries
+    = xcalloc (new_allocated, sizeof (table->entries[0]));
+
+  uint32_t mask = new_allocated - 1;
+  for (uint32_t i = 0; i < table->allocated; ++i)
+    for (struct stringtable_entry *e = table->entries[i]; e != NULL; )
+      {
+        struct stringtable_entry *next = e->next;
+        uint32_t hash = fnv1a (e->string, e->length);
+        uint32_t new_index = hash & mask;
+        e->next = new_entries[new_index];
+        new_entries[new_index] = e;
+        e = next;
+      }
+
+  free (table->entries);
+  table->entries = new_entries;
+  table->allocated = new_allocated;
+}
+
+struct stringtable_entry *
+stringtable_add (struct stringtable *table, const char *string)
+{
+  /* Check for a zero-initialized table.  */
+  if (table->allocated == 0)
+    stringtable_init (table);
+
+  size_t length = strlen (string);
+  if (length > (1U << 30))
+    error (EXIT_FAILURE, 0, _("String table string is too long"));
+  uint32_t hash = fnv1a (string, length);
+
+  /* Return a previously-existing entry.  */
+  for (struct stringtable_entry *e
+         = table->entries[hash & (table->allocated - 1)];
+       e != NULL; e = e->next)
+    if (e->length == length && memcmp (e->string, string, length) == 0)
+      return e;
+
+  /* Increase the size of the table if necessary.  Keep utilization
+     below two thirds.  */
+  if (table->count >= (1U << 30))
+    error (EXIT_FAILURE, 0, _("String table has too many entries"));
+  if (table->count * 3 > table->allocated * 2)
+    stringtable_rehash (table);
+
+  /* Add the new table entry.  */
+  ++table->count;
+  struct stringtable_entry *e
+    = xmalloc (offsetof (struct stringtable_entry, string) + length + 1);
+  uint32_t index = hash & (table->allocated - 1);
+  e->next = table->entries[index];
+  table->entries[index] = e;
+  e->length = length;
+  e->offset = 0;
+  memcpy (e->string, string, length + 1);
+  return e;
+}
+
+/* Sort reversed strings in reverse lexicographic order.  This is used
+   for tail merging.  */
+static int
+finalize_compare (const void *l, const void *r)
+{
+  struct stringtable_entry *left = *(struct stringtable_entry **) l;
+  struct stringtable_entry *right = *(struct stringtable_entry **) r;
+  size_t to_compare;
+  if (left->length < right->length)
+    to_compare = left->length;
+  else
+    to_compare = right->length;
+  for (size_t i = 1; i <= to_compare; ++i)
+    {
+      unsigned char lch = left->string[left->length - i];
+      unsigned char rch = right->string[right->length - i];
+      if (lch != rch)
+        return rch - lch;
+    }
+  if (left->length == right->length)
+    return 0;
+  else if (left->length < right->length)
+    /* Longer strings should come first.  */
+    return 1;
+  else
+    return -1;
+}
+
+void
+stringtable_finalize (struct stringtable *table,
+                      struct stringtable_finalized *result)
+{
+  if (table->count == 0)
+    {
+      result->strings = xstrdup ("");
+      result->size = 0;
+      return;
+    }
+
+  /* Optimize the order of the strings.  */
+  struct stringtable_entry **array = xcalloc (table->count, sizeof (*array));
+  {
+    size_t j = 0;
+    for (uint32_t i = 0; i < table->allocated; ++i)
+      for (struct stringtable_entry *e = table->entries[i]; e != NULL;
+           e = e->next)
+        {
+          array[j] = e;
+          ++j;
+        }
+    assert (j == table->count);
+  }
+  qsort (array, table->count, sizeof (*array), finalize_compare);
+
+  /* Assign offsets, using tail merging (sharing suffixes) if possible.  */
+  array[0]->offset = 0;
+  for (uint32_t j = 1; j < table->count; ++j)
+    {
+      struct stringtable_entry *previous = array[j - 1];
+      struct stringtable_entry *current = array[j];
+      if (previous->length >= current->length
+          && memcmp (&previous->string[previous->length - current->length],
+                     current->string, current->length) == 0)
+        current->offset = (previous->offset + previous->length
+                           - current->length);
+      else if (__builtin_add_overflow (previous->offset,
+                                       previous->length + 1,
+                                       &current->offset))
+        error (EXIT_FAILURE, 0, _("String table is too large"));
+    }
+
+  /* Allocate the result string.  */
+  {
+    struct stringtable_entry *last = array[table->count - 1];
+    if (__builtin_add_overflow (last->offset, last->length + 1,
+                                &result->size))
+      error (EXIT_FAILURE, 0, _("String table is too large"));
+  }
+  /* The strings are copied from the hash table, so the array is no
+     longer needed.  */
+  free (array);
+  result->strings = xcalloc (result->size, 1);
+
+  /* Copy the strings.  */
+  for (uint32_t i = 0; i < table->allocated; ++i)
+    for (struct stringtable_entry *e = table->entries[i]; e != NULL;
+         e = e->next)
+      if (result->strings[e->offset] == '\0')
+        memcpy (&result->strings[e->offset], e->string, e->length + 1);
+}