commit 785969a047ad2f23f758901c6816422573544453
Author: Florian Weimer <fweimer@redhat.com>
Date: Fri Dec 4 09:13:43 2020 +0100
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>
diff --git a/elf/Makefile b/elf/Makefile
index f795617780b393ec..abb3e9d1179ef5cd 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -163,7 +163,7 @@ tests-container = \
tests := tst-tls9 tst-leaks1 \
tst-array1 tst-array2 tst-array3 tst-array4 tst-array5 \
- tst-auxv
+ tst-auxv tst-stringtable
tests-internal := tst-tls1 tst-tls2 $(tests-static-internal)
tests-static := $(tests-static-normal) $(tests-static-internal)
diff --git a/elf/stringtable.c b/elf/stringtable.c
new file mode 100644
index 0000000000000000..099347d73ee70b8f
--- /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,
+ ¤t->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);
+}
diff --git a/elf/stringtable.h b/elf/stringtable.h
new file mode 100644
index 0000000000000000..7d57d1bda9602947
--- /dev/null
+++ b/elf/stringtable.h
@@ -0,0 +1,64 @@
+/* String tables for ld.so.cache construction.
+ 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/>. */
+
+#ifndef _STRINGTABLE_H
+#define _STRINGTABLE_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+/* An entry in the string table. Only the length and string fields are
+ expected to be used outside the string table code. */
+struct stringtable_entry
+{
+ struct stringtable_entry *next; /* For collision resolution. */
+ uint32_t length; /* Length of then string. */
+ uint32_t offset; /* From start of finalized table. */
+ char string[]; /* Null-terminated string. */
+};
+
+/* A string table. Zero-initialization produces a valid atable. */
+struct stringtable
+{
+ struct stringtable_entry **entries; /* Array of hash table buckets. */
+ uint32_t count; /* Number of elements in the table. */
+ uint32_t allocated; /* Length of the entries array. */
+};
+
+/* Adds STRING to TABLE. May return the address of an existing entry. */
+struct stringtable_entry *stringtable_add (struct stringtable *table,
+ const char *string);
+
+/* Result of stringtable_finalize. SIZE bytes at STRINGS should be
+ written to the file. */
+struct stringtable_finalized
+{
+ char *strings;
+ size_t size;
+};
+
+/* Assigns offsets to string table entries and computes the serialized
+ form of the string table. */
+void stringtable_finalize (struct stringtable *table,
+ struct stringtable_finalized *result);
+
+/* Deallocate the string table (but not the TABLE pointer itself).
+ (The table can be re-used for adding more strings without
+ initialization.) */
+void stringtable_free (struct stringtable *table);
+
+#endif /* _STRINGTABLE_H */
diff --git a/elf/stringtable_free.c b/elf/stringtable_free.c
new file mode 100644
index 0000000000000000..8588a254705d4df8
--- /dev/null
+++ b/elf/stringtable_free.c
@@ -0,0 +1,33 @@
+/* String tables for ld.so.cache construction. Deallocation (for tests only).
+ 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 <stdlib.h>
+#include <stringtable.h>
+
+void
+stringtable_free (struct stringtable *table)
+{
+ 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;
+ free (e);
+ e = next;
+ }
+ free (table->entries);
+ *table = (struct stringtable) { 0, };
+}
diff --git a/elf/tst-stringtable.c b/elf/tst-stringtable.c
new file mode 100644
index 0000000000000000..3731086037567d57
--- /dev/null
+++ b/elf/tst-stringtable.c
@@ -0,0 +1,181 @@
+/* Unit test for ldconfig string tables.
+ 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 <array_length.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stringtable.h>
+#include <support/check.h>
+#include <support/support.h>
+
+static int
+do_test (void)
+{
+ /* Empty string table. */
+ {
+ struct stringtable s = { 0, };
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+ TEST_COMPARE_STRING (f.strings, "");
+ TEST_COMPARE (f.size, 0);
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ /* String table with one empty string. */
+ {
+ struct stringtable s = { 0, };
+ struct stringtable_entry *e = stringtable_add (&s, "");
+ TEST_COMPARE_STRING (e->string, "");
+ TEST_COMPARE (e->length, 0);
+ TEST_COMPARE (s.count, 1);
+
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+ TEST_COMPARE (e->offset, 0);
+ TEST_COMPARE_STRING (f.strings, "");
+ TEST_COMPARE (f.size, 1);
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ /* String table with one non-empty string. */
+ {
+ struct stringtable s = { 0, };
+ struct stringtable_entry *e = stringtable_add (&s, "name");
+ TEST_COMPARE_STRING (e->string, "name");
+ TEST_COMPARE (e->length, 4);
+ TEST_COMPARE (s.count, 1);
+
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+ TEST_COMPARE (e->offset, 0);
+ TEST_COMPARE_STRING (f.strings, "name");
+ TEST_COMPARE (f.size, 5);
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ /* Two strings, one is a prefix of the other. Tail-merging can only
+ happen in one way in this case. */
+ {
+ struct stringtable s = { 0, };
+ struct stringtable_entry *suffix = stringtable_add (&s, "suffix");
+ TEST_COMPARE_STRING (suffix->string, "suffix");
+ TEST_COMPARE (suffix->length, 6);
+ TEST_COMPARE (s.count, 1);
+
+ struct stringtable_entry *prefix
+ = stringtable_add (&s, "prefix-suffix");
+ TEST_COMPARE_STRING (prefix->string, "prefix-suffix");
+ TEST_COMPARE (prefix->length, strlen ("prefix-suffix"));
+ TEST_COMPARE (s.count, 2);
+
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+ TEST_COMPARE (prefix->offset, 0);
+ TEST_COMPARE (suffix->offset, strlen ("prefix-"));
+ TEST_COMPARE_STRING (f.strings, "prefix-suffix");
+ TEST_COMPARE (f.size, sizeof ("prefix-suffix"));
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ /* String table with various shared prefixes. Triggers hash
+ resizing. */
+ {
+ enum { count = 1500 };
+ char *strings[2 * count];
+ struct stringtable_entry *entries[2 * count];
+ struct stringtable s = { 0, };
+ for (int i = 0; i < count; ++i)
+ {
+ strings[i] = xasprintf ("%d", i);
+ entries[i] = stringtable_add (&s, strings[i]);
+ TEST_COMPARE (entries[i]->length, strlen (strings[i]));
+ TEST_COMPARE_STRING (entries[i]->string, strings[i]);
+ strings[i + count] = xasprintf ("prefix/%d", i);
+ entries[i + count] = stringtable_add (&s, strings[i + count]);
+ TEST_COMPARE (entries[i + count]->length, strlen (strings[i + count]));
+ TEST_COMPARE_STRING (entries[i + count]->string, strings[i + count]);
+ }
+
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+
+ for (int i = 0; i < 2 * count; ++i)
+ {
+ TEST_COMPARE (entries[i]->length, strlen (strings[i]));
+ TEST_COMPARE_STRING (entries[i]->string, strings[i]);
+ TEST_COMPARE_STRING (f.strings + entries[i]->offset, strings[i]);
+ free (strings[i]);
+ }
+
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ /* Verify that maximum tail merging happens. */
+ {
+ struct stringtable s = { 0, };
+ const char *strings[] = {
+ "",
+ "a",
+ "b",
+ "aa",
+ "aaa",
+ "aa",
+ "bb",
+ "b",
+ "a",
+ "ba",
+ "baa",
+ };
+ struct stringtable_entry *entries[array_length (strings)];
+ for (int i = 0; i < array_length (strings); ++i)
+ entries[i] = stringtable_add (&s, strings[i]);
+ for (int i = 0; i < array_length (strings); ++i)
+ TEST_COMPARE_STRING (entries[i]->string, strings[i]);
+
+ struct stringtable_finalized f;
+ stringtable_finalize (&s, &f);
+
+ /* There are only four different strings, "aaa", "ba", "baa",
+ "bb". The rest is shared in an unspecified fashion. */
+ TEST_COMPARE (f.size, 4 + 3 + 4 + 3);
+
+ for (int i = 0; i < array_length (strings); ++i)
+ {
+ TEST_COMPARE_STRING (entries[i]->string, strings[i]);
+ TEST_COMPARE_STRING (f.strings + entries[i]->offset, strings[i]);
+ }
+
+ free (f.strings);
+ stringtable_free (&s);
+ }
+
+ return 0;
+}
+
+#include <support/test-driver.c>
+
+/* Re-compile the string table implementation here. It is not
+ possible to link against the actual build because it was built for
+ use in ldconfig. */
+#define _(arg) arg
+#include "stringtable.c"
+#include "stringtable_free.c"