Re: [PATCH 24/28] elf: Implement a string table for ldconfig, with tail merging

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 01/10/2020 13:34, Florian Weimer via Libc-alpha wrote:
> This will be used in ldconfig to reduce the ld.so.cache size slightly.

Could you extend the commit message to explain why the 32-bit FNV-1a hash
is used and how the hash table is organized (how collisions are handled,
expected memory usage, strategy used to increase/decrease the bucket size)?

It could help also to explain the usercase a bit more, the 'tail merging'
is not straightforward to understand without dig deep in the code.  Also
why kind of cache size decrease do you expect by using strategy?

> ---
>  elf/Makefile           |   2 +-
>  elf/stringtable.c      | 201 +++++++++++++++++++++++++++++++++++++++++
>  elf/stringtable.h      |  61 +++++++++++++
>  elf/stringtable_free.c |  32 +++++++
>  elf/tst-stringtable.c  | 140 ++++++++++++++++++++++++++++
>  5 files changed, 435 insertions(+), 1 deletion(-)
>  create mode 100644 elf/stringtable.c
>  create mode 100644 elf/stringtable.h
>  create mode 100644 elf/stringtable_free.c
>  create mode 100644 elf/tst-stringtable.c
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index 2c36b08c73..ad50a3e16e 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -172,7 +172,7 @@ 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 0000000000..f9ade50249
> --- /dev/null
> +++ b/elf/stringtable.c
> @@ -0,0 +1,201 @@
> +/* String tables for ld.so.cache construction.  Implementation.

This file misses the Copyright year.

> +   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;
> +  table->allocated = 16;
> +  table->entries = xcalloc (table->allocated, sizeof (table->entries[0]));
> +}
> +

Why 16 elements as initial size? 

> +/* 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;
> +}

Why fnv1a was chosen? 

> +
> +/* Double the capacity of the hash table.  */
> +static void
> +stringtable_rehash (struct stringtable *table)
> +{
> +  /* Cannot overflow because the old allocation size (in bytes) is
> +     larger.  */
> +  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_intern (struct stringtable *table, const char *string)
> +{
> +  if (table->allocated == 0)
> +    stringtable_init (table);

How this could happen? Is it expect the caller to set 'allocated' explicitly?

> +
> +  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 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 (ssize_t i = to_compare - 1; i >= 0; --i)
> +    {
> +      unsigned char lch = left->string[i];
> +      unsigned char rch = right->string[i];
> +      if (lch != rch)
> +        return lch - rch;
> +    }
> +  if (left->length == right->length)
> +    return 0;
> +  else if (left->length < right->length)
> +    /* Longer strings should come first.  */
> +    return 1;
> +  else
> +    return -1;
> +}

Ok.

> +
> +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 table sharing 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);
> +}

Ok, I guess allocating a new stringtable_finalized should be simpler than
operating the table itself. 

> diff --git a/elf/stringtable.h b/elf/stringtable.h
> new file mode 100644
> index 0000000000..e35b6c67fd
> --- /dev/null
> +++ b/elf/stringtable.h
> @@ -0,0 +1,61 @@
> +/* String tables for ld.so.cache construction.
> +   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;
> +  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_intern (struct stringtable *table,
> +                                              const char *string);

I think this name is confusing, why not just 'stringtable_add' or
'stringtable_add_element'?

> +
> +/* 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).  */
> +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 0000000000..0e5296e429
> --- /dev/null
> +++ b/elf/stringtable_free.c
> @@ -0,0 +1,32 @@
> +/* String tables for ld.so.cache construction.  Deallocation (for tests only).
> +   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, };
> +}

Ok.

> diff --git a/elf/tst-stringtable.c b/elf/tst-stringtable.c
> new file mode 100644
> index 0000000000..78ca5434df
> --- /dev/null
> +++ b/elf/tst-stringtable.c
> @@ -0,0 +1,140 @@
> +/* Unit test for ldconfig string tables.
> +   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 <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_intern (&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_intern (&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_intern (&s, "suffix");
> +    TEST_COMPARE_STRING (suffix->string, "suffix");
> +    TEST_COMPARE (suffix->length, 6);
> +    TEST_COMPARE (s.count, 1);
> +
> +    struct stringtable_entry *prefix
> +      = stringtable_intern (&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_intern (&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_intern (&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);
> +  }
> +
> +  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"
> 

Ok.