-
Notifications
You must be signed in to change notification settings - Fork 364
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
stdlib: The qsort implementation needs to use heapsort in more cases
The existing logic avoided internal stack overflow. To avoid a denial-of-service condition with adversarial input, it is necessary to fall over to heapsort if tail-recursing deeply, too, which does not result in a deep stack of pending partitions. The new test stdlib/tst-qsort5 is based on Douglas McIlroy's paper on this subject. Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
- Loading branch information
1 parent
55364e1
commit 64e4acf
Showing
3 changed files
with
187 additions
and
4 deletions.
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,171 @@ | ||
/* Adversarial test for qsort_r. | ||
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/>. */ | ||
|
||
/* The approach follows Douglas McIlroy, A Killer Adversary for | ||
Quicksort. Software—Practice and Experience 29 (1999) 341-344. | ||
Downloaded <http://www.cs.dartmouth.edu/~doug/mdmspe.pdf> | ||
(2023-11-17). */ | ||
|
||
#include <math.h> | ||
#include <stdlib.h> | ||
#include <stdio.h> | ||
#include <support/check.h> | ||
#include <support/support.h> | ||
|
||
struct context | ||
{ | ||
/* Called the gas value in the paper. This value is larger than all | ||
other values (length minus one will do), so comparison with any | ||
decided value has a known result. */ | ||
int undecided_value; | ||
|
||
/* If comparing undecided values, one of them as to be assigned a | ||
value to ensure consistency with future comparisons. This is the | ||
value that will be used. Starts out at zero. */ | ||
int next_decided; | ||
|
||
/* Used to trick pivot selection. Deciding the value for the last | ||
seen undcided value in a decided/undecided comparison happens | ||
to trick the many qsort implementations. */ | ||
int last_undecided_index; | ||
|
||
/* This array contains the actually asigned values. The call to | ||
qsort_r sorts a different array that contains indices into this | ||
array. */ | ||
int *decided_values; | ||
}; | ||
|
||
static int | ||
compare_opponent (const void *l1, const void *r1, void *ctx1) | ||
{ | ||
const int *l = l1; | ||
const int *r = r1; | ||
struct context *ctx = ctx1; | ||
int rvalue = ctx->decided_values[*r]; | ||
int lvalue = ctx->decided_values[*l]; | ||
|
||
if (lvalue == ctx->undecided_value) | ||
{ | ||
if (rvalue == ctx->undecided_value) | ||
{ | ||
/* Both values are undecided. In this case, make a decision | ||
for the last-used undecided value. This is tweak is very | ||
specific to quicksort. */ | ||
if (*l == ctx->last_undecided_index) | ||
{ | ||
ctx->decided_values[*l] = ctx->next_decided; | ||
++ctx->next_decided; | ||
/* The undecided value or *r is greater. */ | ||
return -1; | ||
} | ||
else | ||
{ | ||
ctx->decided_values[*r] = ctx->next_decided; | ||
++ctx->next_decided; | ||
/* The undecided value for *l is greater. */ | ||
return 1; | ||
} | ||
} | ||
else | ||
{ | ||
ctx->last_undecided_index = *l; | ||
return 1; | ||
} | ||
} | ||
else | ||
{ | ||
/* *l is a decided value. */ | ||
if (rvalue == ctx->undecided_value) | ||
{ | ||
ctx->last_undecided_index = *r; | ||
/* The undecided value for *r is greater. */ | ||
return -1; | ||
} | ||
else | ||
return lvalue - rvalue; | ||
} | ||
} | ||
|
||
/* Return a pointer to the adversarial permutation of length N. */ | ||
static int * | ||
create_permutation (size_t n) | ||
{ | ||
struct context ctx = | ||
{ | ||
.undecided_value = n - 1, /* Larger than all other values. */ | ||
.decided_values = xcalloc (n, sizeof (int)), | ||
}; | ||
for (size_t i = 0; i < n; ++i) | ||
ctx.decided_values[i] = ctx.undecided_value; | ||
int *scratch = xcalloc (n, sizeof (int)); | ||
for (size_t i = 0; i < n; ++i) | ||
scratch[i] = i; | ||
qsort_r (scratch, n, sizeof (*scratch), compare_opponent, &ctx); | ||
free (scratch); | ||
return ctx.decided_values; | ||
} | ||
|
||
/* Callback function for qsort which counts the number of invocations | ||
in *CLOSURE. */ | ||
static int | ||
compare_counter (const void *l1, const void *r1, void *closure) | ||
{ | ||
const int *l = l1; | ||
const int *r = r1; | ||
unsigned long long int *counter = closure; | ||
++*counter; | ||
return *l - *r; | ||
} | ||
|
||
/* Count the comparisons required for an adversarial permutation of | ||
length N. */ | ||
static unsigned long long int | ||
count_comparisons (size_t n) | ||
{ | ||
int *array = create_permutation (n); | ||
unsigned long long int counter = 0; | ||
qsort_r (array, n, sizeof (*array), compare_counter, &counter); | ||
free (array); | ||
return counter; | ||
} | ||
|
||
/* Check the scaling factor for one adversarial permutation of length | ||
N, and report some statistics. */ | ||
static void | ||
check_one_n (size_t n) | ||
{ | ||
unsigned long long int count = count_comparisons (n); | ||
double factor = count / (n * log (count)); | ||
printf ("info: length %zu: %llu comparisons ~ %f * n * log (n)\n", | ||
n, count, factor); | ||
/* This is an arbitrary factor which is true for the current | ||
implementation across a wide range of sizes. */ | ||
TEST_VERIFY (factor <= 4.5); | ||
} | ||
|
||
static int | ||
do_test (void) | ||
{ | ||
check_one_n (100); | ||
check_one_n (1000); | ||
for (int i = 1; i <= 15; ++i) | ||
check_one_n (i * 10 * 1000); | ||
return 0; | ||
} | ||
|
||
#include <support/test-driver.c> |