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The mergesort removal from qsort implementation (commit 03bf8357e8)
had the side-effect of making sorting nonstable. Although neither
POSIX nor C standard specify that qsort should be stable, it seems
that it has become an instance of Hyrum's law where multiple programs
expect it.
Also, the resulting introsort implementation is not faster than
the previous mergesort (which makes the change even less appealing).
This patch restores the previous mergesort implementation, with the
exception of machinery that checks the resulting allocation against
the _SC_PHYS_PAGES (it only adds complexity and the heuristic not
always make sense depending on the system configuration and load).
The alloca usage was replaced with a fixed-size buffer.
For the fallback mechanism, the implementation uses heapsort. It is
simpler than quicksort, and it does not suffer from adversarial
inputs. With memory overcommit, it should be rarely triggered.
The drawback is mergesort requires O(n) extra space, and since it is
allocated with malloc the function is AS-signal-unsafe. It should be
feasible to change it to use mmap, although I am not sure how urgent
it is. The heapsort is also nonstable, so programs that require a
stable sort would still be subject to this latent issue.
The tst-qsort5 is removed since it will not create quicksort adversarial
inputs with the current qsort_r implementation.
Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
arm/gcs
Adhemerval Zanella
2 years ago
7 changed files with 232 additions and 446 deletions
@ -1,171 +0,0 @@ |
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/* Adversarial test for qsort_r.
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Copyright (C) 2023-2024 Free Software Foundation, Inc. |
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This file is part of the GNU C Library. |
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The GNU C Library is free software; you can redistribute it and/or |
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modify it under the terms of the GNU Lesser General Public |
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License as published by the Free Software Foundation; either |
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version 2.1 of the License, or (at your option) any later version. |
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The GNU C Library is distributed in the hope that it will be useful, |
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but WITHOUT ANY WARRANTY; without even the implied warranty of |
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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Lesser General Public License for more details. |
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You should have received a copy of the GNU Lesser General Public |
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License along with the GNU C Library; if not, see |
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<http://www.gnu.org/licenses/>. */
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/* The approach follows Douglas McIlroy, A Killer Adversary for
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Quicksort. Software—Practice and Experience 29 (1999) 341-344. |
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Downloaded <http://www.cs.dartmouth.edu/~doug/mdmspe.pdf>
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(2023-11-17). */ |
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#include <math.h> |
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#include <stdlib.h> |
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#include <stdio.h> |
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#include <support/check.h> |
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#include <support/support.h> |
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struct context |
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{ |
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/* Called the gas value in the paper. This value is larger than all
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other values (length minus one will do), so comparison with any |
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decided value has a known result. */ |
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int undecided_value; |
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/* If comparing undecided values, one of them as to be assigned a
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value to ensure consistency with future comparisons. This is the |
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value that will be used. Starts out at zero. */ |
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int next_decided; |
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/* Used to trick pivot selection. Deciding the value for the last
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seen undcided value in a decided/undecided comparison happens |
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to trick the many qsort implementations. */ |
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int last_undecided_index; |
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/* This array contains the actually asigned values. The call to
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qsort_r sorts a different array that contains indices into this |
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array. */ |
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int *decided_values; |
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}; |
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static int |
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compare_opponent (const void *l1, const void *r1, void *ctx1) |
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{ |
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const int *l = l1; |
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const int *r = r1; |
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struct context *ctx = ctx1; |
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int rvalue = ctx->decided_values[*r]; |
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int lvalue = ctx->decided_values[*l]; |
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if (lvalue == ctx->undecided_value) |
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{ |
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if (rvalue == ctx->undecided_value) |
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{ |
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/* Both values are undecided. In this case, make a decision
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for the last-used undecided value. This is tweak is very |
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specific to quicksort. */ |
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if (*l == ctx->last_undecided_index) |
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{ |
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ctx->decided_values[*l] = ctx->next_decided; |
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++ctx->next_decided; |
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/* The undecided value or *r is greater. */ |
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return -1; |
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} |
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else |
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{ |
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ctx->decided_values[*r] = ctx->next_decided; |
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++ctx->next_decided; |
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/* The undecided value for *l is greater. */ |
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return 1; |
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} |
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} |
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else |
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{ |
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ctx->last_undecided_index = *l; |
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return 1; |
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} |
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} |
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else |
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{ |
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/* *l is a decided value. */ |
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if (rvalue == ctx->undecided_value) |
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{ |
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ctx->last_undecided_index = *r; |
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/* The undecided value for *r is greater. */ |
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return -1; |
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} |
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else |
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return lvalue - rvalue; |
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} |
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} |
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/* Return a pointer to the adversarial permutation of length N. */ |
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static int * |
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create_permutation (size_t n) |
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{ |
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struct context ctx = |
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{ |
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.undecided_value = n - 1, /* Larger than all other values. */ |
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.decided_values = xcalloc (n, sizeof (int)), |
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}; |
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for (size_t i = 0; i < n; ++i) |
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ctx.decided_values[i] = ctx.undecided_value; |
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int *scratch = xcalloc (n, sizeof (int)); |
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for (size_t i = 0; i < n; ++i) |
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scratch[i] = i; |
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qsort_r (scratch, n, sizeof (*scratch), compare_opponent, &ctx); |
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free (scratch); |
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return ctx.decided_values; |
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} |
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/* Callback function for qsort which counts the number of invocations
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in *CLOSURE. */ |
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static int |
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compare_counter (const void *l1, const void *r1, void *closure) |
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{ |
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const int *l = l1; |
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const int *r = r1; |
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unsigned long long int *counter = closure; |
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++*counter; |
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return *l - *r; |
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} |
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/* Count the comparisons required for an adversarial permutation of
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length N. */ |
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static unsigned long long int |
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count_comparisons (size_t n) |
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{ |
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int *array = create_permutation (n); |
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unsigned long long int counter = 0; |
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qsort_r (array, n, sizeof (*array), compare_counter, &counter); |
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free (array); |
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return counter; |
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} |
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/* Check the scaling factor for one adversarial permutation of length
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N, and report some statistics. */ |
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static void |
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check_one_n (size_t n) |
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{ |
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unsigned long long int count = count_comparisons (n); |
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double factor = count / (n * log (count)); |
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printf ("info: length %zu: %llu comparisons ~ %f * n * log (n)\n", |
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n, count, factor); |
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/* This is an arbitrary factor which is true for the current
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implementation across a wide range of sizes. */ |
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TEST_VERIFY (factor <= 4.5); |
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} |
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static int |
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do_test (void) |
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{ |
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check_one_n (100); |
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check_one_n (1000); |
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for (int i = 1; i <= 15; ++i) |
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check_one_n (i * 10 * 1000); |
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return 0; |
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} |
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#include <support/test-driver.c> |
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