Consider writing implementation for some not-so-obvious algorithm in C. For example let it be recursive quicksort, that I have found in K. N. King's "C Programming: A Modern Approach, 2nd Edition" book, that it's available from here. The most interesting part consist of two following definitions:
void quicksort(int a[], int low, int high)
{
int middle;
if (low >= high)
return;
middle = split(a, low, high);
quicksort(a, low, middle - 1);
quicksort(a, middle + 1, high);
}
int split(int a[], int low, int high)
{
int part_element = a[low];
for (;;) {
while (low < high && part_element <= a[high])
high--;
if (low >= high)
break;
a[low++] = a[high];
while (low < high && a[low] <= part_element)
low++;
if (low >= high)
break;
a[high--] = a[low];
}
a[high] = part_element;
return high;
}
Both while loops can be optimized by removing low < high tests:
for (;;) {
while (part_element < a[high])
high--;
if (low >= high)
break;
a[low++] = a[high];
a[high] = part_element;
while (a[low] <= part_element)
low++;
if (low >= high)
break;
a[high--] = a[low];
a[low] = part_element;
}
What is the recommended way to make sure that every access or write to array (allocated on stack) is actually valid (i.e. not provoking undefined behaviour) ? What I already tried is to:
gdb on some actual datasplit or cppcheckvalgrind with --tool=exp-sgcheck switchFor example having five elements array {8, 1, 2, 3, 4}:
#define N 5
int main(void)
{
int a[N] = {8, 1, 2, 3, 4}, i;
quicksort(a, 0, N - 1);
printf("After sort:");
for (i = 0; i < N; i++)
printf(" %d", a[i]);
putchar('\n');
return 0;
}
Result is (most certainly it's implemention dependent):
After sort: 1 1 2 4 8
(gdb) p low
$1 = 3
(gdb) p high
$2 = 4
(gdb) p a[low]
$3 = 1
(gdb) p part_element
$4 = 8
(gdb) s
47 low++;
(gdb) s
46 while (a[low] <= part_element)
(gdb) s
47 low++;
(gdb) s
46 while (a[low] <= part_element)
(gdb) p low
$5 = 5
(gdb) p high
$6 = 4
(gdb) bt full
#0 split (a=0x7fffffffe140, low=5, high=4) at qsort.c:46
part_element = 8
#1 0x00000000004005df in quicksort (a=0x7fffffffe140, low=0, high=4) at qsort.c:30
middle = <value optimized out>
#2 0x0000000000400656 in main () at qsort.c:14
a = {4, 1, 2, 1, 8}
i = <value optimized out>
As you see low variable went outside boundary:
(gdb) p low
$5 = 5
$ splint -retvalint -exportlocal qsort.c
Splint 3.1.2 --- 07 Feb 2011
Finished checking --- no warnings
$ cppcheck qsort.c
Checking qsort.c...
--tool=exp-sgcheck$ valgrind --tool=exp-sgcheck ./a.out
==5480== exp-sgcheck, a stack and global array overrun detector
==5480== NOTE: This is an Experimental-Class Valgrind Tool
==5480== Copyright (C) 2003-2012, and GNU GPL'd, by OpenWorks Ltd et al.
==5480== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==5480== Command: ./a.out
==5480==
==5480== Invalid read of size 4
==5480== at 0x4005A0: split (qsort.c:46)
==5480== by 0x4005DE: quicksort (qsort.c:30)
==5480== by 0x400655: main (qsort.c:14)
==5480== Address 0x7ff000114 expected vs actual:
==5480== Expected: stack array "a" of size 20 in frame 2 back from here
==5480== Actual: unknown
==5480== Actual: is 0 after Expected
==5480==
After sort: 1 1 2 4 8
==5480==
==5480== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
The location at 0x4005A0: split (qsort.c:46) is matching to the same place as I found by gdb manually.
What is the recommended way to make sure that every access or write to array (allocated on stack) is actually valid (i.e. not provoking undefined behaviour) ?
What if use clang on Linux with the options -fsanitize=addressand -fsanitize=undefined? It is also available in gcc: http://gcc.gnu.org/gcc-4.8/changes.html.
clang with the option -fsanitize=undefined
This is an example:
#include <stdlib.h>
#define N 5
int main(int argc, char *argv[])
{
int a[N] = {8, 1, 2, 3, 4}, i;
int r =0;
int end = atoi(argv[1]);
for (int i = 0; i != end; ++i)
r += a[i];
return r;
}
Then
clang -fno-omit-frame-pointer -fsanitize=undefined -g out_boundary.c -o out_boundary_clang
$ ./out_boundary_clang 5
$ ./out_boundary_clang 6
out_boundary.c:12:10: runtime error: index 5 out of bounds for type 'int [5]'
Illegal instruction (core dumped)
And then analyze a core file
Program terminated with signal 4, Illegal instruction.
#0 main (argc=2, argv=0x7fff3a1c28c8) at out_boundary.c:12
12 r += a[i];
(gdb) p i
$1 = 5
clang with the option -fsanitize=address
This is a quote:
The tool can detect the following types of bugs:
* Out-of-bounds accesses to heap, stack and globals
* Use-after-free
* Use-after-return (to some extent)
* Double-free, invalid free
* Memory leaks (experimental)
clang -fno-omit-frame-pointer -fsanitize=address -g out_boundary.c -o out_boundary_clang
And then:
$ ./out_boundary_clang 6 2>&1 | asan_symbolize.py
=================================================================
==9634==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff91bb2ad4 at pc 0x459c67 bp 0x7fff91bb2910 sp 0x7fff91bb2908
READ of size 4 at 0x7fff91bb2ad4 thread T0
#0 0x459c66 in main out_boundary.c:12
#1 0x3a1d81ed1c in __libc_start_main ??:0
#2 0x4594ac in _start ??:0
Address 0x7fff91bb2ad4 is located in stack of thread T0 at offset 244 in frame
#0 0x45957f in main out_boundary.c:6
This frame has 8 object(s):
[32, 36) ''
[96, 100) ''
[160, 168) ''
[224, 244) 'a'
[288, 292) 'i'
[352, 356) 'r'
[416, 420) 'end'
[480, 484) 'i1'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
(longjmp and C++ exceptions *are* supported)
Shadow bytes around the buggy address:
0x10007236e500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007236e510: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007236e520: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007236e530: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
0x10007236e540: 04 f4 f4 f4 f2 f2 f2 f2 04 f4 f4 f4 f2 f2 f2 f2
=>0x10007236e550: 00 f4 f4 f4 f2 f2 f2 f2 00 00[04]f4 f2 f2 f2 f2
0x10007236e560: 04 f4 f4 f4 f2 f2 f2 f2 04 f4 f4 f4 f2 f2 f2 f2
0x10007236e570: 04 f4 f4 f4 f2 f2 f2 f2 04 f4 f4 f4 f3 f3 f3 f3
0x10007236e580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007236e590: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007236e5a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Heap right redzone: fb
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack partial redzone: f4
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
ASan internal: fe
==9634==ABORTING
Or you can use both this options. Useful links: