Messages in this thread Patch in this message | | | From | André Goddard Rosa <> | Subject | [PATCH v2 2/2] bsearch: prevent overflow when computing middle comparison element | Date | Mon, 9 Nov 2009 12:53:49 -0200 |
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It's really difficult to occur in practice because the sum of the lower and higher limits must overflow an int variable, but it can occur when working with large arrays. We'd better safe than sorry by avoiding this overflow situation when computing the middle element for comparison.
See: http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5045582
The program below demonstrates the issue:
$ ./a.out (glibc) Element is at the last position (patched) Element is at the last position Segmentation fault
=== #include <search.h> #include <stdlib.h> #include <stdio.h>
/* A number that when doubled will be bigger than 2^31 - 1 */ #define BIG_NUMBER_TO_OVERFLOW_INT (1100000000)
static int cmp_char(const void *p1, const void *p2) { char v1 = (*(char *)p1); char v2 = (*(char *)p2);
if (v1 < v2) return -1; else if (v1 > v2) return 1; else return 0; }
void *lib_bsearch(const void *key, const void *base, size_t num, size_t size , int (*cmp)(const void *key, const void *elt)) { int start = 0, end = num - 1, mid, result;
while (start <= end) { mid = (start + end) / 2; result = cmp(key, base + mid * size); if (result < 0) end = mid - 1; else if (result > 0) start = mid + 1; else return (void *)base + mid * size; }
return NULL; }
void *patch_lib_bsearch(const void *key, const void *base, size_t num, size_t size , int (*cmp)(const void *key, const void *elt)) { size_t start = 0, end = num, mid; int result;
while (start < end) { mid = (start + end) / 2; result = cmp(key, base + mid * size); if (result < 0) end = mid - 1; else if (result > 0) start = mid + 1; else return (void *)base + mid * size; }
return NULL; }
int main(void) { char key = 1; char *array = calloc(BIG_NUMBER_TO_OVERFLOW_INT, sizeof(char)); void *ptr;
if (!array) { printf("%s\n", "no memory"); return EXIT_FAILURE; } array[BIG_NUMBER_TO_OVERFLOW_INT - 1] = 1;
ptr = bsearch(&key, array, BIG_NUMBER_TO_OVERFLOW_INT, sizeof(char), cmp_char); printf("(glibc) Element is%sat the last position\n" , (ptr == &array[BIG_NUMBER_TO_OVERFLOW_INT - 1]) ? " " : " NOT ");
ptr = patch_lib_bsearch(&key, array, BIG_NUMBER_TO_OVERFLOW_INT, sizeof(char), cmp_char); printf("(patched) Element is%sat the last position\n" , (ptr == &array[BIG_NUMBER_TO_OVERFLOW_INT - 1]) ? " " : " NOT ");
ptr = lib_bsearch(&key, array, BIG_NUMBER_TO_OVERFLOW_INT, sizeof(char), cmp_char); printf("(unpatched) Element is%sat the last position\n" , (ptr == &array[BIG_NUMBER_TO_OVERFLOW_INT - 1]) ? " " : " NOT ");
free(array);
return EXIT_SUCCESS; }
Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com> --- lib/bsearch.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/lib/bsearch.c b/lib/bsearch.c index 33cbba6..af40c03 100644 --- a/lib/bsearch.c +++ b/lib/bsearch.c @@ -33,7 +33,8 @@ void *bsearch(const void *key, const void *base, size_t num, size_t size, int (*cmp)(const void *key, const void *elt)) { - int start = 0, end = num, mid, result; + size_t start = 0, end = num, mid; + int result; while (start < end) { mid = (start + end) / 2; -- 1.6.5.2.153.g6e31f.dirty -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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