Re: Observe wrapping variables in while loops

From: Roel Kluin
Date: Wed Mar 11 2009 - 15:16:19 EST

Steven Rostedt wrote:
>
>
> On Sun, 8 Mar 2009, Roel Kluin wrote:
>
>> Inspired by your definition of 'if', I came up with something to test variable
>> wrapping in while loops, and was wondering if this was useful for the linux
>> kernel:

> Interesting variation. Now the question is, how many bugs are we aware of
> that were caused by wraps? If there are a few, then I would say this is
> definitely something to look at. Otherwise, it may not be worth putting
> this in mainline.
>
> -- Steve

Hi Steve, Andrew,

Below is a extended version, that does some more tests. it obviously isn't
finished, it requires more work done, but I think it may eventually provide very
interesting runtime debugging observations.

Roel
------------------------------>8-------------8<---------------------------------

/* (C) Roel Kluin, 2009 GPL v.2 */

#include <stdio.h>
#include <string.h>

#define ULONG_MAX (~0UL)
typedef enum {false, true} bool;
#define NULL ((void *)0)

struct while_data {
const char *func;
const char *file;
const char *str;
unsigned line;
bool signd;
bool postfix;
bool incr;
unsigned long last;
bool printed;
struct while_data *next;
};

static struct while_data *while_data_head;

int while_unsigned(struct while_data *w, int val)
{
if (!(w->printed & 1)) {
w->next = while_data_head;
while_data_head = w;
w->signd = false;

const char* decrstr = strstr(w->str, "--");
if (decrstr != NULL) {
if (strcmp(decrstr, "--") == 0) {
w->incr = false;
w->postfix = true;
} else if (strcmp(decrstr, w->str) == 0) {
/* FIXME: we have to ensure that
the first part is a variable name
and nothing else */
w->incr = false;
w->postfix = false;
} else {
printf("decrement not recognized.\n");
}
}
else
printf("no decrement found.\n");
w->printed |= 1;
}
if (w->last < val && !(w->printed & 2)) {
printf("variable wraps at %s:%u:%s()!\n", w->file, w->line, w->func);
printf("while(%s) with a variable that is %d!\n", w->str, val);
w->printed |= 2;
return 0;
}
return val;
}


int while_int(struct while_data *w, int val)
{
if (!(w->printed & 1)) {
w->next = while_data_head;
while_data_head = w;
w->signd = true;

const char* decrstr = strstr(w->str, "--");
if (decrstr != NULL) {
if (strcmp(decrstr, "--") == 0) {
w->incr = false;
w->postfix = true;
} else if (strcmp(decrstr, w->str) == 0) {
/* FIXME: we have to ensure that
the first part is a variable name
and nothing else */
w->incr = false;
w->postfix = false;
} else {
printf("decrement not recognized.\n");
}
}
else
printf("no decrement found.\n");
w->printed |= 1;
}
/* other test for wrapping here */
return val;
}

#define __while_check__(x) ({ \
static struct while_data ______w = { \
.func = __func__, \
.file = __FILE__, \
.str = #x, \
.line = __LINE__, \
.last = ULONG_MAX, \
.printed = 0, \
}; \
__builtin_choose_expr( \
__builtin_constant_p(x), \
(x), \
__builtin_choose_expr( \
__builtin_types_compatible_p(typeof(x), int), \
while_int(&______w, (x)), \
__builtin_choose_expr( \
__builtin_types_compatible_p(typeof(x), unsigned), \
while_unsigned(&______w, (x)), \
(x) \
))); \
})

#define while(x) while(__while_check__(x))


int check_if(int val, const char* test, const char* func, int line)
{
struct while_data *w = while_data_head;

if (strcmp(func, while_data_head->func) != 0)
goto end;

/* FIXME: we have to verify that the variable
to the left is the same as the one used in
the while loop */
/* TODO: also test "!= 0", "== 0" (and variants)*/
if (w->incr ) {
if(w->postfix) {
if (w->signd) {
;
} else {
;
}
} else {
if (w->signd) {
;
} else {
;
}
}
} else {
if(w->postfix) {
if (w->signd) {
if (strstr(test, "< 0") != 0) {
printf("after while(%s) at %s:%u:%s()\n", w->str,
w->file, w->line, w->func);
printf("if(%s) at line %u is off by one.\n", test, line);
} else if (strstr(test, ">= 0") != 0){
printf("after while(%s) at %s:%u:%s()\n", w->str,
w->file, w->line, w->func);
printf("if(%s) at line %u is off by one.\n", test, line);
}
} else {
if (strstr(test, "< 0") != 0) {
printf("if(%s) at line %u is always false, since variable is unsigned.\n", test, line);
} else if (strstr(test, ">= 0") != 0){
printf("if(%s) at line %u is always true, since variable is unsigned.\n", test, line);
}
}
} else {
if (w->signd) {
if (strstr(test, "<= 0") != 0) {
printf("after while(%s) at %s:%u:%s()\n", w->str,
w->file, w->line, w->func);
printf("if(%s) at line %u is off by one.\n", test, line);
} else if (strstr(test, "> 0") != 0){
printf("after while(%s) at %s:%u:%s()\n", w->str,
w->file, w->line, w->func);
printf("if(%s) at line %u is off by one.\n", test, line);
}
} else {
if (strstr(test, "< 0") != 0) {
printf("if(%s) at line %u is always false, since variable is unsigned.\n", test, line);
} else if (strstr(test, ">= 0") != 0){
printf("if(%s) at line %u is always true, since variable is unsigned.\n", test, line);
} else if (strstr(test, "> 0") != 0){
printf("if(%s) at line %u is off by one.\n", test, line);
}
}
}
}
end:
return val;
}

#define __if_check__(x) ({ \
check_if((x), #x, __func__, __LINE__); \
})

#define if(x) if(__if_check__(x))

int main(int argc, char* argv[])
{
//#define x 0
unsigned x = 10;
int y;
// these alternatives, should work

while (x--)
/*printf("in loop:%d\n", x)*/;

if (x < 0)
printf("won't print!\n");

x = 10;
while (--x)
/*printf("in loop:%d\n", x)*/;

printf("after loop:%d\n", x);

if (x < 0)
printf("won't print!\n");

y = 10;
while (y--)
/*printf("in loop:%d\n", x)*/;

if (y < 0)
printf("will print.\n");

y = 10;
while (--y)
/*printf("in loop:%d\n", x)*/;

if (y < 0)
printf("won't print!\n");

return 0;
}

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