Re: [PATCH] sched/fair: Use 'unsigned long' for group_shares,group_runnable

[chengjian (D)]

Hi, Peter


On 2019/4/15 20:46, Peter Zijlstra wrote:

I write a demo about this, which I described it as overflow.

#cat test.c

//test.c
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <assert.h>

int main(void)
{
ÂÂÂ long a = 1048576 * 9144968455305; /* shares = tg_shares * load */
ÂÂÂ unsigned long b = a;
ÂÂÂ //unsigned long e = 1048576 * 9144968455305;

ÂÂÂ printf("LONG_MAX = %ld, 0x%lx\n", LONG_MAX, LONG_MAX);
ÂÂÂ printf("ULONG_MAX = %lu, 0x%lx\n", ULONG_MAX, ULONG_MAX);

ÂÂÂ if (b > LONG_MAX)
ÂÂÂ ÂÂÂ printf("==overflow!!!==\n");ÂÂ // OVERFLOW
ÂÂÂ printf("a = %20ld,0x%016lx\n", a, a);
ÂÂÂ printf("b = %20lu,0x%016lx\n", b, b);

ÂÂÂ /* shares /= tg_weight */
ÂÂÂ printf("a/3 = 0x%016lx\n", (a / 3)); ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ // WRONG
ÂÂÂ printf("a/3 = 0x%016lx\n", (((unsigned long)a) / 3));ÂÂ // unsigned
ÂÂÂ printf("a/3 = 0x%016lx\n", (a / (unsigned long)3));ÂÂÂ // unsigned
ÂÂÂ printf("b/3 = 0x%016lx\n", b / 3);ÂÂÂ ÂÂÂ ÂÂÂ // unsigned

ÂÂÂ return EXIT_SUCCESS;
}



#./test

LONG_MAX = 9223372036854775807, 0x7fffffffffffffff
ULONG_MAX = 18446744073709551615, 0xffffffffffffffff
==overflow!!!==
a = -8857549630719655936,0x8513a98a48900000
b =Â 9589194442989895680,0x8513a98a48900000
a/3 = 0xd7068dd8c2daaaabÂÂÂÂ //Â WRONG
a/3 = 0x2c5be32e18300000
a/3 = 0x2c5be32e18300000
b/3 = 0x2c5be32e18300000



> On Sat, Apr 13, 2019 at 03:32:34AM +0000, Cheng Jian wrote:
> > group_share and group_runnable are tracked as 'unsigned long',
> > however some functions using them as 'long' which is ultimately
> > assigned back to 'unsigned long' variables in reweight_entity.
> >
> > Since there is not scope on using a different and signed type,
> > this change improves code consistency and avoids further type
> > conversions. More important, to prevent undefined behavior
> > caused by overflow.
> There is no undefined behaviour due to overflow.UBSAN is broken,
> upgrade to GCC8 or later.
>
> .


So, function calc_group_shares will return the wrong value.


```cpp

static long calc_group_shares(struct cfs_rq *cfs_rq)
{

ÂÂÂ // ......

ÂÂÂ shares = (tg_shares * load);
ÂÂÂ if (tg_weight)
ÂÂÂ ÂÂÂ shares /= tg_weight;
}

```


The same to calc_group_runnable.


Thanks.

ÂÂÂ CHENG Jian.