time: signed integer overflow in ktime_add_safe

[Dmitry Vyukov]

Hello,

UBSAN reports undefined behavior in ktime_add_safe:

UBSAN: Undefined behaviour in kernel/time/hrtimer.c:310:16
signed integer overflow:
9223372036854775807 + 100000000 cannot be represented in type 'long long int'
CPU: 3 PID: 26438 Comm: syzkaller_execu Tainted: G    B
4.4.0-rc3+ #141
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 0000000000000003 ffff88005a62f518 ffffffff82c65588 0000000041b58ab3
 ffffffff8769c1b6 ffffffff82c654d6 ffff88005a62f4e0 ffff88005a62f618
 0000000005f5e100 0000000000000001 ffff88005a62f520 ffffffff82d540c7
Call Trace:
 [<ffffffff82d54f69>] __ubsan_handle_add_overflow+0x2a/0x31 lib/ubsan.c:199
 [<     inline     >] ktime_add_safe kernel/time/hrtimer.c:310
 [<     inline     >] hrtimer_set_expires_range_ns include/linux/hrtimer.h:224
 [<ffffffff86820fce>] schedule_hrtimeout_range_clock+0x4ae/0x580
kernel/time/hrtimer.c:1731
 [<ffffffff868210ca>] schedule_hrtimeout_range+0x2a/0x40
kernel/time/hrtimer.c:1779
 [<ffffffff81833112>] poll_schedule_timeout+0xd2/0x180 fs/select.c:241
 [<     inline     >] do_poll fs/select.c:861
 [<ffffffff8183706b>] do_sys_poll+0xa4b/0xfc0 fs/select.c:911
 [<     inline     >] SYSC_ppoll fs/select.c:1019
 [<ffffffff81837d79>] SyS_ppoll+0x1a9/0x420 fs/select.c:991

On commit 31ade3b83e1821da5fbb2f11b5b3d4ab2ec39db8.

For:

ktime_t ktime_add_safe(const ktime_t lhs, const ktime_t rhs)
{
        ktime_t res = ktime_add(lhs, rhs);
        if (res.tv64 < 0 || res.tv64 < lhs.tv64 || res.tv64 < rhs.tv64)
                res = ktime_set(KTIME_SEC_MAX, 0);
        return res;
}

compiler is within its rights to assume that res.tv64 < rhs.tv64 is
always false (after inlining ktime_add). And compilers already do
this. For example, if you compile the following program with clang -O2
(clang version 3.8.0 (trunk 252895)), it does not print OVERFLOW:

#include <stdio.h>
#include <limits.h>
int main() {
        volatile int x = 0;
        int a = INT_MAX + x;
        int b = 1 + x;
        if (a + b < a)
                printf("OVERFLOW\n");
        return 0;
}

Proper overflow checking for signed integers is quite hairy and easy
to mess up. Do we have any helper functions for this? I've seen some
patches from Hannes, not sure what's their status.
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