On Thu, Oct 22, 2020 at 10:03:41AM +0800, Luo Meng wrote:
> When the sum of fl->fl_start and l->l_len overflows,
> UBSAN shows the following warning:
>
> UBSAN: Undefined behaviour in fs/locks.c:482:29
> signed integer overflow: 2 + 9223372036854775806
> cannot be represented in type 'long long int'
> Call Trace:
> __dump_stack lib/dump_stack.c:77 [inline]
> dump_stack+0xe4/0x14e lib/dump_stack.c:118
> ubsan_epilogue+0xe/0x81 lib/ubsan.c:161
> handle_overflow+0x193/0x1e2 lib/ubsan.c:192
> flock64_to_posix_lock fs/locks.c:482 [inline]
> flock_to_posix_lock+0x595/0x690 fs/locks.c:515
> fcntl_setlk+0xf3/0xa90 fs/locks.c:2262
> do_fcntl+0x456/0xf60 fs/fcntl.c:387
> __do_sys_fcntl fs/fcntl.c:483 [inline]
> __se_sys_fcntl fs/fcntl.c:468 [inline]
> __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:468
> do_syscall_64+0xc8/0x5a0 arch/x86/entry/common.c:293
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Fix it by moving -1 forward.
>
> Signed-off-by: Luo Meng <luomeng12@xxxxxxxxxx>
> ---
> fs/locks.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/locks.c b/fs/locks.c
> index 1f84a03601fe..8489787ca97e 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -542,7 +542,7 @@ static int flock64_to_posix_lock(struct file *filp, struct file_lock *fl,
> if (l->l_len > 0) {
> if (l->l_len - 1 > OFFSET_MAX - fl->fl_start)
> return -EOVERFLOW;
> - fl->fl_end = fl->fl_start + l->l_len - 1;
> + fl->fl_end = fl->fl_start - 1 + l->l_len;
>
Given what the bounds check just above does, wouldn't it make more sense to
parenthesize 'l->l_len - 1' instead? So:
fl->fl_end = fl->fl_start + (l->l_len - 1);
Also FWIW, the Linux kernel uses the -fwrapv compiler flag, so signed integer
overflow is defined. IMO it's still best avoided though...
- Eric
