On Thu, 2020-10-22 at 10:25 -0700, Eric Biggers wrote:
> 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...
>
That does seem less ambiguous.
Luo, if you're OK with that approach, I can just fix it up in-tree.
--
Jeff Layton <jlayton@xxxxxxxxxx>
