Re: [PATCH] libfs: fix accidental overflow in offset calculation

Justin Stitt
2024-05-10T03:26:13Z

Hi,

On Fri, May 10, 2024 at 02:04:51AM +0100, Al Viro wrote:
> On Fri, May 10, 2024 at 01:49:06AM +0100, Al Viro wrote:
> > On Fri, May 10, 2024 at 12:35:51AM +0000, Justin Stitt wrote:
> > > @@ -147,7 +147,9 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
> > > struct dentry *dentry = file->f_path.dentry;
> > > switch (whence) {
> > > case 1:
> > > - offset += file->f_pos;
> > > + /* cannot represent offset with loff_t */
> > > + if (check_add_overflow(offset, file->f_pos, &offset))
> > > + return -EOVERFLOW;
> >
> > Instead of -EINVAL it correctly returns in such cases? Why?
>
> We have file->f_pos in range 0..LLONG_MAX. We are adding a value in
> range LLONG_MIN..LLONG_MAX. The sum (in $\Bbb Z$) cannot be below
> LLONG_MIN or above 2 * LLONG_MAX, so if it can't be represented by loff_t,
> it must have been in range LLONG_MAX + 1 .. 2 * LLONG_MAX. Result of
> wraparound would be equal to that sum - 2 * LLONG_MAX - 2, which is going
> to be in no greater than -2. We will run
> fallthrough;
> case 0:
> if (offset >= 0)
> break;
> fallthrough;
> default:
> return -EINVAL;
> and fail with -EINVAL.

This feels like a case of accidental correctness. You demonstrated that
even with overflow we end up going down a control path that returns an
error code so all is good. However, I think finding the solution
shouldn't require as much mental gymnastics. We clearly don't want our
file offsets to wraparound and a plain-and-simple check for that lets
readers of the code understand this.

>
> Could you explain why would -EOVERFLOW be preferable and why should we
> engage in that bit of cargo cult?

I noticed some patterns in fs/ regarding -EOVERFLOW and thought this was
a good application of this particular error code.

Some examples:

read_write.c::do_sendfile()
...
if (unlikely(pos + count > max)) {
retval = -EOVERFLOW;
if (pos >= max)
goto fput_out;
count = max - pos;
}

read_write.c::generic_copy_file_checks()
...
/* Ensure offsets don't wrap. */
if (pos_in + count < pos_in || pos_out + count < pos_out)
return -EOVERFLOW;

... amongst others.

So to answer your question, I don't have strong feelings about what the
error code should be; I was just attempting to match patterns I had seen
within this section of the codebase when handling overflow/wraparound.

If -EOVERFLOW is technically incorrect or if it is just bad style, I'm
happy to send a new version swapping it over to -EINVAL

Thanks
Justin