Re: [PATCH] xfs: avoid underflow in xfs_ioc_trim()

Dave Chinner
2012-10-10T23:21:12Z

On Wed, Oct 10, 2012 at 09:50:38AM +0200, Lukáš Czerner wrote:
> On Wed, 10 Oct 2012, Dave Chinner wrote:
>
> > Date: Wed, 10 Oct 2012 06:39:08 +1100
> > From: Dave Chinner <david@fromorbit.com>
> > To: Lukas Czerner <lczerner@redhat.com>
> > Cc: xfs@oss.sgi.com
> > Subject: Re: [PATCH] xfs: avoid underflow in xfs_ioc_trim()
> >
> > On Tue, Oct 09, 2012 at 02:15:45PM +0200, Lukas Czerner wrote:
> > > Currently if len argument in xfs_ioc_trim() is smaller than one BB
> > > (basic block) the 'end' variable underflow. Avoid that by bailing out if
> > > len is smaller than BB.
> > >
> > > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > > ---
> > > fs/xfs/xfs_discard.c | 7 ++++++-
> > > 1 files changed, 6 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
> > > index 69cf4fc..54dc58a 100644
> > > --- a/fs/xfs/xfs_discard.c
> > > +++ b/fs/xfs/xfs_discard.c
> > > @@ -183,8 +183,12 @@ xfs_ioc_trim(
> > > range.minlen > XFS_FSB_TO_B(mp, XFS_ALLOC_AG_MAX_USABLE(mp)))
> > > return -XFS_ERROR(EINVAL);
> > >
> > > + end = BTOBBT(range.len);
> > > + if (0 == end)
> > > + goto out;
> >
> > Uggh. "if (end == 0)", please.
>
> Not a Star Wars fan then ? ;). Ok, I'll change that.

I might be, but I don't get the reference - something to do with the
second moon of Endor, or just letting the wookie win so you don't
come to an armless end? :)

As it is, I don't like reverse logic notation because it makes code
strange and unreadable. It doesn't provide any real benefit because
the compiler will warn about typos that result in assignments in
such statements (i.e. if (end = 0)). Hence writing code so that the
compilation will fail if you make a typo isn't necessary - Clear,
consistent and easily readable code is much more important....

> > > +
> > > start = BTOBB(range.start);
> > > - end = start + BTOBBT(range.len) - 1;
> > > + end += start - 1;
> >
> > Better would be to check if end <= start. That way it also catches
> > start+len overflows.
>
> That's not possible. Even if range.start and range.len would be
> 2^64-1 (which is not possible for range.start) we always do the
> BTOBB conversion and ((2^64-1) / 512) * 2 always fits the _s64
> type.

Regardless, checking that the end of the range is after the start is a
much more robust check than individual underflow/overflow tests. We
do that all over the place in XFS when converting byte ranges to
block based start/end pairs.

> > > minlen = BTOBB(max_t(u64, granularity, range.minlen));
> > >
> > > if (end > XFS_FSB_TO_BB(mp, mp->m_sb.sb_dblocks) - 1)
> > > @@ -203,6 +207,7 @@ xfs_ioc_trim(
> > > if (last_error)
> > > return last_error;
> > >
> > > +out:
> > > range.len = XFS_FSB_TO_B(mp, blocks_trimmed);
> > > if (copy_to_user(urange, &range, sizeof(range)))
> > > return -XFS_ERROR(EFAULT);
> >
> > I think it should return EINVAL, not silently do nothing. If the
> > user application uses a loop that increments start/len based on the
> > returned amount of blocks trimmed, returning zero could send it into
> > an endless loop.
>
> That's not what the application would do. At least it would not set
> len to what's returned as number of blocs discarded, it does not
> make sense.

Oh, right, my mistake.

> However if user specify length smaller than what we're able to
> discard (in xfs it is BBSIZE if I am not mistaken?), then it
> probably make sense to return -EINVAL. It is similar situation of
> minlen, where we return -EINVAL if it is bigger than AG. However
> this will make it to fail at different threshold on different file
> system / block sizes so I am on the fence about it. What do you
> think, is it worth it ?

Actually, the minimum length that can be discarded is the filesystem
block size, and applications can get that easily enough from
stat(2). IOWs, we probably should be rejecting minlen < 1 FSB,
range < 1 FSB with EINVAL....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com