Re: [RFC v2 12/14] xfs: make the calculation generic in xfs_sb_validate_fsb_count()

[Pankaj Raghav (Samsung)]

On Tue, Feb 13, 2024 at 08:26:11AM -0800, Darrick J. Wong wrote:
> On Tue, Feb 13, 2024 at 10:37:11AM +0100, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> > 
> > Instead of assuming that PAGE_SHIFT is always higher than the blocklog,
> > make the calculation generic so that page cache count can be calculated
> > correctly for LBS.
> > 
> > Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> > ---
> >  fs/xfs/xfs_mount.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> > index aabb25dc3efa..bfbaaecaf668 100644
> > --- a/fs/xfs/xfs_mount.c
> > +++ b/fs/xfs/xfs_mount.c
> > @@ -133,9 +133,13 @@ xfs_sb_validate_fsb_count(
> >  {
> >  	ASSERT(PAGE_SHIFT >= sbp->sb_blocklog);
> >  	ASSERT(sbp->sb_blocklog >= BBSHIFT);
> > +	unsigned long mapping_count;
> 
> Nit: indenting
> 
> 	unsigned long		mapping_count;

I will add this in the next revision.
> 
> > +	uint64_t bytes = nblocks << sbp->sb_blocklog;
> 
> What happens if someone feeds us a garbage fs with sb_blocklog > 64?
> Or did we check that previously, so an overflow isn't possible?
> 
I was thinking of possibility of an overflow but at the moment the 
blocklog is capped at 16 (65536 bytes) right? mkfs refuses any block
sizes more than 64k. And we have check for this in xfs_validate_sb_common()
in the kernel, which will catch it before this happens?

> > +
> > +	mapping_count = bytes >> PAGE_SHIFT;
> 
> Does this result in truncation when unsigned long is 32 bits?

Ah, good point. So it is better to use uint64_t for mapping_count as
well to be on the safe side?

> 
> --D
> 
> >  
> >  	/* Limited by ULONG_MAX of page cache index */
> > -	if (nblocks >> (PAGE_SHIFT - sbp->sb_blocklog) > ULONG_MAX)
> > +	if (mapping_count > ULONG_MAX)
> >  		return -EFBIG;
> >  	return 0;
> >  }
> > -- 
> > 2.43.0
> > 
> >