linux-kernel - Re: [PATCH] xfs: fix diff_two_keys calculation for cnt btree

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Message-ID: <kip7kv57wpvhft65vsbrddakjva66nyld7i2lrp6cnax4t6wbw@ywc6e6yvzqwl>
Date: Sun, 27 Apr 2025 10:25:23 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Fedor Pchelkin <pchelkin@...ras.ru>
Cc: "Darrick J. Wong" <djwong@...nel.org>, 
	Carlos Maiolino <cem@...nel.org>, Chandan Babu R <chandanbabu@...nel.org>, 
	Brian Foster <bfoster@...hat.com>, linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	lvc-project@...uxtesting.org, Alexey Nepomnyashih <sdl@...ct.ru>, stable@...r.kernel.org, 
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] xfs: fix diff_two_keys calculation for cnt btree

On Sun, Apr 27, 2025 at 02:56:39PM +0300, Fedor Pchelkin wrote:
> Hi,
> 
> On Sat, 26. Apr 08:03, Darrick J. Wong wrote:
> > On Sat, Apr 26, 2025 at 04:42:31PM +0300, Fedor Pchelkin wrote:
> > > Currently the difference is computed on 32-bit unsigned values although
> > > eventually it is stored in a variable of int64_t type. This gives awkward
> > > results, e.g. when the diff _should_ be negative, it is represented as
> > > some large positive int64_t value.
> > > 
> > > Perform the calculations directly in int64_t as all other diff_two_keys
> > > routines actually do.
> > > 
> > > Found by Linux Verification Center (linuxtesting.org) with Svace static
> > > analysis tool.
> > > 
> > > Fixes: 08438b1e386b ("xfs: plumb in needed functions for range querying of the freespace btrees")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Fedor Pchelkin <pchelkin@...ras.ru>
> > > ---
> > >  fs/xfs/libxfs/xfs_alloc_btree.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
> > > index a4ac37ba5d51..b3c54ae90e25 100644
> > > --- a/fs/xfs/libxfs/xfs_alloc_btree.c
> > > +++ b/fs/xfs/libxfs/xfs_alloc_btree.c
> > > @@ -238,13 +238,13 @@ xfs_cntbt_diff_two_keys(
> > >  	ASSERT(!mask || (mask->alloc.ar_blockcount &&
> > >  			 mask->alloc.ar_startblock));
> > >  
> > > -	diff =  be32_to_cpu(k1->alloc.ar_blockcount) -
> > > -		be32_to_cpu(k2->alloc.ar_blockcount);
> > > +	diff = (int64_t)be32_to_cpu(k1->alloc.ar_blockcount) -
> > > +			be32_to_cpu(k2->alloc.ar_blockcount);
> > 
> > Perhaps it's time to hoist cmp_int to include/ and refactor all these
> > things to use it?
> > 
> > #define cmp_int(l, r)          ((l > r) - (l < r))
> > 
> > --D
> > 
> 
> Thanks, that would be worth it, I think. Though the current xfs
> ***diff_two_keys() implementations try to compute and return the actual
> difference between two values, not the result of their comparison. Now
> looking at diff_two_keys() use cases, I see only the latter one is needed
> anyway so a good bit to refactor.
> 
> 
> The thing I'm pondering over now is whether the macro in its current
> form is okay to move up to include/. There is no argument restrictions and
> typechecking intended to catch up obviously misleading usage patterns
> though we'd need some if this is hoisted to a generic header and exported
> for potential use by others?
> 
> There are four places where cmp_int is defined at the moment:
> - bcachefs
> - md/bcache
> - xfs_zone_gc
> - pipe.c
> 
> bcachefs is the largest user having all kinds of different arguments
> providing to the macro, bitfields included. It also has several rather
> generic wrappers, like u64_cmp, unsigned_cmp, u8_cmp, cmp_le32 and
> others..
> 
> AF_UNIX code even has
> 
> 	#define cmp_ptr(l, r)	(((l) > (r)) - ((l) < (r)))
> 
> for pointer comparisons.
> 
> 
> So in my opinion we'd probably need to come up with something like a new
> include/linux/cmp.h header where all this stuff will be gathered in a
> generic way.
> 
> Any objections/suggestions on that? Or just moving
> 
> 	#define cmp_int(l, r)          ((l > r) - (l < r))

Ack. It avoids underflow issues when using a subtract and gcc generates
good code for it.