btrfs: fix signedness issue in min()

[PATCH] btrfs: fix signedness issue in min()

Posted by Arnd Bergmann 9 months, 1 week ago

From: Arnd Bergmann <arnd@arndb.de>

Comparing a u64 to an loff_t causes a warning in min()

fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
 2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
      |                           ^~~

Use min_t() instead.

Fixes: f286b1c72175 ("btrfs: prepare extent_io.c for future larger folio support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/btrfs/extent_io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index c2451194be66..88bced0bfa51 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2468,7 +2468,7 @@ void extent_write_locked_range(struct inode *inode, const struct folio *locked_f
 			continue;
 		}
 
-		cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
+		cur_end = min_t(u64, folio_pos(folio) + folio_size(folio) - 1, end);
 		cur_len = cur_end + 1 - cur;
 
 		ASSERT(folio_test_locked(folio));
-- 
2.39.5

Re: [PATCH] btrfs: fix signedness issue in min()

Posted by David Laight 9 months ago

On Fri, 14 Mar 2025 16:54:41 +0100
Arnd Bergmann <arnd@kernel.org> wrote:

> From: Arnd Bergmann <arnd@arndb.de>
> 
> Comparing a u64 to an loff_t causes a warning in min()
> 
> fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
> include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
> fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
>  2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
>       |                           ^~~
> 
> Use min_t() instead.

It would be slightly better to use min_unsigned() since, regardless of the types
involved, it can't discard significant bits.

OTOH the real problem here is that both folio_pos() and folio_size() return signed types.

	David

> 
> Fixes: f286b1c72175 ("btrfs: prepare extent_io.c for future larger folio support")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  fs/btrfs/extent_io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index c2451194be66..88bced0bfa51 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2468,7 +2468,7 @@ void extent_write_locked_range(struct inode *inode, const struct folio *locked_f
>  			continue;
>  		}
>  
> -		cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
> +		cur_end = min_t(u64, folio_pos(folio) + folio_size(folio) - 1, end);
>  		cur_len = cur_end + 1 - cur;
>  
>  		ASSERT(folio_test_locked(folio));

Re: [PATCH] btrfs: fix signedness issue in min()

Posted by David Sterba 9 months ago

On Mon, Mar 17, 2025 at 02:16:37PM +0000, David Laight wrote:
> On Fri, 14 Mar 2025 16:54:41 +0100
> Arnd Bergmann <arnd@kernel.org> wrote:
> 
> > From: Arnd Bergmann <arnd@arndb.de>
> > 
> > Comparing a u64 to an loff_t causes a warning in min()
> > 
> > fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
> > include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
> > fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
> >  2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
> >       |                           ^~~
> > 
> > Use min_t() instead.
> 
> It would be slightly better to use min_unsigned() since, regardless of the types
> involved, it can't discard significant bits.
> 
> OTOH the real problem here is that both folio_pos() and folio_size() return signed types.

folio_size() returns size_t:

static inline size_t folio_size(const struct folio *folio)
{
	return PAGE_SIZE << folio_order(folio);
}

Otherwise the min_t with force u64 is ok and lots of min() use (in
btrfs) was converted to the typed variant in case the types don't match.

Re: [PATCH] btrfs: fix signedness issue in min()

Posted by David Laight 9 months ago

On Mon, 17 Mar 2025 20:26:39 +0100
David Sterba <dsterba@suse.cz> wrote:

> On Mon, Mar 17, 2025 at 02:16:37PM +0000, David Laight wrote:
> > On Fri, 14 Mar 2025 16:54:41 +0100
> > Arnd Bergmann <arnd@kernel.org> wrote:
> >   
> > > From: Arnd Bergmann <arnd@arndb.de>
> > > 
> > > Comparing a u64 to an loff_t causes a warning in min()
> > > 
> > > fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
> > > include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
> > > fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
> > >  2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
> > >       |                           ^~~
> > > 
> > > Use min_t() instead.  
> > 
> > It would be slightly better to use min_unsigned() since, regardless of the types
> > involved, it can't discard significant bits.
> > 
> > OTOH the real problem here is that both folio_pos() and folio_size() return signed types.  
> 
> folio_size() returns size_t:
> 
> static inline size_t folio_size(const struct folio *folio)
> {
> 	return PAGE_SIZE << folio_order(folio);
> }
> 
> Otherwise the min_t with force u64 is ok and lots of min() use (in
> btrfs) was converted to the typed variant in case the types don't match.

That is just broken.
min_t(u64, x, y) is just min((u64)x, (u64)y) and you wouldn't do the
same casts anywhere else unless you really had to.
So you really shouldn't use min_t() unless there is no other way around the problem.

Ok (u64) are unlikely to be a problem, but there are plenty of places where
(u8) get used and can (and actually has) discard significant bits and cause bugs.

	David

Re: [PATCH] btrfs: fix signedness issue in min()

Posted by David Sterba 9 months, 1 week ago

On Fri, Mar 14, 2025 at 04:54:41PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Comparing a u64 to an loff_t causes a warning in min()
> 
> fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
> include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
> fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
>  2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
>       |                           ^~~
> 
> Use min_t() instead.
> 
> Fixes: f286b1c72175 ("btrfs: prepare extent_io.c for future larger folio support")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Thanks, there was another report and the upcoming for-next will have it
fixed.