[PATCH v1 1/1] regmap: spi: Fix potential off-by-one when calculating reserved size

[PATCH v1 1/1] regmap: spi: Fix potential off-by-one when calculating reserved size

[Andy Shevchenko]

If we ever meet a hardware that uses weird register bits and padding,
we may end up in off-by-one error since x/8 + y/8 might not be equal
to (x + y)/8 in some cases.

bits    pad   x/8+y/8 (x+y)/8
4..7    0..3    0       0 // x + y from 4 up to 7
4..7    4..7    0       1 // x + y from 8 up to 11
4..7    8..11   1       1 // x + y from 12 up to 15
8..15   0..7    1       1 // x + y from 8 up to 15
8..15   8..15   2       2 // x + y from 16 up to 23

Fix this by using (x+y)/8.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
---
 drivers/base/regmap/regmap-spi.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/base/regmap/regmap-spi.c b/drivers/base/regmap/regmap-spi.c
index 094cf2a2ca3c..14b1d88997cb 100644
--- a/drivers/base/regmap/regmap-spi.c
+++ b/drivers/base/regmap/regmap-spi.c
@@ -122,8 +122,7 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
 			return ERR_PTR(-ENOMEM);
 
 		max_msg_size = spi_max_message_size(spi);
-		reg_reserve_size = config->reg_bits / BITS_PER_BYTE
-				 + config->pad_bits / BITS_PER_BYTE;
+		reg_reserve_size = (config->reg_bits + config->pad_bits) / BITS_PER_BYTE;
 		if (max_size + reg_reserve_size > max_msg_size)
 			max_size -= reg_reserve_size;
 
-- 
2.45.2

Re: [PATCH v1 1/1] regmap: spi: Fix potential off-by-one when calculating reserved size

[Mark Brown]

On Wed, 05 Jun 2024 23:53:15 +0300, Andy Shevchenko wrote:
> If we ever meet a hardware that uses weird register bits and padding,
> we may end up in off-by-one error since x/8 + y/8 might not be equal
> to (x + y)/8 in some cases.
> 
> bits    pad   x/8+y/8 (x+y)/8
> 4..7    0..3    0       0 // x + y from 4 up to 7
> 4..7    4..7    0       1 // x + y from 8 up to 11
> 4..7    8..11   1       1 // x + y from 12 up to 15
> 8..15   0..7    1       1 // x + y from 8 up to 15
> 8..15   8..15   2       2 // x + y from 16 up to 23
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next

Thanks!

[1/1] regmap: spi: Fix potential off-by-one when calculating reserved size
      commit: d4ea1d504d2701ba04412f98dc00d45a104c52ab

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark