[PATCH AUTOSEL 6.16 32/85] iio: adc: ad_sigma_delta: don't overallocate scan buffer

[Sasha Levin <sashal@kernel.org>]

From: David Lechner <dlechner@baylibre.com>

[ Upstream commit 5a2f15c5a8e017d0951e6dc62aa7b5b634f56881 ]

Fix overallocating the size of the scan buffer by converting bits to
bytes. The size is meant to be in bytes, so scanbits needs to be
divided by 8.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250701-iio-adc-ad7173-add-spi-offload-support-v3-1-42abb83e3dac@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**Backport Status: YES**

This commit fixes a clear memory overallocation bug in the
ad_sigma_delta ADC driver that has been present since kernel v5.19. The
analysis shows:

1. **Bug Description**: The code incorrectly calculates buffer size by
   treating `storagebits` (which is in bits) as if it were in bytes. The
   `storagebits` field in `struct iio_scan_type` represents the number
   of bits needed to store a sample, typically 8, 16, 24, or 32 bits.

2. **Impact**: The bug causes the driver to allocate 8x more memory than
   needed. For example:
   - If `storagebits = 16` (2 bytes per sample) and `slot = 4`
   - Buggy calculation: `ALIGN(4 * 16, 8) = 64 bytes`
   - Correct calculation: `ALIGN(4 * 16 / 8, 8) = 8 bytes`

   This wastes kernel memory and could potentially lead to memory
exhaustion in systems with many IIO devices.

3. **Fix Quality**: The fix is minimal and correct - simply dividing by
   8 to convert bits to bytes. This is consistent with how `storagebits`
   is used throughout the IIO subsystem, as evidenced by the grep
   results showing `sizeof(u32) * 8` assignments.

4. **Affected Versions**: The bug was introduced in commit 8bea9af887de4
   ("iio: adc: ad_sigma_delta: Add sequencer support") which first
   appeared in v5.19 and is present in all releases since then (v5.19,
   v6.0, v6.1, v6.10, v6.11).

5. **Stable Criteria Met**:
   - ✓ Fixes a real bug (memory overallocation)
   - ✓ Small, contained fix (single line change)
   - ✓ No architectural changes
   - ✓ Low risk of regression
   - ✓ Bug affects users (wastes memory)
   - ✓ Clear and obvious fix

The commit should be backported to all stable kernels from v5.19 onwards
to fix this memory waste issue in the industrial I/O subsystem.

 drivers/iio/adc/ad_sigma_delta.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index 4c5f8d29a559..6b3ef7ef403e 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -489,7 +489,7 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
 			return ret;
 	}
 
-	samples_buf_size = ALIGN(slot * indio_dev->channels[0].scan_type.storagebits, 8);
+	samples_buf_size = ALIGN(slot * indio_dev->channels[0].scan_type.storagebits / 8, 8);
 	samples_buf_size += sizeof(int64_t);
 	samples_buf = devm_krealloc(&sigma_delta->spi->dev, sigma_delta->samples_buf,
 				    samples_buf_size, GFP_KERNEL);
-- 
2.39.5