[PATCH 0/2] iio: consumers: ensure read buffers for labels and ext_info are page aligned

[Matteo Martelli <matteomartelli3@gmail.com>]

This patch series is a follow up of [1], where I described an issue
related to the fact that devm_k*alloc() functions do not provide
alignment to the requested power of two size, leading to potential
errors when used together with sysfs_emit* helpers which expect
page-aligned buffers.

From that discussion, it became clear that this issue currently only
affects iio consumer drivers, as they can directly access providers
attribute formatted using sysfs_emit* helpers. For instance, the iio-mux
driver allocates a buffer with devm_kzalloc(PAGE_SIZE) to read provider
ext_info attributes, which could be handled via sysfs_emit* helpers.
This leads to an error in the provider ext_info read callback since the
allocated buffer is not page-aligned.

Summary:
- Patch 1: harden the consumers APIs to ensure read buffers are page
  aligned for attributes which could be formatted with sysfs_emit*
  helpers by the providers. Currently labels and ext_info attributes.

- Patch 2: fix iio-mux consumer by switching from devm_kzalloc to
  kzalloc for the ext_info buffer.

Tested with the iio-mux consumer driver alongside the pac1921 driver,
which provides an ext_info attribute (the shunt resistor in this case).
After applying patch-1, the error was detected during the iio-mux probe
rather than in the pac1921 ext_info read callback. After applying
patch-2, the error condition no longer occurred. Additionally, the extra
check in iio_read_channel_label() was tested with the iio_hwmon consumer
driver temporarily modified to allocate the buffer for retrieving
provider labels using devm_kzalloc(PAGE_SIZE) instead of
devm_get_free_pages(). The error was correctly detected during the
iio_hwmon probe when attempting to retrieve pac1921 channel labels.

[1]: https://lore.kernel.org/all/c486a1cf98a8b9ad093270543e8d2007@gmail.com

Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
---
Matteo Martelli (2):
      iio: consumers: ensure read buffers for labels and ext_info are page aligned
      iio: iio-mux: kzalloc instead of devm_kzalloc to ensure page alignment

 drivers/iio/inkern.c              | 11 +++++
 drivers/iio/multiplexer/iio-mux.c | 84 +++++++++++++++++++++------------------
 include/linux/iio/consumer.h      |  4 +-
 3 files changed, 59 insertions(+), 40 deletions(-)
---
base-commit: 20fd1383cd616d61b2a79967da1221dc6cfb8430
change-id: 20241127-iio-kmalloc-align-ac7bfcfe5ec0

Best regards,
-- 
Matteo Martelli <matteomartelli3@gmail.com>