Re: [PATCH 00/28] iio: zero init stack with { } instead of memset()

Nicolas Frattaroli
2025-06-12T13:20:36Z

Hello,

I thought I'd chime in as someone uninvolved because this seemed
interesting.

On Thursday, 12 June 2025 11:17:52 Central European Summer Time Pavel Machek wrote:
> Hi!
>
> > Jonathan mentioned recently that he would like to get away from using
> > memset() to zero-initialize stack memory in the IIO subsystem. And we
> > have it on good authority that initializing a struct or array with = { }
> > is the preferred way to do this in the kernel [1]. So here is a series
> > to take care of that.
>
> 1) Is it worth the churn?
>
> 2) Will this fail to initialize padding with some obscure compiler?

as of right now, the only two C compilers that are supported are
GCC >= 8.1, and Clang >= 13.0.1. If anyone even manages to get the kernel
to finish a build with something else, I think the compiler not
implementing the C standard correctly is the least of their worries.

My bigger worry is that = { } is only guaranteed to be as correct as
memset on C23, and the kernel's standard right now is C11. For that
reason alone, I don't think memset should be moved away from for now,
unless someone can verify that every GCC release >= 8.1 and every
Clang release >= 13.0.1 does the right thing here regardless.

>
> 3) Why do you believe that {} is the preffered way? All we have is
> Kees' email that explains that = {} maybe works in configs he tested.

= { } is guaranteed to work in C23, as per the standard, but again we're
not on C23.

The reason to prefer this is likely that it's easier for static analysis
to see the struct as initialised, but that's me making assumptions here.

A more human-centric argument is that once we're on a C standards version
where = { } is guaranteed to be correct, then = { } is much more obviously
correct to a reader than a memset with a value and a size somewhere later
in the code. This argument is evident from the number of patches in this
series where the memset and the declaration are not in the same hunk.
That's the kind of stuff that keeps me awake at night, sweating profusely.

Kind regards,
Nicolas Frattaroli

>
> BR,
> Pavel
>
> > [1]:
> > https://lore.kernel.org/linux-iio/202505090942.48EBF01B@keescook/
>
>
>
> > ---
> > David Lechner (28):
> > iio: accel: adxl372: use = { } instead of memset()
> > iio: accel: msa311: use = { } instead of memset()
> > iio: adc: dln2-adc: use = { } instead of memset()
> > iio: adc: mt6360-adc: use = { } instead of memset()
> > iio: adc: rockchip_saradc: use = { } instead of memset()
> > iio: adc: rtq6056: use = { } instead of memset()
> > iio: adc: stm32-adc: use = { } instead of memset()
> > iio: adc: ti-ads1015: use = { } instead of memset()
> > iio: adc: ti-ads1119: use = { } instead of memset()
> > iio: adc: ti-lmp92064: use = { } instead of memset()
> > iio: adc: ti-tsc2046: use = { } instead of memset()
> > iio: chemical: scd4x: use = { } instead of memset()
> > iio: chemical: scd30: use = { } instead of memset()
> > iio: chemical: sunrise_co2: use = { } instead of memset()
> > iio: dac: ad3552r: use = { } instead of memset()
> > iio: imu: inv_icm42600: use = { } instead of memset()
> > iio: imu: inv_mpu6050: use = { } instead of memset()
> > iio: light: bh1745: use = { } instead of memset()
> > iio: light: ltr501: use = { } instead of memset()
> > iio: light: opt4060: use = { } instead of memset()
> > iio: light: veml6030: use = { } instead of memset()
> > iio: magnetometer: af8133j: use = { } instead of memset()
> > iio: pressure: bmp280: use = { } instead of memset()
> > iio: pressure: mpl3115: use = { } instead of memset()
> > iio: pressure: mprls0025pa: use = { } instead of memset()
> > iio: pressure: zpa2326: use = { } instead of memset()
> > iio: proximity: irsd200: use = { } instead of memset()
> > iio: temperature: tmp006: use = { } instead of memset()
> >
> > drivers/iio/accel/adxl372.c | 3 +--
> > drivers/iio/accel/msa311.c | 4 +---
> > drivers/iio/adc/dln2-adc.c | 4 +---
> > drivers/iio/adc/mt6360-adc.c | 3 +--
> > drivers/iio/adc/rockchip_saradc.c | 4 +---
> > drivers/iio/adc/rtq6056.c | 4 +---
> > drivers/iio/adc/stm32-adc.c | 3 +--
> > drivers/iio/adc/ti-ads1015.c | 4 +---
> > drivers/iio/adc/ti-ads1119.c | 4 +---
> > drivers/iio/adc/ti-lmp92064.c | 4 +---
> > drivers/iio/adc/ti-tsc2046.c | 3 +--
> > drivers/iio/chemical/scd30_core.c | 3 +--
> > drivers/iio/chemical/scd4x.c | 3 +--
> > drivers/iio/chemical/sunrise_co2.c | 6 ++----
> > drivers/iio/dac/ad3552r.c | 3 +--
> > drivers/iio/imu/inv_icm42600/inv_icm42600_accel.c | 5 ++---
> > drivers/iio/imu/inv_icm42600/inv_icm42600_gyro.c | 5 ++---
> > drivers/iio/imu/inv_mpu6050/inv_mpu_acpi.c | 4 +---
> > drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c | 6 ++----
> > drivers/iio/light/bh1745.c | 4 +---
> > drivers/iio/light/ltr501.c | 4 +---
> > drivers/iio/light/opt4060.c | 4 +---
> > drivers/iio/light/veml6030.c | 4 +---
> > drivers/iio/magnetometer/af8133j.c | 4 +---
> > drivers/iio/pressure/bmp280-core.c | 5 +----
> > drivers/iio/pressure/mpl3115.c | 3 +--
> > drivers/iio/pressure/mprls0025pa_i2c.c | 5 +----
> > drivers/iio/pressure/zpa2326.c | 4 +---
> > drivers/iio/proximity/irsd200.c | 3 +--
> > drivers/iio/temperature/tmp006.c | 4 +---
> > 30 files changed, 34 insertions(+), 85 deletions(-)
> > ---
> > base-commit: 4c6073fec2fee4827fa0dd8a4ab4e6f7bbc05ee6
> > change-id: 20250611-iio-zero-init-stack-with-instead-of-memset-0d12d41a7ecb
> >
> > Best regards,
>
>