Re: [PATCH v2 03/13] s390/chsc_sch: Use kzalloc() for CHSC requests

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Hardware Alignment Violation: Replacing get_zeroed_page() with kzalloc(PAGE_SIZE) drops the strict 4K page-alignment guarantee required by the CHSC hardware instruction.
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commit 605eecc8759317b0b4a005d290e0db9d7f1fd0a3
Author: Mike Rapoport (Microsoft) <rppt@kernel.org>

s390/chsc_sch: Use kzalloc() for CHSC requests

This commit replaces get_zeroed_page() and free_page() with kzalloc() and
kfree() for CHSC request and response areas. The goal is to simplify the
code and use a more standard API since the memory is assumed not to require
page allocator specifics.

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com

> diff --git a/drivers/s390/cio/chsc_sch.c b/drivers/s390/cio/chsc_sch.c
> --- a/drivers/s390/cio/chsc_sch.c
> +++ b/drivers/s390/cio/chsc_sch.c
> @@ -292,7 +292,7 @@ static int chsc_ioctl_start(void __user *user_area)
>  	if (!css_general_characteristics.dynio)
>  		/* It makes no sense to try. */
>  		return -EOPNOTSUPP;
> -	chsc_area = (void *)get_zeroed_page(GFP_DMA | GFP_KERNEL);
> +	chsc_area = kzalloc(PAGE_SIZE, GFP_DMA | GFP_KERNEL);

[Severity: High]
Does this kzalloc() break the strict 4KB page alignment required by the
CHSC hardware instruction?

When slab debugging (like redzoning) is enabled, the allocated memory block
can be shifted by the redzone size, breaking page alignment.

Passing this unaligned pointer to the CHSC instruction unconditionally
triggers a hardware specification exception (program check) which breaks
channel subsystem I/O functionality.

[ ... ]

> @@ -340,7 +340,7 @@ static int chsc_ioctl_on_close_set(void __user *user_area)
>  		ret = -ENOMEM;
>  		goto out_unlock;
>  	}
> -	on_close_chsc_area = (void *)get_zeroed_page(GFP_DMA | GFP_KERNEL);
> +	on_close_chsc_area = kzalloc(PAGE_SIZE, GFP_DMA | GFP_KERNEL);

[Severity: High]
Will this also lose the hardware-required page alignment for the on-close
CHSC command area?

[ ... ]

> @@ -392,7 +392,7 @@ static int chsc_ioctl_start_sync(void __user *user_area)
>  	struct chsc_sync_area *chsc_area;
>  	int ret, ccode;
>  
> -	chsc_area = (void *)get_zeroed_page(GFP_KERNEL | GFP_DMA);
> +	chsc_area = kzalloc(PAGE_SIZE, GFP_KERNEL | GFP_DMA);

[Severity: High]
Is there a risk of a hardware exception here as well due to the same
alignment issue in chsc_ioctl_start_sync()?

[ ... ]

> @@ -438,7 +438,7 @@ static int chsc_ioctl_info_channel_path(void __user *user_cd)
>  		u8 data[PAGE_SIZE - 20];
>  	} __attribute__ ((packed)) *scpcd_area;
>  
> -	scpcd_area = (void *)get_zeroed_page(GFP_KERNEL | GFP_DMA);
> +	scpcd_area = kzalloc(PAGE_SIZE, GFP_KERNEL | GFP_DMA);

[Severity: High]
Could this silent drop of the 4K page-alignment guarantee cause I/O failure
for the command block allocation in chsc_ioctl_info_channel_path()?

[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20260910-s390-cio-ready-v2-0-e931086fdde1@kernel.org?part=3