[PATCH] cxl/region: Use do_div() for 64-bit modulo operation

[PATCH] cxl/region: Use do_div() for 64-bit modulo operation

[Alison Schofield]

div64_u64_rem() was the wrong choice for doing a modulo operation
and it was used incorrectly, causing a kernel oops by passing NULL
as the remainder parameter. Replace it with the do_div() helper
that does the intended math (gran_offset = offset % gran) and is
architecture safe.

This bug appeared during testing of unaligned address translations.
The visibility to userspace would be limited to folks doing poison
injection or clear by HPA on unaligned regions.

Fixes: 78b50b598462 ("cxl/region: Translate HPA to DPA and memdev in unaligned regions")
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---
 drivers/cxl/core/region.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index d5979000fba1..96888d87a8df 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -3324,6 +3324,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
 	u64 interleave_width, interleave_index;
 	u64 gran, gran_offset, dpa_offset;
 	u64 hpa = p->res->start + offset;
+	u64 tmp = offset;
 
 	/*
 	 * Unaligned addresses are not algebraically invertible. Calculate
@@ -3333,7 +3334,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
 	gran = cxld->interleave_granularity;
 	interleave_width = gran * cxld->interleave_ways;
 	interleave_index = div64_u64(offset, interleave_width);
-	gran_offset = div64_u64_rem(offset, gran, NULL);
+	gran_offset = do_div(tmp, gran);
 
 	dpa_offset = interleave_index * gran + gran_offset;
 

base-commit: 78b50b5984629d362f826e9615ce4599f429716e
-- 
2.37.3

Re: [PATCH] cxl/region: Use do_div() for 64-bit modulo operation

[Dave Jiang]

On 1/16/26 9:47 PM, Alison Schofield wrote:
> div64_u64_rem() was the wrong choice for doing a modulo operation
> and it was used incorrectly, causing a kernel oops by passing NULL
> as the remainder parameter. Replace it with the do_div() helper
> that does the intended math (gran_offset = offset % gran) and is
> architecture safe.
> 
> This bug appeared during testing of unaligned address translations.
> The visibility to userspace would be limited to folks doing poison
> injection or clear by HPA on unaligned regions.
> 
> Fixes: 78b50b598462 ("cxl/region: Translate HPA to DPA and memdev in unaligned regions")
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>

Thanks Alison. I'll get this applied to next
> ---
>  drivers/cxl/core/region.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index d5979000fba1..96888d87a8df 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3324,6 +3324,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
>  	u64 interleave_width, interleave_index;
>  	u64 gran, gran_offset, dpa_offset;
>  	u64 hpa = p->res->start + offset;
> +	u64 tmp = offset;
>  
>  	/*
>  	 * Unaligned addresses are not algebraically invertible. Calculate
> @@ -3333,7 +3334,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
>  	gran = cxld->interleave_granularity;
>  	interleave_width = gran * cxld->interleave_ways;
>  	interleave_index = div64_u64(offset, interleave_width);
> -	gran_offset = div64_u64_rem(offset, gran, NULL);
> +	gran_offset = do_div(tmp, gran);
>  
>  	dpa_offset = interleave_index * gran + gran_offset;
>  
> 
> base-commit: 78b50b5984629d362f826e9615ce4599f429716e

Re: [PATCH] cxl/region: Use do_div() for 64-bit modulo operation

[Dave Jiang]

On 1/16/26 9:47 PM, Alison Schofield wrote:
> div64_u64_rem() was the wrong choice for doing a modulo operation
> and it was used incorrectly, causing a kernel oops by passing NULL
> as the remainder parameter. Replace it with the do_div() helper
> that does the intended math (gran_offset = offset % gran) and is
> architecture safe.
> 
> This bug appeared during testing of unaligned address translations.
> The visibility to userspace would be limited to folks doing poison
> injection or clear by HPA on unaligned regions.
> 
> Fixes: 78b50b598462 ("cxl/region: Translate HPA to DPA and memdev in unaligned regions")
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>

Applied to cxl/next
345f23df5d08fe58fb865cd17b696cd345455224

> ---
>  drivers/cxl/core/region.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index d5979000fba1..96888d87a8df 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3324,6 +3324,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
>  	u64 interleave_width, interleave_index;
>  	u64 gran, gran_offset, dpa_offset;
>  	u64 hpa = p->res->start + offset;
> +	u64 tmp = offset;
>  
>  	/*
>  	 * Unaligned addresses are not algebraically invertible. Calculate
> @@ -3333,7 +3334,7 @@ static int unaligned_region_offset_to_dpa_result(struct cxl_region *cxlr,
>  	gran = cxld->interleave_granularity;
>  	interleave_width = gran * cxld->interleave_ways;
>  	interleave_index = div64_u64(offset, interleave_width);
> -	gran_offset = div64_u64_rem(offset, gran, NULL);
> +	gran_offset = do_div(tmp, gran);
>  
>  	dpa_offset = interleave_index * gran + gran_offset;
>  
> 
> base-commit: 78b50b5984629d362f826e9615ce4599f429716e