Subject: Re: [PATCH v14 1/8] uaccess: add generic fallback version of copy_mc_to_user()

From: Shuai Xue <[email-masked]>
Date: Wed, 27 May 2026 17:16:14 +0800

On 5/18/26 4:49 PM, Ruidong Tian wrote:
> From: Tong Tiangen <[email-masked]>
> 
> x86/powerpc has it's implementation of copy_mc_to_user(), we add generic
> fallback in include/linux/uaccess.h prepare for other architechures to
> enable CONFIG_ARCH_HAS_COPY_MC.
> 
> Signed-off-by: Tong Tiangen <[email-masked]>
> Acked-by: Michael Ellerman <[email-masked]>
> Reviewed-by: Mauro Carvalho Chehab <[email-masked]>
> Reviewed-by: Jonathan Cameron <[email-masked]>
> ---
>   arch/powerpc/include/asm/uaccess.h | 1 +
>   arch/x86/include/asm/uaccess.h     | 1 +
>   include/linux/uaccess.h            | 8 ++++++++
>   3 files changed, 10 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index e98c628e3899..073de098d45a 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -432,6 +432,7 @@ copy_mc_to_user(void __user *to, const void *from, unsigned long n)
>   
>   	return n;
>   }
> +#define copy_mc_to_user copy_mc_to_user
>   #endif
>   
>   extern size_t copy_from_user_flushcache(void *dst, const void __user *src, size_t size);
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 3a0dd3c2b233..308b0854d1d5 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -496,6 +496,7 @@ copy_mc_to_kernel(void *to, const void *from, unsigned len);
>   
>   unsigned long __must_check
>   copy_mc_to_user(void __user *to, const void *from, unsigned len);
> +#define copy_mc_to_user copy_mc_to_user
>   #endif
>   
>   /*
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 56328601218c..c53a65394f80 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -250,6 +250,14 @@ copy_mc_to_kernel(void *dst, const void *src, size_t cnt)
>   }
>   #endif
>   
> +#ifndef copy_mc_to_user
> +static inline unsigned long __must_check
> +copy_mc_to_user(void *dst, const void *src, size_t cnt)
> +{
The prototype of this generic fallback diverges from both copy_to_user()
and the existing arch implementations of copy_mc_to_user() in two
non-trivial ways:

   1. The destination pointer drops the __user annotation.

      copy_to_user() is declared as:

          unsigned long copy_to_user(void __user *to,
                                     const void *from,
                                     unsigned long n);

      and arch/x86/include/asm/uaccess.h declares the arch version as:

          unsigned long copy_mc_to_user(void __user *to,
                                        const void *from,
                                        unsigned long len);

      With this fallback typed as 'void *dst', sparse will silently
      accept a kernel pointer being passed in, and the address-space
      check is effectively lost for any architecture that ends up using
      the fallback. The annotation is the whole point of __user; please
      keep it.

   2. The size parameter is 'size_t cnt' instead of 'unsigned long n'.

      copy_to_user() and the x86/ppc copy_mc_to_user() implementations
      all use 'unsigned long'. On all supported arches this happens to
      be the same width as size_t, so it is not a functional bug, but
      the inconsistent signature is a foot-gun: when an arch later
      switches from the fallback to its own implementation, every
      caller signature has to be re-checked.

Please align the fallback prototype exactly with the arch versions:

     #ifndef copy_mc_to_user
     static inline unsigned long __must_check
     copy_mc_to_user(void __user *dst, const void *src, unsigned long cnt)
     {
         return copy_to_user(dst, src, cnt);
     }
     #endif

With that fixed:

Reviewed-by: Shuai Xue <[email-masked]>

Thanks.
Shuai