Subject: Re: [PATCH 1/5] uaccess: Add masked_user_{read/write}_access_begin

From: David Laight <[email-masked]>
Date: Sun, 22 Jun 2025 17:35:54 +0100

On Sun, 22 Jun 2025 11:52:39 +0200
Christophe Leroy <[email-masked]> wrote:

> Allthough masked_user_access_begin() seems to only be used when reading
> data from user at the moment, introduce masked_user_read_access_begin()
> and masked_user_write_access_begin() in order to match
> user_read_access_begin() and user_write_access_begin().
> 
> Have them default to masked_user_access_begin() when they are
> not defined.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  fs/select.c             | 2 +-
>  include/linux/uaccess.h | 8 ++++++++
>  kernel/futex/futex.h    | 4 ++--
>  lib/strncpy_from_user.c | 2 +-
>  lib/strnlen_user.c      | 2 +-
>  5 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index 9fb650d03d52..d8547bedf5eb 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -777,7 +777,7 @@ static inline int get_sigset_argpack(struct sigset_argpack *to,
>  	// the path is hot enough for overhead of copy_from_user() to matter
>  	if (from) {
>  		if (can_do_masked_user_access())
> -			from = masked_user_access_begin(from);
> +			from = masked_user_read_access_begin(from);
>  		else if (!user_read_access_begin(from, sizeof(*from)))
>  			return -EFAULT;
>  		unsafe_get_user(to->p, &from->p, Efault);
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 7c06f4795670..682a0cd2fe51 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -41,6 +41,14 @@

>  #ifdef masked_user_access_begin
>   #define can_do_masked_user_access() 1
>  #else
>   #define can_do_masked_user_access() 0
>   #define masked_user_access_begin(src) NULL
>   #define mask_user_address(src) (src)
>  #endif
>  
> +#ifndef masked_user_write_access_begin
> +#define masked_user_write_access_begin masked_user_access_begin
> +#endif
> +#ifndef masked_user_read_access_begin
> +#define masked_user_read_access_begin masked_user_access_begin
> +#endif

I think that needs merging with the bit above.
Perhaps generating something like:

#ifdef masked_user_access_begin
#define masked_user_read_access_begin masked_user_access_begin
#define masked_user_write_access_begin masked_user_access_begin
#endif

#ifdef masked_user_read_access_begin
  #define can_do_masked_user_access() 1
#else
  #define can_do_masked_user_access() 0
  #define masked_user_read_access_begin(src) NULL
  #define masked_user_write_access_begin(src) NULL
  #define mask_user_address(src) (src)
#endif

Otherwise you'll have to #define masked_user_access_begin even though
it is never used.

Two more patches could change x86-64 to define both and then remove
the 'then unused' first check - but that has to be for later.

	David