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

[Christophe Leroy]

Le 22/06/2025 à 18:35, David Laight a écrit :
> On Sun, 22 Jun 2025 11:52:39 +0200
> Christophe Leroy <christophe.leroy@xxxxxxxxxx> 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 <christophe.leroy@xxxxxxxxxx>
> > ---
> >   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.

I'm not sure I understand what you mean.

masked_user_access_begin() is used, for instance in 
arch/x86/include/asm/futex.h so it will remain.

masked_user_access_begin() is the analogy of user_access_begin(), it 
starts a read-write user access and is worth it.

> 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.

Christophe