[v3] sysfs: prepare the constification of struct attribute

[PATCH v3 1/7] sysfs: attribute_group: allow registration of const attribute

Posted by Thomas Weißschuh 1 month, 3 weeks ago

To be able to constify instances of struct attribute it has to be
possible to add them to struct attribute_group.
The current type of the attrs member however is not compatible with that.
Introduce a union that allows registration of both const and non-const
attributes to enable a piecewise transition.
As both union member types are compatible no logic needs to be adapted.

Technically it is now possible register a const struct
attribute and receive it as mutable pointer in the callbacks.
This is a soundness issue.
But this same soundness issue already exists today in
sysfs_create_file().
Also the struct definition and callback implementation are always
closely linked and are meant to be moved to const in lockstep.

Similar to commit 906c508afdca ("sysfs: attribute_group: allow registration of const bin_attribute")

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 include/linux/sysfs.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index f418aae4f1134f8126783d9e8eb575ba4278e927..a47092e837d9eb014894d1f7e49f0fd0f9a2e350 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -105,7 +105,10 @@ struct attribute_group {
 	size_t			(*bin_size)(struct kobject *,
 					    const struct bin_attribute *,
 					    int);
-	struct attribute	**attrs;
+	union {
+		struct attribute	**attrs;
+		const struct attribute	*const *attrs_new;
+	};
 	union {
 		const struct bin_attribute	*const *bin_attrs;
 		const struct bin_attribute	*const *bin_attrs_new;

-- 
2.50.1

Re: [PATCH v3 1/7] sysfs: attribute_group: allow registration of const attribute

Posted by Greg Kroah-Hartman 1 month, 2 weeks ago

On Mon, Aug 11, 2025 at 11:14:27AM +0200, Thomas Weißschuh wrote:
> To be able to constify instances of struct attribute it has to be
> possible to add them to struct attribute_group.
> The current type of the attrs member however is not compatible with that.
> Introduce a union that allows registration of both const and non-const
> attributes to enable a piecewise transition.
> As both union member types are compatible no logic needs to be adapted.
> 
> Technically it is now possible register a const struct
> attribute and receive it as mutable pointer in the callbacks.
> This is a soundness issue.
> But this same soundness issue already exists today in
> sysfs_create_file().
> Also the struct definition and callback implementation are always
> closely linked and are meant to be moved to const in lockstep.
> 
> Similar to commit 906c508afdca ("sysfs: attribute_group: allow registration of const bin_attribute")
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  include/linux/sysfs.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> index f418aae4f1134f8126783d9e8eb575ba4278e927..a47092e837d9eb014894d1f7e49f0fd0f9a2e350 100644
> --- a/include/linux/sysfs.h
> +++ b/include/linux/sysfs.h
> @@ -105,7 +105,10 @@ struct attribute_group {
>  	size_t			(*bin_size)(struct kobject *,
>  					    const struct bin_attribute *,
>  					    int);
> -	struct attribute	**attrs;
> +	union {
> +		struct attribute	**attrs;
> +		const struct attribute	*const *attrs_new;

I know you will drop the "_new" prefix after a while, but "new" is
relative, and not very descriptive.  How about "_const"?

> +	};
>  	union {
>  		const struct bin_attribute	*const *bin_attrs;
>  		const struct bin_attribute	*const *bin_attrs_new;

There is no bin_attrs_new anymore.  Finally.  sorry about that...

greg k-h

Re: [PATCH v3 1/7] sysfs: attribute_group: allow registration of const attribute

Posted by Thomas Weißschuh 1 month, 2 weeks ago

On 2025-08-19 13:22:55+0200, Greg Kroah-Hartman wrote:
> On Mon, Aug 11, 2025 at 11:14:27AM +0200, Thomas Weißschuh wrote:
> > To be able to constify instances of struct attribute it has to be
> > possible to add them to struct attribute_group.
> > The current type of the attrs member however is not compatible with that.
> > Introduce a union that allows registration of both const and non-const
> > attributes to enable a piecewise transition.
> > As both union member types are compatible no logic needs to be adapted.
> > 
> > Technically it is now possible register a const struct
> > attribute and receive it as mutable pointer in the callbacks.
> > This is a soundness issue.
> > But this same soundness issue already exists today in
> > sysfs_create_file().
> > Also the struct definition and callback implementation are always
> > closely linked and are meant to be moved to const in lockstep.
> > 
> > Similar to commit 906c508afdca ("sysfs: attribute_group: allow registration of const bin_attribute")
> > 
> > Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> > ---
> >  include/linux/sysfs.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> > index f418aae4f1134f8126783d9e8eb575ba4278e927..a47092e837d9eb014894d1f7e49f0fd0f9a2e350 100644
> > --- a/include/linux/sysfs.h
> > +++ b/include/linux/sysfs.h
> > @@ -105,7 +105,10 @@ struct attribute_group {
> >  	size_t			(*bin_size)(struct kobject *,
> >  					    const struct bin_attribute *,
> >  					    int);
> > -	struct attribute	**attrs;
> > +	union {
> > +		struct attribute	**attrs;
> > +		const struct attribute	*const *attrs_new;
> 
> I know you will drop the "_new" prefix after a while, but "new" is
> relative, and not very descriptive.

That is somewhat intentional to express that it is a transitional thing.

> How about "_const"?

At some point the regular variant will be const too, so "_const" would
be a bit weird.

> > +	};
> >  	union {
> >  		const struct bin_attribute	*const *bin_attrs;
> >  		const struct bin_attribute	*const *bin_attrs_new;
> 
> There is no bin_attrs_new anymore.  Finally.  sorry about that...

Thanks! No worries.


Thomas

Re: [PATCH v3 1/7] sysfs: attribute_group: allow registration of const attribute

Posted by Greg Kroah-Hartman 1 month, 2 weeks ago

On Tue, Aug 19, 2025 at 03:59:04PM +0200, Thomas Weißschuh wrote:
> On 2025-08-19 13:22:55+0200, Greg Kroah-Hartman wrote:
> > On Mon, Aug 11, 2025 at 11:14:27AM +0200, Thomas Weißschuh wrote:
> > > To be able to constify instances of struct attribute it has to be
> > > possible to add them to struct attribute_group.
> > > The current type of the attrs member however is not compatible with that.
> > > Introduce a union that allows registration of both const and non-const
> > > attributes to enable a piecewise transition.
> > > As both union member types are compatible no logic needs to be adapted.
> > > 
> > > Technically it is now possible register a const struct
> > > attribute and receive it as mutable pointer in the callbacks.
> > > This is a soundness issue.
> > > But this same soundness issue already exists today in
> > > sysfs_create_file().
> > > Also the struct definition and callback implementation are always
> > > closely linked and are meant to be moved to const in lockstep.
> > > 
> > > Similar to commit 906c508afdca ("sysfs: attribute_group: allow registration of const bin_attribute")
> > > 
> > > Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> > > ---
> > >  include/linux/sysfs.h | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> > > index f418aae4f1134f8126783d9e8eb575ba4278e927..a47092e837d9eb014894d1f7e49f0fd0f9a2e350 100644
> > > --- a/include/linux/sysfs.h
> > > +++ b/include/linux/sysfs.h
> > > @@ -105,7 +105,10 @@ struct attribute_group {
> > >  	size_t			(*bin_size)(struct kobject *,
> > >  					    const struct bin_attribute *,
> > >  					    int);
> > > -	struct attribute	**attrs;
> > > +	union {
> > > +		struct attribute	**attrs;
> > > +		const struct attribute	*const *attrs_new;
> > 
> > I know you will drop the "_new" prefix after a while, but "new" is
> > relative, and not very descriptive.
> 
> That is somewhat intentional to express that it is a transitional thing.

Fair, but given the huge quantity here, it's going to take a long time,
so "new" is going to be rough to push through for 6+ months.

> > How about "_const"?
> 
> At some point the regular variant will be const too, so "_const" would
> be a bit weird.

Yes, that's when you "switch it back", right?  You would have to do that
for _new as well.

thanks,

greg k-h

Re: [PATCH v3 1/7] sysfs: attribute_group: allow registration of const attribute

Posted by Thomas Weißschuh 1 month, 2 weeks ago

On 2025-08-19 16:10:42+0200, Greg Kroah-Hartman wrote:
> On Tue, Aug 19, 2025 at 03:59:04PM +0200, Thomas Weißschuh wrote:
> > On 2025-08-19 13:22:55+0200, Greg Kroah-Hartman wrote:
> > > On Mon, Aug 11, 2025 at 11:14:27AM +0200, Thomas Weißschuh wrote:
> > > > To be able to constify instances of struct attribute it has to be
> > > > possible to add them to struct attribute_group.
> > > > The current type of the attrs member however is not compatible with that.
> > > > Introduce a union that allows registration of both const and non-const
> > > > attributes to enable a piecewise transition.
> > > > As both union member types are compatible no logic needs to be adapted.
> > > > 
> > > > Technically it is now possible register a const struct
> > > > attribute and receive it as mutable pointer in the callbacks.
> > > > This is a soundness issue.
> > > > But this same soundness issue already exists today in
> > > > sysfs_create_file().
> > > > Also the struct definition and callback implementation are always
> > > > closely linked and are meant to be moved to const in lockstep.
> > > > 
> > > > Similar to commit 906c508afdca ("sysfs: attribute_group: allow registration of const bin_attribute")
> > > > 
> > > > Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> > > > ---
> > > >  include/linux/sysfs.h | 5 ++++-
> > > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> > > > index f418aae4f1134f8126783d9e8eb575ba4278e927..a47092e837d9eb014894d1f7e49f0fd0f9a2e350 100644
> > > > --- a/include/linux/sysfs.h
> > > > +++ b/include/linux/sysfs.h
> > > > @@ -105,7 +105,10 @@ struct attribute_group {
> > > >  	size_t			(*bin_size)(struct kobject *,
> > > >  					    const struct bin_attribute *,
> > > >  					    int);
> > > > -	struct attribute	**attrs;
> > > > +	union {
> > > > +		struct attribute	**attrs;
> > > > +		const struct attribute	*const *attrs_new;
> > > 
> > > I know you will drop the "_new" prefix after a while, but "new" is
> > > relative, and not very descriptive.
> > 
> > That is somewhat intentional to express that it is a transitional thing.
> 
> Fair, but given the huge quantity here, it's going to take a long time,
> so "new" is going to be rough to push through for 6+ months.

Looking at how 'struct bin_attribute' went probably quite a bit longer.

> > > How about "_const"?
> > 
> > At some point the regular variant will be const too, so "_const" would
> > be a bit weird.
> 
> Yes, that's when you "switch it back", right?  You would have to do that
> for _new as well.

There will probably be some overlap. But in the end it probably
doesn't matter. Let's go with "_const".

Thomas