Re: [PATCH 1/2] uaccess: unify inline vs outline copy_{from,to}_user() selection

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 26/03/2026 à 18:29, Yury Norov a écrit :
> On Thu, Mar 26, 2026 at 02:44:40PM +0100, Christophe Leroy (CS GROUP) wrote:
>>
>>
>> Le 25/03/2026 à 17:33, Yury Norov a écrit :
>>> The kernel allows arches to select between inline and outline
>>> implementations of the copy_{from,to}_user() by defining individual
>>> INLINE_COPY_FROM_USER and INLINE_COPY_TO_USER, correspondingly.
>>> However, all arches enable or disable them always together.
>>>
>>> Without the real use-case for one helper being inlined while the other
>>> outlined, having independent controls is excessive and error prone.
>>>
>>> Switch the codebase to the single unified INLINE_COPY_USER control.
>>
>> Could we use a (non user selectable) Kconfig item instead, e.g.
>> CONFIG_ARCH_WANT_OUTLINE_USER_COPY ?
> 
> This sounds interesting. I need to wrap it around my head for a while.
> Right now, I believe, the best solution is to isolate this setting
> from the sources as much as we can, and your suggestion looks like a
> step forward.
> 
> Overall, I'm puzzled why some arches enable this while the others
> don't. The next question is why copy_{from,to}_user is so special.
> If this function benefits from being inlined for that particular
> arch or compiler, which functions would also benefit and why?
> 
> Reasoning logically, if this WANT_INLINE thing makes sense, it would
> make much more sense if we create a machinery for something like:
> 
>          unsigned long __arch_inline copy_to_user();
> 
>> Also, looks like only powerpc doesn't select INLINE_COPY. Would it be
>> cleaner to change the logic to a flag for OUTLINE_COPY ?
> 
> How that? x86_64 outlines it. Check it yourself:
> 
>    @@ -206,7 +206,9 @@ _inline_copy_to_user(void __user *to, const void *from, unsigned long n)
>     #ifdef INLINE_COPY_USER
>     # define _copy_to_user _inline_copy_to_user
>     # define _copy_from_user _inline_copy_from_user
>    +#error INLINE_COPY_USER
>     #else
>    +#error OUTLINE_COPY_USER
>     extern __must_check unsigned long
>     _copy_from_user(void *, const void __user *, unsigned long);
> 
> If it was really a single arch, it would be worth to discuss what for do
> we need this customization at all.

Hum ... You are right. I don't know why, I thought powerpc was the only one.

Nevertheless, among the 21 architectures in arch/ , 17 of them select 
INLINE_COPY_FROM_USER, which means only 4 don't define it.

So might still be interesting to reverse the logic, then only 4 arches 
will have to select it, similar to the exemple below with powerpc:

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index ad7a2fe63a2a..58743cb0e305 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -186,6 +186,7 @@ config PPC
  	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
  	select ARCH_WANT_LD_ORPHAN_WARN
  	select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP	if PPC_RADIX_MMU
+	select ARCH_WANT_OUTLINE_USER_COPY
  	select ARCH_WANTS_MODULES_DATA_IN_VMALLOC	if PPC_BOOK3S_32 || PPC_8xx
  	select ARCH_WEAK_RELEASE_ACQUIRE
  	select BINFMT_ELF
diff --git a/lib/Kconfig b/lib/Kconfig
index 0f2fb9610647..0106c1facfa4 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -550,6 +550,9 @@ config ARCH_HAS_UACCESS_FLUSHCACHE
  config ARCH_HAS_COPY_MC
  	bool

+config ARCH_WANT_OUTLINE_USER_COPY
+	bool
+
  # Temporary. Goes away when all archs are cleaned up
  config ARCH_STACKWALK
         bool


Christophe