Re: [PATCH] [RFC] instrumented.h: mark instrument_write non-const

[Alexander Gordeev]

On Wed, Nov 11, 2026 at 10:21:31AM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> I got another instance of a -Wmaybe-uninitialized in ntfs, as this is
> the only place that enables the warning:
> 
> In file included from include/asm-generic/rwonce.h:26,
>                  from fs/ntfs3/file.c:10:
> In function 'instrument_copy_from_user_before',
>     inlined from '_inline_copy_from_user' at include/linux/uaccess.h:184:2,
>     inlined from 'copy_from_user' at include/linux/uaccess.h:222:9,
>     inlined from 'ntfs_ioctl_fitrim' at fs/ntfs3/file.c:78:6,
>     inlined from 'ntfs_ioctl' at fs/ntfs3/file.c:246:10:
> include/linux/kasan-checks.h:38:27: error: 'range' may be used uninitialized [-Werror=maybe-uninitialized]
>    38 | #define kasan_check_write __kasan_check_write
> include/linux/instrumented.h:146:9: note: in expansion of macro 'kasan_check_write'
>   146 |         kasan_check_write(to, n);
>       |         ^~~~~~~~~~~~~~~~~
> include/linux/kasan-checks.h: In function 'ntfs_ioctl':
> include/linux/kasan-checks.h:20:6: note: by argument 1 of type 'const volatile void *' to '__kasan_check_write' declared here
>    20 | bool __kasan_check_write(const volatile void *p, unsigned int size);
>       |      ^~~~~~~~~~~~~~~~~~~
> fs/ntfs3/file.c:66:29: note: 'range' declared here
>    66 |         struct fstrim_range range;
>       |                             ^~~~~
> 
> The problem here is that copy_from_user() is called on an uninitialized
> object, but the instrumentation passes the pointer into a function that
> takes a const object. gcc deduces that any access to that object is
> undefined behavior and warns about it.
> 
> The only workaround I could come up with is to remove the 'const' keyword
> from these annotations, which of course is not great either, as the
> functions do not actually access the data at all.
> 
> This also found incorrect 'const' annotations in
> arch/s390/include/asm/fpu-insn.h, which writes to those pointers.
> 
> Any ideas?
> 
> Fixes: 33b75c1d884e ("instrumented.h: allow instrumenting both sides of copy_from_user()")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  arch/s390/include/asm/fpu-insn.h |  8 ++++----
>  include/linux/instrumented.h     | 10 +++++-----
>  include/linux/kasan-checks.h     |  6 +++---
>  mm/kasan/shadow.c                |  2 +-
>  4 files changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/s390/include/asm/fpu-insn.h b/arch/s390/include/asm/fpu-insn.h
> index aaf1faeb3dc5..7cd6e9f30485 100644
> --- a/arch/s390/include/asm/fpu-insn.h
> +++ b/arch/s390/include/asm/fpu-insn.h
> @@ -359,7 +359,7 @@ static __always_inline void fpu_vsrlb(u8 v1, u8 v2, u8 v3)
>  
>  #ifdef CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS
>  
> -static __always_inline void fpu_vst(u8 v1, const void *vxr)
> +static __always_inline void fpu_vst(u8 v1, void *vxr)
>  {
>  	instrument_write(vxr, sizeof(__vector128));
>  	asm volatile("VST	%[v1],%O[vxr],,%R[vxr]"
> @@ -370,7 +370,7 @@ static __always_inline void fpu_vst(u8 v1, const void *vxr)
>  
>  #else /* CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS */
>  
> -static __always_inline void fpu_vst(u8 v1, const void *vxr)
> +static __always_inline void fpu_vst(u8 v1, void *vxr)
>  {
>  	instrument_write(vxr, sizeof(__vector128));
>  	asm volatile(
> @@ -385,7 +385,7 @@ static __always_inline void fpu_vst(u8 v1, const void *vxr)
>  
>  #ifdef CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS
>  
> -static __always_inline void fpu_vstl(u8 v1, u32 index, const void *vxr)
> +static __always_inline void fpu_vstl(u8 v1, u32 index, void *vxr)
>  {
>  	unsigned int size;
>  
> @@ -400,7 +400,7 @@ static __always_inline void fpu_vstl(u8 v1, u32 index, const void *vxr)
>  
>  #else /* CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS */
>  
> -static __always_inline void fpu_vstl(u8 v1, u32 index, const void *vxr)
> +static __always_inline void fpu_vstl(u8 v1, u32 index, void *vxr)
>  {
>  	unsigned int size;
>  

Acked-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx> # s390