Re: [PATCH] arm64: mm: Fix dead assignment of old_pte

[Nathan Chancellor]

On Tue, Jul 02, 2019 at 04:13:02PM -0700, 'Nathan Huckleberry' via Clang Built Linux wrote:
> When analyzed with the clang static analyzer the
> following warning occurs
> 
> line 251, column 2
> Value stored to 'old_pte' is never read
> 
> This warning is repeated every time pgtable.h is
> included by another file and produces ~3500
> extra warnings.
> 
> Moving old_pte into preprocessor guard.
> 
> Cc: clang-built-linux@xxxxxxxxxxxxxxxx
> Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/pgtable.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index fca26759081a..42ca4fc67f27 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -238,8 +238,6 @@ extern void __sync_icache_dcache(pte_t pteval);
>  static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>  			      pte_t *ptep, pte_t pte)
>  {
> -	pte_t old_pte;
> -
>  	if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
>  		__sync_icache_dcache(pte);
>  
> @@ -248,8 +246,11 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>  	 * hardware updates of the pte (ptep_set_access_flags safely changes
>  	 * valid ptes without going through an invalid entry).
>  	 */
> +	#if IS_ENABLED(CONFIG_DEBUG_VM)
> +	pte_t old_pte;
> +
>  	old_pte = READ_ONCE(*ptep);
> -	if (IS_ENABLED(CONFIG_DEBUG_VM) && pte_valid(old_pte) && pte_valid(pte) &&
> +	if (pte_valid(old_pte) && pte_valid(pte) &&
>  	   (mm == current->active_mm || atomic_read(&mm->mm_users) > 1)) {
>  		VM_WARN_ONCE(!pte_young(pte),
>  			     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
> @@ -258,6 +259,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>  			     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
>  			     __func__, pte_val(old_pte), pte_val(pte));
>  	}
> +	#endif
>  
>  	set_pte(ptep, pte);
>  }
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

Hi Nathan,

This does not apply on -next because of https://git.kernel.org/arm64/c/9b604722059039a1a3ff69fb8dfd024264046024.
I would get into the habit of testing -next to see if the warning is
present there first because someone may have independently fixed it
already (I'd be surprised if it wasn't fixed by that commit from a quick
glance).

Additionally, when I do apply this patch to mainline and build, I see
the following warning:

In file included from /home/nathan/cbl/linux/arch/arm64/kernel/asm-offsets.c:10:
In file included from /home/nathan/cbl/linux/include/linux/arm_sdei.h:14:
In file included from /home/nathan/cbl/linux/include/acpi/ghes.h:5:
In file included from /home/nathan/cbl/linux/include/acpi/apei.h:9:
In file included from /home/nathan/cbl/linux/include/linux/acpi.h:34:
In file included from /home/nathan/cbl/linux/include/acpi/acpi_io.h:5:
In file included from /home/nathan/cbl/linux/include/linux/io.h:13:
In file included from /home/nathan/cbl/linux/arch/arm64/include/asm/io.h:18:
/home/nathan/cbl/linux/arch/arm64/include/asm/pgtable.h:250:8: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]
        pte_t old_pte;
              ^
1 warning generated.

Cheers,
Nathan