Subject: Re: [PATCH] arm64: mmu: no write cache for O_SYNC flag

From: Catalin Marinas <[email-masked]>
Date: Thu, 26 Mar 2020 16:55:20 +0000

On Thu, Mar 26, 2020 at 09:36:25AM -0700, Li Wang wrote:
> reproduce steps:
> 1.
> disable CONFIG_STRICT_DEVMEM in linux kernel
> 2.
> Process A gets a Physical Address of global variable by
> "/proc/self/pagemap".
> 3.
> Process B writes a value to the same Physical Address by mmap():
> fd=open("/dev/mem",O_SYNC);
> Virtual Address=mmap(fd);
> 
> problem symptom:
> after Process B write a value to the Physical Address,
> Process A of the value of global variable does not change.
> They both W/R the same Physical Address.
> 
> technical reason:
> Process B writing the Physical Address is by the Virtual Address,
> and the Virtual Address comes from "/dev/mem" and mmap().
> In arm64 arch, the Virtual Address has write cache.
> So, maybe the value is not written into Physical Address.
> 
> fix reason:
> giving write cache flag in arm64 is in phys_mem_access_prot():
> =====
> arch/arm64/mm/mmu.c
> phys_mem_access_prot()
> {
>   if (!pfn_valid(pfn))
>     return pgprot_noncached(vma_prot);
>   else if (file->f_flags & O_SYNC)
>     return pgprot_writecombine(vma_prot);
>   return vma_prot;
> }
> ====
> the other arch and the share function drivers/char/mem.c of phys_mem_access_prot()
> does not add write cache flag.
> So, removing the flag to fix the issue

Other architectures may have transparent caches and don't require
different attributes.

> Signed-off-by: Li Wang <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> ---
>  arch/arm64/mm/mmu.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 128f70852bf3..d7083965ca17 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -81,8 +81,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  {
>  	if (!pfn_valid(pfn))
>  		return pgprot_noncached(vma_prot);
> -	else if (file->f_flags & O_SYNC)
> -		return pgprot_writecombine(vma_prot);
>  	return vma_prot;
>  }
>  EXPORT_SYMBOL(phys_mem_access_prot);

A better solution is for user space not to pass O_SYNC when opening
/dev/mem. We've had this ABI for a long time (arch/arm/ and several
other architectures do the same), why change it now?

-- 
Catalin

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