RE: [PATCH] microblaze: remove CONFIG_SET_FS

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd
> Sent: 09 February 2022 14:49
> 
> Remove the address space override API set_fs().  The microblaze user
> address space is now limited to TASK_SIZE.
> 
> To support this we implement and wire in __get_kernel_nofault and
> __set_kernel_nofault.
> 
> The function user_addr_max is removed as there is a default definition
> provided when CONFIG_SET_FS is not used.
...
>  static inline int access_ok(const void __user *addr, unsigned long size)
>  {
>  	if (!size)
>  		goto ok;
> 
> -	if ((get_fs().seg < ((unsigned long)addr)) ||
> -			(get_fs().seg < ((unsigned long)addr + size - 1))) {
> -		pr_devel("ACCESS fail at 0x%08x (size 0x%x), seg 0x%08x\n",
> -			(__force u32)addr, (u32)size,
> -			(u32)get_fs().seg);
> +	if ((((unsigned long)addr) > TASK_SIZE) ||
> +	    (((unsigned long)addr + size - 1) > TASK_SIZE)) {
> +		pr_devel("ACCESS fail at 0x%08x (size 0x%x)",
> +			(__force u32)addr, (u32)size);
>  		return 0;

Isn't that the wrong check?
If 'size' is big 'addr + size' can wrap.

It needs to be (addr >= TASK_SIZE || size > TASK_SIZE - addr)

Which is pretty much a generic version.
Although typical 64bit architectures can use the faster:
	((addr | size) >> 62)

	David

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