Re: [PATCH V2 2/4] riscv: mm: Fixup compat arch_get_mmap_end

[Leonardo Bras]

On Thu, Dec 21, 2023 at 10:46:59AM -0500, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> When the task is in COMPAT mode, the arch_get_mmap_end should be 2GB,
> not TASK_SIZE_64. The TASK_SIZE has contained is_compat_mode()
> detection, so change the definition of STACK_TOP_MAX to TASK_SIZE
> directly.

ok

> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: add2cc6b6515 ("RISC-V: mm: Restrict address space for sv39,sv48,sv57")
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/processor.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index f19f861cda54..1f538fc4448d 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -16,15 +16,13 @@
>  
>  #ifdef CONFIG_64BIT
>  #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
> -#define STACK_TOP_MAX		TASK_SIZE_64
> +#define STACK_TOP_MAX		TASK_SIZE

It means STACK_TOP_MAX will be in 64BIT:
- TASK_SIZE_32 if compat_mode=y
- TASK_SIZE_64 if compat_mode=n

Makes sense for me.

>  
>  #define arch_get_mmap_end(addr, len, flags)			\
>  ({								\
>  	unsigned long mmap_end;					\
>  	typeof(addr) _addr = (addr);				\
> -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> -		mmap_end = STACK_TOP_MAX;			\
> -	else if ((_addr) >= VA_USER_SV57)			\
> +	if ((_addr) == 0 || (_addr) >= VA_USER_SV57)		\
>  		mmap_end = STACK_TOP_MAX;			\
>  	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) \
>  		mmap_end = VA_USER_SV48;			\


I don't think I got this change, or how it's connected to the commit msg.

Before:
- addr == 0, or addr > 2^57, or compat: mmap_end = STACK_TOP_MAX
- 2^48 < addr < 2^57: mmap_end = 2^48
- 0 < addr < 2^48 : mmap_end = 2^39

Now:
- addr == 0, or addr > 2^57: mmap_end = STACK_TOP_MAX
- 2^48 < addr < 2^57: mmap_end = 2^48
- 0 < addr < 2^48 : mmap_end = 2^39

IIUC compat mode addr will be < 2^32, so will always have mmap_end = 2^39 
if addr != 0. Is that desireable? 
(if not, above change is unneeded)

Also, unrelated to the change:
- 2^48 < addr < 2^57: mmap_end = 2^48
Is the above correct?
It looks like it should be 2^57 instead, and a new if clause for 
2^32 < addr < 2^48 should have mmap_end = 2^48.

Do I get it wrong?

(I will send an RFC 'fixing' the code the way I am whinking it should look 
like)

Thanks, 
Leo





> -- 
> 2.40.1
>