Re: [PATCH V3 1/4] riscv: mm: Fixup compat mode boot failure

[Leonardo Bras]

On Fri, Dec 22, 2023 at 06:57:00AM -0500, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> In COMPAT mode, the STACK_TOP is DEFAULT_MAP_WINDOW (0x80000000), but
> the TASK_SIZE is 0x7fff000. When the user stack is upon 0x7fff000, it
> will cause a user segment fault. Sometimes, it would cause boot
> failure when the whole rootfs is rv32.
> 
> Freeing unused kernel image (initmem) memory: 2236K
> Run /sbin/init as init process
> Starting init: /sbin/init exists but couldn't execute it (error -14)
> Run /etc/init as init process
> ...
> 
> Increase the TASK_SIZE to cover STACK_TOP.
> 
> 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/pgtable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index ab00235b018f..74ffb2178f54 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -881,7 +881,7 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>  #define TASK_SIZE_MIN	(PGDIR_SIZE_L3 * PTRS_PER_PGD / 2)
>  
>  #ifdef CONFIG_COMPAT
> -#define TASK_SIZE_32	(_AC(0x80000000, UL) - PAGE_SIZE)
> +#define TASK_SIZE_32	(_AC(0x80000000, UL))
>  #define TASK_SIZE	(test_thread_flag(TIF_32BIT) ? \
>  			 TASK_SIZE_32 : TASK_SIZE_64)
>  #else
> -- 
> 2.40.1
> 

I am not really involved in the issue this is solving, so I have no 
technical opinion on the solution. 

IIUC there should always be (TASK_SIZE >= STACK_TOP), so by itself this 
is fixing an issue.

I have reviewed the code and it does exactly as stated into the commit 
message, so FWIW:
Reviewed-by: Leonardo Bras <leobras@xxxxxxxxxx>