Re: [PATCH V2 1/4] riscv: mm: Fixup compat mode boot failure

[Leonardo Bras]

Hello Guo Ren,

On Thu, Dec 21, 2023 at 10:46:58AM -0500, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> In COMPAT mode, the STACK_TOP is 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.

Checking if I get the scenario:

In pgtable.h:
#ifdef CONFIG_64BIT
#define TASK_SIZE_64    (PGDIR_SIZE * PTRS_PER_PGD / 2)
#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       (test_thread_flag(TIF_32BIT) ? \
                         TASK_SIZE_32 : TASK_SIZE_64)
#else
[...]

Meaning CONFIG_COMPAT is only available in CONFIG_64BIT, and TASK_SIZE in 
compat mode is either TASK_SIZE_32 or TASK_SIZE_64 depending on the thread_flag.

from processor.h:
#ifdef CONFIG_64BIT
#define DEFAULT_MAP_WINDOW      (UL(1) << (MMAP_VA_BITS - 1))
#define STACK_TOP_MAX           TASK_SIZE_64
[...]
#define STACK_TOP               DEFAULT_MAP_WINDOW


where:
#define MMAP_VA_BITS (is_compat_task() ? VA_BITS_SV32 : MMAP_VA_BITS_64)
with MMAP_VA_BITS_64 being either 48 or 37.

In compat mode,
STACK_TOP = 1 << (32 - 1) 	-> 0x80000000
TASK_SIZE = 0x8000000 - 4k 	-> 0x7ffff000

IIUC, your suggestion is to make TASK_SIZE = STACK_TOP in compat mode only.

Then why not:
#ifdef CONFIG_COMPAT
#define TASK_SIZE_32    STACK_TOP

With some comments explaining why there is no need to reserve a PAGE_SIZE 
in the TASK_SIZE_32.

Does that make sense?

Thanks!
Leo

> 
> 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
> ...
> 
> 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
>