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

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Dec 22, 2023 at 10:57:16AM +0800, Guo Ren wrote:
> On Fri, Dec 22, 2023 at 9:51 AM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > Hello Guo Ren,
> >
> > On Thu, Dec 21, 2023 at 10:46:58AM -0500, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>
> > >
> > > 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.
> Yes, it causes the problem, which causes the boot to fail.

I think what Leonardo is getting at is that it is odd that it would
cause boot issues if TASK_SIZE is not equal STACK_TOP. This seems
indicative of a different problem. While this may fix the issue, it
should be valid for TASK_SIZE to be less than STACK_TOP.

- Charlie

> 
> >
> > Then why not:
> > #ifdef CONFIG_COMPAT
> > #define TASK_SIZE_32    STACK_TOP
> Yes, it's the solution that I think at first. But I didn't find any
> problem with 0x7ffff000 ~ 0x80000000, and then I removed this gap to
> unify it with Sv39 and Sv48.
> 
> >
> > With some comments explaining why there is no need to reserve a PAGE_SIZE
> > in the TASK_SIZE_32.
> At first, I wanted to put a invalid page between the user & kernel
> space, but it seems useless.
> 
> >
> > 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@vger.kernel.org
> > > Fixes: add2cc6b6515 ("RISC-V: mm: Restrict address space for sv39,sv48,sv57")
> > > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > ---
> > >  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
> > >
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren