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

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Date: Fri, 22 Dec 2023 10:57:16 +0800
From: Guo Ren <guoren@...nel.org>
To: Leonardo Bras <leobras@...hat.com>
Cc: linux-kernel@...r.kernel.org, paul.walmsley@...ive.com, palmer@...belt.com, 
	alexghiti@...osinc.com, charlie@...osinc.com, xiao.w.wang@...el.com, 
	david@...hat.com, panqinglin2020@...as.ac.cn, rick.p.edgecombe@...el.com, 
	willy@...radead.org, bjorn@...osinc.com, conor.dooley@...rochip.com, 
	cleger@...osinc.com, linux-riscv@...ts.infradead.org, 
	Guo Ren <guoren@...ux.alibaba.com>, stable@...r.kernel.org
Subject: Re: [PATCH V2 1/4] riscv: mm: Fixup compat mode boot failure

On Fri, Dec 22, 2023 at 9:51 AM Leonardo Bras <leobras@...hat.com> wrote:
>
> Hello Guo Ren,
>
> On Thu, Dec 21, 2023 at 10:46:58AM -0500, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.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.

>
> 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@...r.kernel.org
> > Fixes: add2cc6b6515 ("RISC-V: mm: Restrict address space for sv39,sv48,sv57")
> > Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@...nel.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