linux-kernel - Re: [PATCH V2 2/4] riscv: mm: Fixup compat arch_get_mmap

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Date: Fri, 22 Dec 2023 12:26:19 +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 2/4] riscv: mm: Fixup compat arch_get_mmap_end

On Fri, Dec 22, 2023 at 11:35 AM Leonardo Bras <leobras@...hat.com> wrote:
>
> On Thu, Dec 21, 2023 at 10:46:59AM -0500, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.alibaba.com>
> >
> > 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@...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/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.
The above is just code simplification; if STACK_TOP_MAX is TASK_SIZE, then

     if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
             mmap_end = STACK_TOP_MAX;                       \
    else if ((_addr) >= VA_USER_SV57)                       \

is equal to:

     if ((_addr) == 0 || (_addr) >= VA_USER_SV57)            \

>
> 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?
Maybe I should move this into the optimization part.

>
> (I will send an RFC 'fixing' the code the way I am whinking it should look
> like)
>
> Thanks,
> Leo
>
>
>
>
>
> > --
> > 2.40.1
> >
>


-- 
Best Regards
 Guo Ren