Re: [PATCH V2 2/4] riscv: mm: Fixup compat arch_get_mmap_end

[Leonardo Bras <leobras@redhat.com>]

On Fri, Dec 22, 2023 at 12:26:19PM +0800, Guo Ren wrote:
> On Fri, Dec 22, 2023 at 11:35 AM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > On Thu, Dec 21, 2023 at 10:46:59AM -0500, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.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@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/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)            \

I am failing to understand exactly how are they equal.
I mean, what in your STACK_TOP_MAX change made them equal?

See below, the behavior changed: 
> 
> >
> > 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)
> >

^

With your change on STACK_TOP_MAX only (not changing arch_get_mmap_end), 
you would have:

- compat_mode & (0 < addr < 2^32) 	-> mmap_end = 2^32
- non-compat, addr == 0, or addr > 2^57 -> mmap_end = TASK_SIZE_64
- non-compat, (2^48 < addr < 2^57)	-> mmap_end = 2^48
- non-compat, (0 < addr < 2^48) 	-> mmap_end = 2^39

Which seems more likely, based on Charlie comments.

Thanks,
Leo

> > 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
> 


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