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

[Leonardo Bras <leobras@redhat.com>]

On Fri, Dec 22, 2023 at 12:50:44PM +0800, Guo Ren wrote:
> On Fri, Dec 22, 2023 at 12:43 PM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > 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?
> #define STACK_TOP_MAX TASK_SIZE
> #define TASK_SIZE       (is_compat_task() ? TASK_SIZE_32 : TASK_SIZE_64)
> 

yes, I am aware. Let's do a simple test with the new code and
addr = 2^27 (random 32-bit addr) and compat mode.

if ((_addr) == 0 || (_addr) >= VA_USER_SV57)
	// Evaluates to false: 2^27 != 0, and is < 2^57
else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) 
	// Evaluates to false: 2^27 < 2^48
else				
	mmap_end = VA_USER_SV39;	

mmap_end = VA_USER_SV39, even in compat_mode.

We need the extra is_compat_task() if we want to return 2^32.

Thanks!
Leo


> >
> > 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
> compat_mode      -> mmap_end = 2^32
> 

This is correct! 
Yeah, since you changed STACK_TOP_MAX to be 2^32 in compat mode,
any addr value < 2^32 with compat value will return 2^32.
(without the change in arch_get_mmap_end(), that is.) 

> > - 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
> > >
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren
> 


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